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Gale Encyclopedia of Genetic Disorder / Gale Encyclopedia of Genetic Disorders, Two Volume Set - Volume 2 - M-Z - I

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symptoms. It is important to remember that if a pregnancy inherits a condition that is associated with oligohydramnios sequence, it does not necessarily mean that the pregnancy will develop oligohydramnios sequence. Therefore, for each subsequent pregnancy, the risk is related to inheriting the condition or syndrome, not necessarily to develop oligohydramnios sequence.

Demographics

There is no one group of individuals or one particular sex that have a higher risk to develop oligohydramnios sequence. Although, some of the inherited conditions that have been associated with oligohydramnios sequence may be more common in certain regions of the world or in certain ethnic groups.

Signs and symptoms

With severe oligohydramnios, because of the lack of amniotic fluid, the amniotic cavity remains small, thereby constricting the fetus. As the fetus grows, the amniotic cavity tightens around the fetus, inhibiting normal growth and development. This typically results in the formation of certain facial features, overall small size, wrinkled skin, and prevents the arms and legs from moving.

The facial features seen in oligohydramnios sequence include a flattened face, wide-set eyes, a flattened, beaked nose, ears set lower on the head than expected (low-set ears), and a small, receding chin (micrognathia).

Because the movement of the arms and legs are restricted, a variety of limb deformities can occur, including bilateral clubfoot (both feet turned to the side), dislocated hips, broad flat hands and joint contractures (inability for the joints to fully extend). Contractures tend to be seen more often in fetuses where the oligohydramnios occurred during the second trimester. Broad, flat hands tend to be seen more often in fetuses where the oligohydramnios began during the third trimester.

Fetuses with oligohydramnios sequence also tend to have pulmonary hypoplasia (underdevelopment of the lungs). The pulmonary hypoplasia is felt to occur as a result of the compression of the fetal chest (thorax), although it has been suggested that pulmonary hypoplasia may develop before 16 weeks of pregnancy in some cases. Therefore, regardless of the cause of the severe oligohydramnios, the physical features that develop and are seen in oligohydramnios sequence tend to be the same.

Diagnosis

An ultrasound examination during the second and/or third trimester of a pregnancy is a good tool to help detect

Low set ears are a common feature of infants with olioghydramnios sequence. (Custom Medical Stock Photo, Inc.)

the presence of oligohydramnios. Since oligohydramnios can occur later in a pregnancy, an ultrasound examination performed during the second trimester may not detect the presence of oligohydramnios. In pregnancies affected with oligohydramnios, an ultrasound examination can be difficult to perform because there is less amniotic fluid around the fetus. Therefore, an ultrasound examination may not be able to detect the underlying cause of the oligohydramnios.

In some situations, an amnioinfusion (injection of fluid into the amniotic cavity) is performed. This can sometimes help determine if the cause of the oligohydramnios was leakage of the amniotic fluid. Amnioinfusions may also be used to help visualize the fetus on ultrasound in attempts to detect any fetal abnormalities.

Additionally, maternal serum screening may detect the presence of oligohydramnios in a pregnancy. Maternal serum screening is a blood test offered to preg-

sequence Oligohydramnios

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nant women to help determine the chance that their baby may have Down syndrome, Trisomy 18, and spina bifida. This test is typically performed between the fifteenth and twentith week of a pregnancy. The test works by measuring amount of certain substances in the maternal circulation.

Alpha-fetoprotein (AFP) is a protein produced mainly by the fetal liver and is one of the substances measured in the mother’s blood. The level of AFP in the mother’s blood has been used to help find pregnancies at higher risk to have spina bifida. An elevated AFP in the mother’s blood, which is greater than 2.5 multiples of the median (MoM), has also been associated with several conditions, including the presence of oligohydramnios in a pregnancy. Since oligohydramnios is just one of several explanations for an elevated AFP level, an ultrasound examination is recommended when there is an elevated AFP level. However, not all pregnancies affected with oligohydramnios will have an elevated AFP level, some pregnancies with oligohydramnios will have the AFP level within the normal range.

Because fetuses with oligohydramnios sequence can have other anomalies, a detailed examination of the fetus should be performed. Knowing all the abnormalities a fetus has is important in making an accurate diagnosis. Knowing the cause of the oligohydramnios and if it is related to a syndrome or genetic condition is essential in predicting the chance for the condition to occur again in a future pregnancy. Sometimes the fetal abnormalities can be detected on a prenatal ultrasound examination or on an external examination of the fetus after delivery. However, several studies have shown that an external examination of the fetus can miss some fetal abnormalities and have stressed the importance of performing an autopsy to make an accurate diagnosis.

Treatment and management

There is currently no treatment or prevention for oligohydramnios sequence. Amnioinfusions, which can assist in determining the cause of the oligohydramnios in a pregnancy, is not recommended as a treatment for oligohydramnios sequence.

