Ординатура / Офтальмология / Английские материалы / The Glaucomas Volume 1 Pediatric Glaucomas_Sampaolesi, Zarate_2009

also presents mental retardation, short stature, and cardiac, visceral and general anomalies.

Alterations in the Shape of Chromosomes

There may also be arm deletion, a partial loss of a segment (Fig. 7.3), or translocation, a misplacement of a chromosome fragment or from one to another (Fig. 7.4). Retinoblastoma, a typical example of this type of alteration, must also be remembered.

Gene Abnormalities

What Is a Gene?

A gene is a region of a chromosome, consisting of units of DNA and containing codes of information for producing a particular protein or enzyme. It can potentially mutate, and some genes exist in several copies of the genome (the set of all the genes expressed in DNA), while others are unique. Genes are located at specific sites of the homologous chromosomes and exist in pairs called alleles [2].

When an individual is homozygotic in relation to a phenotypic character (e.g., hair or eye color), this is because both the genes or alleles are identical. When they are heterozygotic, each gene of the pair or allele is different and specifies different characteristics.

There are dominant genes and recessive ones. If one of the genes of the pair is dominant, the phenotypic expression will be exclusively of the dominant one. Within this group, the dominance of a gene can be absolute or partial. Thus in some examples, such as aniridia, dominance is absolute; this means that the existence of just one altered gene is sufficient for the lack of iris to manifest itself.

In cases where the dominance is not absolute, both

the altered genes, maternal and paternal, are needed for the disease to be expressed. This is also known as penetrance. The higher the penetrance, the greater the percentage of descendants affected by the disease.

If both genes are recessive, the phenotypic expression will be the result of that recessive gene. Naturally, if both are dominant, the result in the phenotype will be dominance.

-Genes are classified into various types:

-Structural: according to the protein it generates; Regulator: when it controls an enzyme activity

-within the metabolism;

Topographic: when it determines the location of a protein or enzyme within the cell, for example, that the enzyme be situated in the cell membrane or in

-an organelle.

Temporal: when it determines the activity of other genes at different stages of life.

Thus the pathology of a gene or various genes, simultaneously or otherwise, can cause multiple disorders as a result of mutated proteins, poor enzyme control, wrong location, or wrong stage of life.

The most common genetic defect is mutation, i.e., a change in the information for forming a protein or enzyme (Fig. 7.5).

The gene information is codified in messages by DNA sequences.

Fig. 7.5 Mutation

The term “gene” is given to a definite, limited portion of a chromosome that can be seen sometimes morphologically, and at others, more often used today, by gel densitometry, or by polymerase chain reaction (PCR), which are molecular biology techniques. These are the ones where etiology is related with a modification of the normal structure of more than one gene.

The next question must be: where are genes located? Genes constitute very small parts of the chromosomes, and their anomalies consist of abnormalities of one or more genes (polygenic/multifactorial inheritance). In the first case, the alteration is simply a mutation or modification within the structure of a particular gene, which is expressed by a mutated protein, or some form of morphological expression such as the loss of homozygosity, microsatellite alterations (of the

gene satellites), etc.

Each Mendelian feature is represented by two variants of the same gene, maternal or paternal, which are called alleles and occupy the same locus or place of two homologous chromosomes. Genes in turn may be autosomal if they belong to an autosome, or sex-linked if they are located in the X or Y chromosome. It should be remembered that the father and the mother each provide one of the pair of genes, and this is why it is important to study the Mendelian features of the genealogical tree. Nowadays, the ability to map the genes of the chromosomes enables genetic defects to be linked with specific structural chromosome defects, establishing a biologically very important bridge between Mendelian genetics and modern cytogenetics, based on more precise techniques such as the study of DNA.

Every genetic message is coded in DNA sequences, with the codon as the basic genetic unit, consisting of a triplet of DNA bases. This sequence is transmitted in a series of amino acids, forming a polypeptide chain, i.e., a protein, the basic principle of modern genetics.

To sum up, we have chromosomes that are made up of pairs of genes, which are in turn made up of DNA. Based on this information (of the DNA), the proteins will come out as normal or as mutated.