Prognosis

Pregnancies affected with oligohydramnios sequence can miscarry, be stillborn, or die shortly after birth. This condition is almost always fatal because the lungs do not develop completely (pulmonary hypoplasia).

Resources

BOOKS

Larsen, William J. Human Embryology. Churchill Livingstone,

Inc. 1993.

PERIODICALS

Christianson, C., et. al. “Limb Deformations in Oligohydramnios Sequence.” American Journal of Medical Genetics 86 (1999): 430-433.

Curry, C. J. R., et. al. “The Potter Sequence: A Clinical Analysis of 80 Cases.” American Journal of Medical Genetics 19 (1984): 679-702.

Locatelli, Anna, et. al. “Role of amnioinfusion in the management of premature rupture of the membranes at less than 26 weeks’ gestation.” American Journal of Obstetrics and

Gynecology 183, no. 4 (October 2000): 878-882. Newbould, M. J., et. al. “Oligohydramnios Sequence: The

Spectrum of Renal Malformation.” British Journal of

Obstetrics and Gynaecology 101 (1994): 598-604.

Scott, R. J., and S. F. Goodburn. “Potter’s Syndrome in the Second Trimester-Prenatal Screening and Pathological Findings in 60 cases of Oligohydramnios Sequence.”

Prenatal Diagnosis 15 (1995): 519-525.

Sharon A. Aufox, MS, CGC

Ollier disease see Chondrosarcoma

I Omphalocele

Definition

An omphalocele occurs when the abdominal wall does not close properly during fetal development. The extent to which abdominal contents protrude through the base of the umbilical cord will vary. A membrane usually covers the defect.

Description

An omphalocele is an abnormal closure of the abdominal wall. Between the sixth and tenth weeks of pregnancy, the intestines normally protrude into the umbilical cord as the baby is developing. During the tenth week, the intestines should return and rotate in such a way that the abdomen is closed around the umbilical cord. An omphalocele occurs when the intestines do not return, and this closure does not occur properly.

Genetic profile

In one-third of infants, an omphalocele occurs by itself, and is said to be an isolated abnormality. The cause

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of an isolated omphalocele is suspected to be multifactorial. Multifactorial means that many factors, both genetic and environmental, contribute to the cause. The specific genes involved, as well as the specific environmental factors are largely unknown. The chance for a couple to have another baby with an omphalocele, after they have had one with an isolated omphalocele is approximately one in 100 or 1%.

The remaining two-thirds of babies with an omphalocele have other birth defects, including problems with the heart (heart disease), spine (spina bifida), digestive system, urinary system, and the limbs.

Approximately 30% of babies with an omphalocele have a chromosome abnormality as the underlying cause of the omphalocele. Babies with chromosome abnormalities usually have multiple birth defects, so many babies will have other medical problems in addition to the omphalocele. Chromosomes are structures in the center of the cell that contain our genes; our genes code for our traits, such as blood type or eye color. The normal number of chromosomes is 46; having extra or missing chromosome material is associated with health problems. Babies with an omphalocele may have an extra chromosome number 13, 18, 21, or others. An omphalocele is sometimes said to occur more often in a mother who is older. This is because the chance for a chromosome abnormality to occur increases with maternal age.

Some infants with an omphalocele have a syndrome (collection of health problems). An example is

Beckwith-Wiedemann syndrome, where a baby is born larger than normal (macrosomia), has an omphalocele, and a large tongue (macroglossia). Finally, in some families, an omphalocele has been reported to be inherited as an autosomal dominant, or autosomal recessive trait. Autosomal means that males and females are equally affected. Dominant means that only one gene is necessary to produce the condition, while recessive means that two genes are necessary to have the condition. With autosomal dominant inheritance, there is a 50% chance with each pregnancy to have an affected child, while with autosomal recessive inheritance. the recurrence risk is 25%.

Demographics

Omphalocele is estimated to occur in one in 4,000 to one in 6,000 liveborns. Males are slightly more often affected than females (1.5:1).

Signs and symptoms

Anytime an infant is born with an omphalocele, a thorough physical examination is performed to determine whether the omphalocele is isolated or associated with

other health problems. To determine this, various studies may be performed such as a chromosome study, which is done from a small blood sample. Since the chest cavity may be small in an infant born with an omphalocele, the baby may have underdeveloped lungs, requiring breathing assistance with a ventilator (mechanical breathing machine). In 10–20% of infants, the sac has torn (ruptured), requiring immediate surgical repair, due to the risk of infection.

Diagnosis

During pregnancy, two different signs may cause a physician to suspect an omphalocele: increased fluid around the baby (polyhydramnios) on a fetal ultrasound and/or an abnormal maternal serum screening test, showing an elevated amount of alpha-fetoprotein (AFP). Maternal serum screening, measuring analytes present in the mother’s bloodstream only during pregnancy, is offered to pregnant women usually under the age of 35, to screen for various disorders such as Down syndrome, trisomy 18, and abnormalities of the spine (such as spina bifida). Other abnormalities can give an abnormal test result, and an omphalocele is an example.

An ultrasound is often performed as the first step when a woman’s maternal serum screening is abnormal, if one has not already been performed. Omphalocele is usually identifiable on fetal ultrasound. If a woman’s fetal ultrasound showed an omphalocele, polyhydramnios, or if she had an abnormal maternal serum screening test, an amniocentesis may be offered.

Amniocentesis is a procedure done under ultrasound guidance where a long thin needle is inserted into the mother’s abdomen, then into the uterus, to withdraw a couple tablespoons of amniotic fluid (fluid surrounding the developing baby) to study. Measurement of the AFP in the amniotic fluid can then be done to test for problems such as omphalocele. In addition, a chromosome analysis for the baby can be performed on the cells contained in the amniotic fluid. When the AFP in the amniotic fluid is elevated, an additional test is used to look for the presence or absence of an enzyme found in nerve tissue, called acetylcholinesterase, or ACHE. ACHE is present in the amniotic fluid only when a baby has an opening such as spina bifida or an omphalocele. Not all babies with an omphalocele will cause the maternal serum screening test to be abnormal or to cause extra fluid accumulation, but many will. At birth, an omphalocele is diagnosed by visual/physical examination.

Treatment and management

Treatment and management of an omphalocele depends upon the size of the abnormality, whether the sac

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K E Y T E R M S

Acetylcholinesterase (ACHE)—An enzyme found in nerve tissue.

Alpha-fetoprotein (AFP)—A chemical substance produced by the fetus and found in the fetal circulation. AFP is also found in abnormally high concentrations in most patients with primary liver cancer.

Amniocentesis—A procedure performed at 16-18 weeks of pregnancy in which a needle is inserted through a woman’s abdomen into her uterus to draw out a small sample of the amniotic fluid from around the baby. Either the fluid itself or cells from the fluid can be used for a variety of tests to obtain information about genetic disorders and other medical conditions in the fetus.

Amniotic fluid—The fluid which surrounds a developing baby during pregnancy.

Analyte—A chemical substance such as an enzyme, hormone, or protein.

Autosomal dominant—A pattern of genetic inheritance where only one abnormal gene is needed to display the trait or disease.

Autosomal recessive—A pattern of genetic inheritance where two abnormal genes are needed to display the trait or disease.

Beckwith-Wiedemann syndrome—A collection of health problems present at birth including an omphalocele, large tongue, and large body size.

Chromosome—A microscopic thread-like structure found within each cell of the body and consists of a complex of proteins and DNA. Humans have 46 chromosomes arranged into 23 pairs. Changes in either the total number of chromosomes or their shape and size (structure) may lead to physical or mental abnormalities.

Gastroschisis—A small defect in the abdominal wall normally located to the right of the umbilicus, and not covered by a membrane, where intestines and other organs may protrude.

Gene—A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.

Macroglossia—A large tongue.

Macrosomia—Overall large size due to overgrowth.

Maternal serum screening—A blood test offered to pregnant women usually under the age of 35, which measures analytes in the mother’s blood that are present only during pregnancy, to screen for Down syndrome, trisomy 18, and neural tube defects.

Multifactorial—Describes a disease that is the product of the interaction of multiple genetic and environmental factors.

Omphalocele—A birth defect where the bowel and sometimes the liver, protrudes through an opening in the baby’s abdomen near the umbilical cord.

Polyhydramnios—A condition in which there is too much fluid around the fetus in the amniotic sac.

Thoracic cavity—The chest.

Ultrasound—An imaging technique that uses sound waves to help visualize internal structures in the body.

Ventilator—Mechanical breathing machine.

Ventral wall defect—An opening in the abdomen (ventral wall). Examples include omphalocele and gastroschisis.

is intact or ruptured, and whether other health problems are present. A small omphalocele is usually repaired by surgery shortly after birth, where an operation is performed to return the organs to the abdomen and close the opening in the abdominal wall. If the omphalocele is large, where most of the intestines, liver, and/or spleen are present outside of the body, the repair is done in stages because the abdomen is small and may not be able to hold all of the organs at once. Initially, sterile protective gauze is placed over the abdominal organs whether the omphalocele is large or small. The exposed organs are then gradually moved back into the abdomen over

several days or weeks. The abdominal wall is surgically closed once all of the organs have been returned to the abdomen. Infants are often on a breathing machine (ventilator) until the abdominal cavity increases in size since returning the organs to the abdomen may crowd the lungs in the chest area.

Prognosis

The prognosis of an infant born with an omphalocele depends upon the size of the defect, whether there was a loss of blood flow to part of the intestines or other organs,

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and the extent of other abnormalities. The survival rate overall for an infant born with an isolated omphalocele has improved greatly over the past forty years, from 60% to over 90%.