In glaucoma, the study of these gene alterations is being fully applied [3], through numerous mappings of loci in different genes, such as the lq23, Chr2, 3q21q24, 8q23, as well as other genes on Chr7 and Chr10. Thus primary open-angle glaucoma (POAG) could be a variety of diseases at the molecular level (molecular pathology), explaining certain cases of different clinical behavior, based simply on forms that are molecularly different [4–6]. In some cases, chromosomes 2, 14, 17, and 19 may be involved in this pathology, suggesting for example that POAG constitutes a heterogeneous genetic disorder.

An ophthalmologist who wishes to put this research into practice could do the chromosome, gene, and DNA sequence studies using different methods, and the studies can be done on affected tissues removed for therapeutic reasons, such as filtering operations, or on the patient’s blood cells. In the not too distant future, gene therapy may be included in the alternative protocols.

To apply the knowledge of genetics in glaucoma, we will classify it as follows:

1.Congenital glaucomas:

–Primary [7];

–Primary refractory [8];

–Associated with ocular alterations: Rieger, Peters, Axenfeld, aniridia, sclerocornea, cornea plana, isolated retinal dysplasia, persistent hyperplastic primary vitreous;

–Associated with ocular and somatic alterations: neurofibromatosis, Sturge–Weber and Klippel– Trenaunay syndromes, Norrie disease, Warburg syndrome, and retinal dysplasia;

–Associated with metabolism errors: hyperaminoaciduria, Lowe syndrome – (oculocerebrorenal syndrome), Fanconi syndrome, homocystinuria, Hurler syndrome (mucopolysaccharidosis), ochronosis (alkaptonuria);

–Associated with mesodermic dystrophies: Marfan syndrome, Marchesani syndrome;

–Associated with goniodysgeneses [9, 10]: late congenital glaucoma, Busacca metaplasia, pigmentary;

–Associated with the nevus of Ota and other ocular melanocytosis;

2.Nonhereditary congenital glaucomas:

–Rubeola;

–Toxoplasmosis;

3.Secondary pediatric glaucomas:

–Retinopathy of prematurity.

Therefore, if we examine the above and our practical experience in observation, especially of the angle, we

-will recognize:

Glaucomas with an altered phenotype: Barkan’s - membrane, pathological mesoderm remains, etc.

Glaucomas with a normal phenotype and normal gonioscopic angle.

In both cases, there is gene alteration (in one or more genes) and consequently the molecule, the filtrationrelated protein or proteins in question, are altered; knowing their names will help gene therapy [11].

In the international nomenclature, it was arbitrarily decided that the chromosomes affecting open-angle

glaucomas should be identified as GLC1, those of closed-angle as GLC2, and congenital glaucomas as GLC3. Stone [4, 5] for example, described a family with congenital glaucoma in which the alteration was found on chromosome 1, on its long arm, so the name given was GLC3,1q, where GLC3 means congenital glaucoma, the number “1” indicates that the chromosome affected is chromosome 1, and the letter “q” means the long arm.

There are numerous chromosomes already detected -in congenital glaucomas, for example:

- GLC3,1q21-q31;

- GLC3,1p36-6p25;

GLC3,2p21.

In other disorders such as Rieger syndrome, the affected chromosomes have been recognized: 4q25 and d13q14.

In aniridia, there are two types of aniridia: (1) an1 in Chr. 2p and (2) an2 (wagr) 11p 13 – where the “w” means Wilms tumor, the “a” is for aniridia, the “g” for genitourinary disorders, and the “r” for mental retardation.

Ophthalmology is one of the areas where the greatest number of genes have been found to be affected in ophthalmologic diseases. It is calculated that there are currently from 50–100,000 genes, 10% of which may be involved in some ocular disease (between 5 and 10,000). See Table 7.1 for an example.

Genetic Counseling

The mode of inheritance in primary congenital glaucoma there is autosomal recessive, and prenatal testing can determine the risk for this disease. With a CYP1B1 (GLC31) mutation, an individual has a 25% chance of being affected, a 50% of being asymptomatic, and a 25% chance of being unaffected. However, further studies are necessary for the proper implementation of this risk evaluation method. The detection of phe-

notype (goniodysgenesis) is very important for the genetic counseling in ophthalmological practice.