Resources

ORGANIZATIONS

Foundation for Blood Research. PO Box 190, 69 US Route One, Scarborough, ME 04070-0190. (207) 883-4131. Fax: (207) 883-1527. http://www.fbr.org .

WEBSITES

Adam.com. “Omphalocele.” Medlineplus. U.S. National Library of Medicine. http://medlineplus.adam.com/ency/ article/000994.htm .

Catherine L. Tesla, MS, CGC

I Oncogene

Definition

In a cell with normal control regulation (non-cancer- ous), genes produce proteins that provide regulated cell division. Cancer is the disease caused by cells that have lost their ability to control their regulation. The abnormal proteins allowing the non-regulated cancerous state are produced by genes known as oncogenes. The normal gene from which the oncogene evolved is called a protooncogene.

Description

History

The word oncogene comes from the Greek term oncos, which means tumor. Oncogenes were originally discovered in certain types of animal viruses that were capable of inducing tumors in the animals they infected. These viral oncogenes, called v-onc, were later found in human tumors, although most human cancers do not appear to be caused by viruses. Since their original discovery, hundreds of oncogenes have been found, but only a small number of them are known to affect humans. Although different oncogenes have different functions, they are all somehow involved in the process of transformation (change) of normal cells to cancerous cells.

The transformation of normal cells into cancerous cells

The process by which normal cells are transformed into cancerous cells is a complex, multi-step process

involving a breakdown in the normal cell cycle. Normally, a somatic cell goes through a growth cycle in which it produces new cells. The two main stages of this cycle are interphase (genetic material in the cell duplicates) and mitosis (the cell divides to produce two other identical cells). The process of cell division is necessary for the growth of tissues and organs of the body and for the replacement of damaged cells. Normal cells have a limited life span and only go through the cell cycle a limited number of times.

Different cell types are produced by the regulation of which genes in a given cell are allowed to be expressed. One way cancer is caused, is by de-regulation of those genes related to control of the cell cycle; the development of oncogenes. If the oncogene is present in a skin cell, the patient will have skin cancer; in a breast cell, breast cancer will result, and so on.

Cells that loose control of their cell cycle and replicate out of control are called cancer cells. Cancer cells undergo many cell divisions often at a quicker rate than normal cells and do not have a limited life span. This allows them to eventually overwhelm the body with a large number of abnormal cells and eventually affect the functioning of the normal cells.

A cell becomes cancerous only after changes occur in a number of genes that are involved in the regulation of its cell cycle. A change in a regulatory gene can cause it to stop producing a normal regulatory protein or can produce an abnormal protein which does not regulate the cell in a normal manner. When changes occur in one regulatory gene this often causes changes in other regulatory genes. Cancers in different types of cells can be caused by changes in different types of regulatory genes.

Proto-oncogenes and tumor-suppressor genes are the two most common genes involved in regulating the cell cycle. Proto-oncogenes and tumor-suppressor genes have different functions in the cell cycle. Tumor-suppressor genes produce proteins that are involved in prevention of uncontrolled cell growth and division. Since two of each type of gene are inherited two of each type of tumorsuppressor gene are inherited. Both tumor suppressor genes of a pair need to be changed in order for the protein produced to stop functioning as a tumor suppressor. Mutated tumor-suppressor genes therefore act in an autosomal recessive manner.

Proto-oncogenes produce proteins that are largely involved in stimulating the growth and division of cells in a controlled manner. Each proto-oncogene produces a different protein that has a unique role in regulating the cell cycles of particular types of cells. We inherit two of each type of proto-oncogene. A change in only one protooncogene of a pair converts it into an oncogene. The

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oncogene produces an abnormal protein, which is somehow involved in stimulating uncontrolled cell growth. An oncogene acts in an autosomal dominant manner since only one proto-oncogene of a pair needs to be changed in the formation of an oncogene.

Classes of proto-oncogene

There are five major classes of proto-oncogene/ oncogenes: (1) growth factors, (2) growth factor receptors, (3) signal transducers (4) transcription factors, and

(5) programmed cell death regulators.

GROWTH FACTORS Some proto-oncogenes produce proteins, called growth factors, which indirectly stimulate growth of the cell by activating receptors on the surface of the cell. Different growth factors activate different receptors, found on different cells of the body. Mutations in growth factor proto-oncogene result in oncogenes that promote uncontrolled growth in cells for which they have a receptor. For example, platelet-derived growth factor (PDGF) is a proto-oncogene that helps to promote wound healing by stimulating the growth of cells around a wound. PDGF can be mutated into an oncogene called v- sis (PDGFB) which is often present in connective-tissue tumors.

GROWTH FACTOR RECEPTORS Growth factor receptors are found on the surface of cells and are activated by growth factors. Growth factors send signals to the center of the cell (nucleus) and stimulate cells that are at rest to enter the cell cycle. Different cells have different growth factors receptors. Mutations in a proto-oncogene that are growth factor receptors can result in oncogenes that produce receptors that do not require growth factors to stimulate cell growth. Overstimulation of cells to enter the cell cycle can result and promote uncontrolled cell growth. Most proto-oncogene growth factor receptors are called tyrosine kinases and are very involved in controlling cell shape and growth. One example of a tyrosine kinase is called GDFNR. The RET (rearranged during transfection) oncogene is a mutated form of GDFNR and is commonly found in cancerous thyroid cells.

SIGNAL TRANSDUCERS Signal transducers are proteins that relay cell cycle stimulation signals, from growth factor receptors to proteins in the nucleus of the cell. The transfer of signals to the nucleus is a stepwise process that involves a large number of proto-oncogenes and is often called the signal transduction cascade. Mutations in proto-oncogene involved in this cascade can cause unregulated activity, which can result in abnormal cell proliferation. Signal transducer oncogenes are the largest class of oncogenes. The RAS family is a group of 50 related signal transducer oncogenes that are found in approximately 20% of tumors.

TRANSCRIPTION FACTORS Transcription factors are proteins found in the nucleus of the cell which ultimately receive the signals from the growth factor receptors. Transcription factors directly control the expression of genes that are involved in the growth and proliferation of cells. Transcription factors produced by oncogenes typically do not require growth factor receptor stimulation and thus can result in uncontrolled cell proliferation. Transcription factor proto-oncogenes are often changed into oncogenes by chromosomal translocations in leukemias, lymphomas, and solid tumors. C-myc is a common transcription factor oncogene that results from a chromosomal translocation and is often found in leukemias and lymphomas.

PROGRAMMED CELL DEATH REGULATORS Normal cells have a predetermined life span and different genes regulate their growth and death. Cells that have been damaged or have an abnormal cell cycle may develop into cancer cells. Usually these cells are destroyed through a process called programmed cell death (apoptosis). Cells that have developed into cancer cells, however, do not undergo apoptosis. Mutated proto-oncogenes may inhibit the death of abnormal cells, which can lead to the formation and spread of cancer. The bcl-2 oncogene, for example, inhibits cell death in cancerous cells of the immune system.

Mechanisms of transformation of proto-oncogene into oncogenes

It is not known in most cases what triggers a particular proto-oncogene to change into an oncogene. There appear to be environmental triggers such as exposure to toxic chemicals. There also appear to be genetic triggers since changes in other genes in a particular cell can trigger changes in proto-oncogenes.

The mechanisms through which proto-oncogenes are changed into oncogenes are, however, better understood. Proto-oncogenes are transformed into oncogenes through: 1) mutation 2) chromosomal translocation, and 3) gene amplification.

A tiny change, called a mutation, in a proto-oncogene can convert it into an oncogene. The mutation results in an oncogene that produces a protein with an abnormal structure. These mutations often make the protein resistant to regulation and cause uncontrolled and continuous activity of the protein. The RAS family of oncogenes, found in approximately 20% of tumors, are examples of oncogenes caused by mutations.

Chromosomal translocations, which result from errors in mitosis, have also been implicated in the transformation of proto-oncogenes into oncogenes. Chromosomal translocations result in the transfer of a

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K E Y T E R M S

Autosomal dominant manner—An abnormal gene on one of the 22 pairs of non-sex chromosomes that will display the defect when only one copy is inherited.

Benign—A non-cancerous tumor that does not spread and is not life-threatening.

Cell—The smallest living units of the body which group together to form tissues and help the body perform specific functions.

Chromosome—A microscopic thread-like structure found within each cell of the body and consists of a complex of proteins and DNA. Humans have 46 chromosomes arranged into 23 pairs. Changes in either the total number of chromosomes or their shape and size (structure) may lead to physical or mental abnormalities.

Gene—A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.

Leukemia—Cancer of the blood forming organs which results in an overproduction of white blood cells.

Lymphoma—A malignant tumor of the lymph nodes.

Mitosis—The process by which a somatic cell—a cell not destined to become a sperm or egg—dupli- cates its chromosomes and divides to produce two new cells.

Mutation—A permanent change in the genetic material that may alter a trait or characteristic of an individual, or manifest as disease, and can be transmitted to offspring.

Nucleus—The central part of a cell that contains most of its genetic material, including chromosomes and DNA.

Parathyroid glands—A pair of glands adjacent to the thyroid gland that primarily regulate blood calcium levels.

Pheochromocytoma—A small vascular tumor of the inner region of the adrenal gland. The tumor causes uncontrolled and irregular secretion of certain hormones.

Proliferation—The growth or production of cells.