References

1.Santillo C, Brinelli M (2003) Eziopatogenesi dei glaucomi infantili. Boll Oculi 82:103–116

2.Musarella MA (1992) Gene mapping of ocular diseases. Surv Ophthalmol 36:285–312

3.Gonzalez EO, Rodriguez MM, Gonzalez Garcia AD y Cruz AL (1999) Avances en la genética de los glaucomas. Rev Cubana Oftalmol 12:77–83

4.Stone E.M, Fingert JH, Alward WLM, Nguyen TD et al (1997) Identification of a gene that causes primary open glaucoma. Science 275:668–670

5.Sheffield VC, Stone EM, Alward WLM, Drack AV, Johnson AT, Streb LM, Nichols BE (1993) Genetic linkage of familial open-angle glaucoma to chromosome 1q21-q31. Nat Genet 4:47–50

6.Aldred MA, Baumber L, Hill A, Schwalbe EC, Goh K, Karwatowski W, Trembath RC (2004) Low prevalence of MYOC mutations in UK primary open-angle glaucoma patients limits the utility of genetic testing. Hum Genet 115:428–431

7.Walton DS, Katsavounidou G (2005) Newborn primary congenital glaucoma: 2005 update. J Pediatr Ophthalmol Strabismus 42:333–341

8.Cohn AC, Kearns LS, Savarirayan R, Ryan J, Craig JE, Mackey DA (2005) Chromosomal abnormalities and glaucoma: a case of congenital glaucoma with trisomy 8q22qter/monosomy 9p23-pter. Ophthalmic Genet 26:45–53

9.Kniestedt C, Kammann MTT, Stürmer J, Gloor BP (2000) Dysgenetische Kammerwinkelveränderungen bei Patienten mit Glaukom oder Verdacht auf Glaukom aufgetreten vor dem 40. Lebensjahr. Klin Monatsbl Augenheilkd 216:377–387

10.Richards JE, Lichter PR, Boehnke M, Justine LA, Uro, Torrez D, Wong D, Johnson T (1994) Mapping of a gene for autosomal dominant juvenile-onset open-angle glaucoma to chromosome I q”. Am J Hum Genet 54:62–70

11.Bergen AA, Leschot NJ, Husman CA, De Smet MD, De Jong PT (2004) From gene to disease: primary open-angle glaucoma and three known genes: MYOC, CYP1B1 and OPTN. Ned Tijdschr Geneeskd 148:1343–1344

12.Bejjani BA, Edward DP (2007) Primary congenital glaucoma. Gene Reviews. http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=glc. Cited 29 July 2008. University of Washington, Seattle

Contents

Embryology of the Chamber Angle . . . . . . . . . . . . . . . . . . . 61

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Embryology of the Chamber Angle

Embryological

Development of the Chamber Angle. The Normal and Abnormal Chamber Angle in Newborns up to 1 Year

of Age and Its Importance with Respect to Pathology

We will leave aside the discussion of the two most widely accepted theories regarding the mechanism by which the chamber angle is formed: whether it results from atrophy and resorption [1] or from cleavage and separation into layers [2].

The primordium of the chamber angle appears between the 3rd and 5th month of gestation. It is ringshaped and its periphery is bounded by a triangular area with its base facing it.

In 1906, Seefelder and Wolfrum [3] described the formation of the anterior chamber and chamber angle, which can be summarized as follows:

1.The ciliary processes develop at the end of the 3rd month of gestation.

2.The Schlemm canal appears in the second half of the 4th month.

3.The anterior chamber appears at the end of the 5th month and its development finishes in the middle of the 6th month.

At the end of the 3rd month, a primordium of the anterior chamber can be seen at the periphery, even when there is no mesoderm there.

At the beginning of the 6th month, the lens touches the posterior surface of the cornea only at its posterior pole.

The iridopupillary membrane, the central part of which later gives rise to the pupillary membrane, and its peripheral part to the iris stroma, is located in front of the lens capsule.