Protein—Important building blocks of the body, composed of amino acids, involved in the formation of body structures and controlling the basic functions of the human body.

Proto-oncogene—A gene involved in stimulating the normal growth and division of cells in a controlled manner.

Replicate—Produce identical copies of itself.

Somatic cells—All the cells of the body except for the egg and sperm cells.

Translocation—The transfer of one part of a chromosome to another chromosome during cell division. A balanced translocation occurs when pieces from two different chromosomes exchange places without loss or gain of any chromosome material. An unbalanced translocation involves the unequal loss or gain of genetic information between two chromosomes.

Tumor suppressor gene—Genes involved in controlling normal cell growth and preventing cancer.

proto-oncogene from its normal location on a chromosome to a different location on another chromosome. Sometimes this translocation results in the transfer of a proto-oncogene next to a gene involved in the immune system. This results in an oncogene that is controlled by the immune system gene and as a result becomes deregulated. One example of this mechanism is the transfer of the c-myc proto-oncogene from its normal location on chromosome 8 to a location near an immune system gene on chromosome 14. This translocation results in the deregulation of c-myc and is involved in the development of Burkitt’s lymphoma. The translocated c-myc protooncogene is found in the cancer cells of approximately 85% of people with Burkitt’s lymphoma.

In other cases, the translocation results in the fusion of a proto-oncogene with another gene. The resulting oncogene produces an unregulated protein that is involved in stimulating uncontrolled cell proliferation. The first discovered fusion oncogene resulted from a Philadelphia chromosome translocation. This type of translocation is found in the leukemia cells of greater than 95% of patients with a chronic form of leukemia. The Philadelphia chromosome translocation results in the fusion of the c-abl proto-oncogene, normally found on chromosome 9 to the bcr gene found on chromosome 22. The fused gene produces an unregulated transcription factor protein that has a different structure than the normal protein. It is not

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known how this protein contributes to the formation of cancer cells.

Some oncogenes result when multiple copies of a proto-oncogene are created (gene amplification). Gene amplification often results in hundreds of copies of a gene, which results in increased production of proteins and increased cell growth. Multiple copies of proto-onco- genes are found in many tumors. Sometimes amplified genes form separate chromosomes called double minute chromosomes and sometimes they are found within normal chromosomes.

Inherited oncogenes

In most cases, oncogenes result from changes in proto-oncogenes in select somatic cells and are not passed on to future generations. People with an inherited oncogene, however, do exist. They possess one changed proto-oncogene (oncogene) and one unchanged protooncogene in all of their somatic cells. The somatic cells have two of each chromosome and therefore two of each gene since one of each type of chromosome is inherited from the mother in the egg cell and one of each is inherited from the father in the sperm cell. The egg and sperm cells have undergone a number of divisions in their cell cycle and therefore only contain one of each type of chromosome and one of each type of gene. A person with an inherited oncogene has a changed proto-oncogene in approximately 50% of their egg or sperm cells and an unchanged proto-oncogene in the other 50% of their egg or sperm cells and therefore has a 50% chance of passing this oncogene on to their children.

A person only has to inherit a change in one protooncogene of a pair to have an increased risk of cancer. This is called autosomal dominant inheritance. Not all people with an inherited oncogene develop cancer, since mutations in other genes that regulate the cell cycle need to occur in a cell for it to be transformed into a cancerous cell. The presence of an oncogene in a cell does, however, make it more likely that changes will occur in other regulatory genes. The degree of cancer risk depends on the type of oncogene inherited as well as other genetic factors and environmental exposures. The type of cancers that are likely to develop depend on the type of oncogene that has been inherited.

Multiple endocrine neoplasia type II (MENII) is one example of a condition caused by an inherited oncogene. People with MENII have usually inherited the RET oncogene. They have approximately a 70% chance of developing thyroid cancer, a 50% chance of developing a tumor of the adrenal glands (pheochromocytoma) and about a 5-10% chance of developing symptomatic parathyroid disease.

Oncogenes as targets for cancer treatment

The discovery of oncogenes approximately 20 years ago has played an important role in developing an understanding of cancer. Oncogenes promise to play an even greater role in the development of improved cancer therapies since oncogenes may be important targets for drugs that are used for the treatment of cancer. The goal of these therapies is to selectively destroy cancer cells while leaving normal cells intact. Many anti-can- cer therapies currently under development are designed to interfere with oncogenic signal transducer proteins, which relay the signals involved in triggering the abnormal growth of tumor cells. Other therapies hope to trigger specific oncogenes to cause programmed cell death in cancer cells. Whatever the mechanism by which they operate, it is hoped that these experimental therapies will offer a great improvement over current cancer treatments.

Resources

BOOKS

Park, Morag. “Oncogenes.” In The Genetic Basis of Human Cancer, edited by Bert Vogelstein and Kenneth Kinzler. New York: McGraw-Hill, 1998, pp. 205-228.

PERIODICALS

Stass, S. A., and J. Mixson. “Oncogenes and tumor suppressor genes: therapeutic implications.” Clinical Cancer Research 3 (12 Pt 2) (December 1997): 2687-2695.

“What you need to know about Cancer.” Scientific America (September 1996).

Wong, Todd. “Oncogenes.” Anticancer Research 6(A) (NovDec 1999): 4729-4726.

WEBSITES

Aharchi, Joseph. “Cell division–Overview.” Western Illinois University. Biology 150. http://www.wiu.edu/users/ mfja/cell1.htm . (1998).

“The genetics of cancer–an overview.” (February 17, 1999). Robert H. Lurie Comprehensive Cancer Center of Northwestern University. http://www.cancergenetics

.org/gncavrvu.htm .

Kimball, John. “Oncogenes.” Kimball’s Biology Pages. (March 22, 2000). http://www.ultranet.com/ jkimball/ BiologyPages/O/Oncogenes.html .

Schichman, Stephen, and Carlo Croce. “Oncogenes.” (1999) Cancer Medicine. http://www.cancernetwork.com/ CanMed/Ch005/005-0.htm .

Lisa Maria Andres, MS, CGC

Onychoosteodysplasia see Nail-Patella syndrome

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I Opitz syndrome

Definition

Opitz syndrome is a heterogeneous genetic condition characterized by a range of midline birth defects such as hypertelorism, clefts in the lips and larynx, heart defects, hypospadias and agenesis of the corpus callosum.

Description

Opitz syndrome or Opitz G/BBB syndrome, as it is sometimes called, includes G syndrome and BBB syndrome, which were originally thought to be two different syndromes. In 1969, Dr. John Opitz described two similar conditions that he called G syndrome and BBB syndrome. G syndrome was named after one family affected with this syndrome whose last name began with the initial G and BBB syndrome was named after the surname of three different families. Subsequent research suggested that these two conditions were one disorder but researchers could not agree on how this disorder was inherited. It wasn’t until 1995 that Dr. Nathaniel Robin and his colleagues demonstrated that Opitz syndrome had both X-linked and autosomal dominant forms.

Opitz syndrome is a complex condition that has many symptoms, most of which affect organs along the midline of the body such as clefts in the lip and larynx, heart defects, hypospadias and agenesis of the corpus callosum. Opitz syndrome has variable expressivity, which means that different people with the disorder can have different symptoms. This condition also has decreased penetrance, which means that not all people who inherit this disorder will have symptoms.

Genetic profile

Opitz syndrome is a genetically heterogeneous condition. There appear to be at least two to three genes that can cause Opitz syndrome when changed (mutated) or deleted. Opitz syndrome can be caused by changes in genes found on the X chromosome (X-linked) and changes in or deletion of a gene found on chromosome 22 (autosomal dominant).

Chromosomes, genes, and proteins

Each cell of the body, except for the egg and sperm cells contain 23 pairs of chromosomes—46 chromosomes in total. The egg and sperm cells contain only one of each type of chromosome and therefore contain 23 chromosomes in total. Males and females have 22 pairs of chromosomes, called the autosomes, numbered one to twenty-two in order of decreasing size. The other pair of chromosomes, called the sex chromosomes, determines

the sex of the individual. Women possess two identical chromosomes called the X chromosomes while men possess one X chromosome and one Y chromosome. Since every egg cell contains an X chromosome, women pass on the X chromosome to their daughters and sons. Some sperm cells contain an X chromosome and some sperm cells contain a Y chromosome. Men pass the X chromosome on to their daughters and the Y chromosome on to their sons. Each type of chromosome contains different genes that are found at specific locations along the chromosome. Men and women inherit two of each type of autosomal gene since they inherit two of each type of autosome. Women inherit two of each type of X-linked gene since they possess two X chromosomes. Men inherit only one of each X-linked gene since they posses only one X chromosome.

Each gene contains the instructions for the production of a particular protein. The proteins produced by genes have many functions and work together to create the traits of the human body such as hair and eye color and are involved in controlling the basic functions of the human body. Changes or deletions of genes can cause them to produce abnormal protein, less protein or no protein. This can prevent the protein from functioning normally.

Autosomal dominant Opitz syndrome

The gene responsible for the autosomal dominant form of Opitz syndrome has not been discovered yet, but it appears to result from a deletion in a segment of chromosome 22 containing the Opitz gene or a change in the gene responsible for Opitz syndrome. In some cases the deletion or gene change is inherited from either the mother or father who have the gene change or deletion in one chromosome 22 in their somatic cells. The other chromosome 22 found in each of their somatic cells is normal. Some of their egg or sperm cells contain the gene change or deletion in chromosome 22 and some contain a normal chromosome 22. In other cases the deletion has occurred spontaneously during conception or is only found in some of the egg or sperm cells of either parent but not found in the other cells of their body.