“Mesodermal tissue can be found at the chamber angle, between the corneoscleral trabecular meshwork (ciliary muscle tendon) and the ciliary processes and the iris. In the fetal stage, this corneoscleral trabecular meshwork reduces the anterior chamber” [3]. These authors named it the pectinate ligament, as did Hüeck, since it is very similar to this characteristic formation of ungulates (rabbits and horses). Since the Symposium on Congenital Glaucoma held in Venice, we have preferred to call it normal mesodermal tissue.

Figure 8.1 shows the histologic appearance of the primordium of the chamber angle at the 7th, 8th, and 9th months of gestation with corresponding graphics.

The chamber angle develops by enlarging in two directions: toward the periphery and backward. It has a loose and definite mesenchymal tissue (squared in the graphic).

It is triangular in shape, bounded toward the front by the longitudinal part of the ciliary muscle and by the scleral trabecular meshwork, which is its tendon, and to the rear by the future iris root, the bundles of the radiated ciliary muscle (Ivanoff ’s muscle) and the ciliary processes. The third side is formed by the anterior chamber at that location (Fig. 8.1). As shown in the figure, in the 7th month, both the Schlemm canal and the spur are clearly distinguishable: the black circles represent the circular part of the ciliary muscle, which is starting to develop, and the thick black band, its longitudinal part. The squared areas represent the mesodermal tissue.

It should be noted that the Schlemm canal extends posteriorly to the peripheral limit of the anterior chamber.

By the 8th month (Fig. 8.1b), the anterior chamber has extended toward the periphery, the radial part of the ciliary muscle has developed further (in an anteroposterior direction), the mesodermal tissue has shortened and thickened, and the separation between its layers is greater (large squares).

Fig. 8.1a–c Intrauterine development of the chamber angle. a Chamber angle at the 7th month; b chamber angle at the 8th month. In a, the anterior chamber just reaches the Schwalbe line. Behind it, the mesodermal tissue has a very tight mesh (small squares); it extends over the whole area from the scleral trabecular meshwork (dotted line) to the radiated muscle and the ciliary process. In b, the anterior chamber enlarges in a distal direction. The meshes of the mesodermal tissue are looser

(larger squares). In c, at birth, the anterior chamber enlarges even further and the mesodermal tissue is reduced to a thin layer that will later become the Busacca trabecular conjunctival layer, also known as the Rohen iridoscleral membrane. (The histologic sections on the right, from [3]). The corresponding graphics are displayed on the left. On the right (in color) an original specimen confirming the specimen from Seefelder and Wolfrum [3]

At birth, the 9th month (Fig. 8.1c), the limit of the anterior chamber has extended past the spur, and the radial part of the ciliary muscle has become attached to the longitudinal part (the circular part will derive from the radial part, since it develops after birth). The mesodermal tissue has reduced to one layer with bigger inner spaces (the largest squares in the graph), which is the Busacca trabecular conjunctival layer or the Rohen iridoscleral membrane.

Figure 8.2 shows the great similarity between the histologic appearance of the normal chamber angle at

the 7th month of gestation and the chamber angle in congenital glaucomas.

Figure 8.3 shows a sequential histology of different histological sections in the chamber angle in the 7th month: a broad anterior chamber showing the abundant mesenchyma tissue and the Barkan membrane (Fig. 8.3a); an image similar to the previous one, with a more closed angle (H-E) (Fig. 8.3b); the Masson trichrome stain shows the position of the ciliary muscle and also marks the Barkan membrane in an even more closed angle (Fig. 8.3c); an image with H-E stain-

Fig. 8.2 The histology of congenital glaucoma has the same appearance as that of the chamber angle at the seventh month of gestation. I Chamber angle of the normal fetus at the seventh month [3]. II Chamber angle in congenital glaucoma. III Graph

representing I and II. tm mesodermal tissue, Sch Schlemm canal, mc radiated ciliary muscle, c anterior ciliary vein and collector. IV and V original specimen from Sampaolesi and Zarate confirming Seefelder and Wolfrum

ing shows an extremely narrow angle with mesodermal remains simulating a high iris insertion (Fig. 8.3d).

Chronodynamics of Normal Anterior

Segment Development

After ovulation, the ovum passes into the fallopian tube where it meets the sperm cells, one of which fertilizes it. As a consequence of fertilization, the diploid number of chromosomes is restored, the sex is determined, and the cleavage process starts. This is a succession of mitoses that determine the formation of a number of smaller cells called blastomeres, which thus take on a volume in accordance with the usual human tissue cells (remembering that the fertilized egg is a large cell). Seventy-two hours after ovulation, the egg has reached approximately 16 cells. Then, as liquid penetrates between the cells, which continue dividing, the blastocyst develops, which is a cystic structure in which the peripheral cells become flattened (they will

constitute the trophoblast), with a group of them remaining concentrated in one of the poles (inner cell mass) (Fig. 8.4a). The trophoblast will form the caul and the placenta, while the inner cell mass gives rise to the embryo and the amnion.

Implantation occurs in the endometrium at this blastocyst stage where an inner cell mass and a cavity surrounded by the trophoblast is found. The inner cell mass gives rise to the ectoand endodermic embryo layers. A space appears between the former (ectoderm), which are cells arranged in a flat shape, and amniogenic cells derived from the trophoblast, and this will constitute the amniotic cavity. Thus in a 15-day-old embryo, we have the amniotic cavity, the embryonic ectoderm and endoderm, and the vitelline sac (Fig. 8.4b). The ectoderm and the endoderm constitute the embryonic disc. In the caudal part of the disc, at only 15 days, a cellular thickening of the ectoderm appears, which swells out into the amniotic cavity, called the primitive streak. It becomes spherical and begins to settle between the ectoderm and the primitive endoderm, to form the

Fig. 8.3a–d Sequential histology of different histological sections in the chamber angle in the 7th month. a Broad anterior chamber showing the abundant mesenchymal tissue and the Barkan membrane. b Image similar to the previous, with a more closed angle (H-E). c The Masson trichrome shows the

position of the ciliary muscle and also marks the Barkan membrane in an even more closed angle. d Image with H-E staining with an extremely narrow angle with mesodermal remains simulating a high iris insertion

third germinal layer, called the intraembryonic mesoderm, which, as it spreads toward the edges, joins the extraembryonic mesoderm, situated in relation to the trophoblast. The embryonic disc is now trilaminar: ectoderm, mesoderm, and endoderm (Fig. 8.4c).

From the ectoderm are successively formed the plate, the groove, and the neural tube. At the same time, the paraxial mesoderm and the somites are formed, which are segmentations of this tissue starting on day 21 at the anterior level and continuing in a caudal direction until by day 30 there are 44 pairs. The endoderm develops into the embryonic intestine.

The anterior (cranial) part of the neural tube constitutes the prosencephalon, and the primitive optic groove is formed on its sides and then the optic vesicle, which is thus a lateral evagination of the prosencephalon.

On day 26, the optic vesicle approaches the embryonic ectoderm (Fig. 8.5a). On day 27, the crystalline plaque can be distinguished in this ectoderm (Fig. 8.5b). This initial crystalline placode goes on to form a crystalline vesicle, which then separates from the primitive ectoderm on day 33, and corneal differentiation begins [4].

The crystalline placode is a thickening of the superficial ectoderm in the region adjacent to the neural ectoderm (optic vesicle). This thickening happens after the 2nd week and is induced by the optic vesicle, although the nature of the agent that acts to cause this is not precisely known and could be related with the extracellular matrix produced either by the neuroectodermic cells of the optic vesicle, or by those of the crystalline placode.

The thickening that makes up the crystalline placode looks like a stratified epithelium, but it is really a monolayer of cells that become columnar, whose nuclei are found in different sites, i.e., they form a true pseudostratified epithelium. This different position of the nuclei is caused by them migrating from the basal layer to the apical sector before mitosis. Later the crystalline placode begins a central depression, starting to form the crystalline vesicle, accompanying the invagination of the optic cup, which thus forms its double layer (Fig. 8.6).

The lens vesicle separates from the (corneal) ectoderm in the 4th week. In this way, the cells making up the lens vesicle have their basal part in the outer side, which explains why the basal lamina they secrete, con-

Fig. 8.6 Invagination of the lens. Invagination of the optic cup coinciding with the penetration and initial differentiation of the crystalline vesicle