Parents who have had a child with an autosomal dominant form of Opitz syndrome may or may not be at increased risk for having other affected children. If one of the parents is diagnosed with Opitz syndrome then each of their children has a 50% chance of inheriting the condition. If neither parent has symptoms of Opitz syndrome nor possesses a deletion, then it becomes more difficult to assess their chances of having other affected children.

In many cases they would not be at increased risk since the gene alteration occurred spontaneously in the embryo during conception. It is possible, however, that one of the parents is a carrier, meaning they possess a

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change in the autosomal dominant Opitz gene but do not have any obvious symptoms. This parent’s children would each have a 50% chance of inheriting the Opitz gene.

X-linked Opitz syndrome

Some people with the X-linked form of Opitz syndrome have a change (mutation) in a gene found on the X chromosome called the MID1 (midline1) gene. Changes in another X-linked gene called the MID2 gene may also cause Opitz syndrome in some cases. It is believed that the MID genes produce proteins involved in the development of midline organs. Changes in the MID gene prevent the production of enough normal protein for normal organ development.

The X-linked form of Opitz syndrome is inherited differently by men and woman. A woman with an X- linked form of Opitz syndrome has typically inherited a changed MID gene from her mother and a changed MID gene from her father. This occurs very infrequently. All of this woman’s sons will have Opitz syndrome and all of her daughters will be carriers for Opitz syndrome. Only women can be carriers for Opitz syndrome since carriers possess one changed MID gene and one unchanged MID gene. Most carriers for the X-linked form of Opitz syndrome do not have symptoms since one normal MID gene is usually sufficient to promote normal development. Some carriers do have symptoms but they tend to be very mild. Daughters of carriers for Opitz syndrome have a 50% chance of being carriers and sons have a 50% chance of being affected with Opitz syndrome. A man with an X-linked form of Opitz syndrome will have normal sons but all of his daughters will be carriers.

Demographics

Opitz syndrome is a rare disorder that appears to affect all ethnic groups. The frequency of this disorder is unknown since people with this disorder exhibit a wide range of symptoms, making it difficult to diagnose and many possess mild or non-detectable symptoms.

Signs and symptoms

People with Opitz syndrome exhibit a wide range of medical problems and in some cases may not exhibit any detectable symptoms. This may be due in part to the genetic heterogeneity of this condition. Even people with Opitz syndrome who are from the same family can have different problems. This may mean there are other genetic and non-genetic factors that influence the development of symptoms in individuals who have inherited a changed or deleted Opitz gene. Most individuals with

Opitz syndrome only have a few symptoms of the disorder such as wide set eyes and a broad prominent forehead. Opitz syndrome can, however, affect many of the organs and structures of the body and primarily affects the development of midline organs. The most common symptoms are: hypertelorism (wide-spaced eyes), broad prominent forehead, heart defects, hypospadias (urinary opening of the penis present on the underside of the penis instead of its normal location at the tip), undescended testicles, an abnormality of the anal opening, agenesis of the corpus callosum (absence of the tissue which connects the two sides of the brain), cleft lip, and clefts and abnormalities of the pharynx (throat) and larynx (voice-box), trachea(wind-pipe) and esophagus.

People with Opitz syndrome usually have a distinctive look to the face such as a broad prominent forehead, cleft lip, wide set eyes that may be crossed, wide noses with upturned nostrils, small chins or jaws, malformed ears, crowded, absent or misplaced teeth and hair that may form a ‘widow’s peak’. In many cases the head may appear large or small and out of proportion to the rest of the body.

Often people with Opitz syndrome have difficulties swallowing because of abnormalities in the pharynx, larynx, trachea, or esophagus. This can sometimes result in food entering the trachea instead of the esophagus, which can cause damage to the lungs and pneumonia, and can sometimes be fatal in small infants. Abnormalities in the trachea can sometimes make breathing difficult and may result in a hoarse or weak voice and wheezing.

Both males and females may have abnormal genitals and abnormalities in the anal opening. Males can have hypospadias and undescended testicles and girls may have minor malformation of their external genitalia. Heart defects are also often present and abnormalities of the kidney can be present as well. Intelligence is usually normal but mild mental retardation can sometimes be present. Twins appear more common in families affected with Opitz syndrome.

Males and females with the dominant form of Opitz syndrome are equally likely to have symptoms whereas carrier females with the X-linked form of Opitz syndrome are less likely to have symptoms then males with the condition. In general, males with the X-linked form of Opitz syndrome tend to be more severely affected than females and males with the autosomal dominant form of Opitz syndrome. People with X-linked Opitz syndrome and dominant Opitz syndrome generally appear to exhibit the same range of symptoms. The only known exceptions are upturned nostrils and clefts at the back of throat, which appear to only occur in people with X-linked Opitz syndrome.

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