What is Functional Genomics Teaching us about Intraocular Pressure Regulation and Glaucoma?

I. OVERVIEW

Glaucoma is a complex disease characterized by the degeneration of the optic nerve and subsequent loss of vision. The disease aVects a large number of people, especially those among the older population. The causes leading to the development of glaucoma can be the result of the dysfunction of several tissues. The major risk for glaucoma is elevated intraocular pressure which is the consequence of an improper regulation of aqueous humor outflow in the anterior segment of the eye (ciliary body and trabecular meshwork). Elevated pressure in turn, aVects the posterior segment by exerting mechanical strain on the optic nerve and provoking deformation, distortion of axonal flow and death of the nerve cells (lamina cribrosa and retinal ganglion cells). The function of any given tissue is governed by the regulation of expression of its genes. Functional genomics oVers an invaluable tool to identify the molecular diVerences aVecting the cells under conditions associated with glaucoma. In this chapter, we have reviewed such changes. Because of the wide scope of the project, we have briefly reviewed studies in all tissues and then concentrated in those conducted on the trabecular meshwork, the keeper of outflow resistance. We have analyzed and correlated our internal data with that of other laboratories, and built a molecular signature for trabecular meshwork pathophysiology. We identified and present a set of 40 genes. These include new genes involved in known trabecular meshwork physiology as well as genes representative of new processes and mechanisms. As the functional genomics studies on RNA, proteins, protein modifications and activities continues this list will be further edited. As of today, this signature provides the opening of a small window into the global mechanisms governing the development of glaucoma.

II. INTRODUCTION

In the beginning, it was just the gene. The gene was embedded into this vast amount of DNA which needed to be deciphered, sequenced, cataloged and arranged in bits of expressed tiny fractions (expressed sequence tags, EST). The task appeared insurmountable but it was accomplished sooner than previously scheduled (Pennisi, 2001). After defining the structure of the gene, came functional genomics. Functional genomics comprises the elucidation of the genes’ function, its transcription into mRNA, its translation into protein and the subsequent protein modifications which continue to occur during the life of proteins in any cell. All these regulations of gene expression, each diVerent level, will determine the function of the tissue during its

physiological and pathological conditions. The functional genomics task is thus specifically targeted to a given tissue or disease and it will take much longer to complete than the ‘‘simple’’ sequence of the genome.

Glaucoma is a blinding disease aVecting 70 million people worldwide (Quigley, 1993, 1996; Kupfer et al., 1994). Glaucoma is diagnosed clinically by a reduction of the visual field and by a change in the structure of the optic nerve, which acquires a characteristic ‘‘cupping’’ shape. The landmark of the development of glaucoma is the death of the retinal ganglion cells (RGC). It is generally accepted that the major risk for the development of glaucoma and thus RGC depletion is elevated intraocular pressure (IOP) (Anderson, 1989; Sommer et al., 1991; Gordon et al., 2002; Kass et al., 2002; Higginbotham et al., 2004). Nevertheless, an increasing number of reports show development of normal tension glaucoma, or individuals that experience glaucoma symptoms within an IOP considered to be inside the normal range (Werner, 1996). Whether these so called ‘‘normal’’ pressures are normal for the aVected individual, or whether they reflect a diVerence pressure threshold of each individual is not yet known. In addition to pressure, other factors, such glucocorticoids have been undoubtedly associated in the clinic with the development of elevated IOP and glaucoma (Armaldy, 1963a,b; Becker and Mills, 1963; Grant, 1963b; Becker and Hank, 1964; Armaldy, 1965; Armaldy and Becker, 1965; Becker, 1965; Armaldy, 1966; Francois, 1977; Weinreb et al., 1985; Jones and Rhee, 2006). Others, such an increased presence of the TGFb2 growth factor has been observed in the aqueous of patients with the disease (Cousins et al., 1991; Tripathi et al., 1994b; Picht et al., 2001). Still others, such the genetically glaucoma linked myocilin have not yet yielded a clear mechanistic correlation (Borra´s et al., 2002, 2006; Tamm, 2002).

In addition to considering general factors aVecting the development of glaucoma, it is important to take into account the number of diVerent cell types and tissues which can contribute to the physiological maintenance of IOP and thus can be considered as contributors to the development of glaucoma. The ciliary epithelium secretes the flowing aqueous humor and has the ability to release molecules that will reach and signal the trabecular meshwork. The trabecular meshwork maintains the pressure through a number of cellular and molecular mechanisms which aVect the physical properties of elasticity, tension and softness. The death of the RGC is aVected not only by pressure but by other factors, like blood flow and ischemia. And the RGC axons forming the optic nerve are supported at their exit from the eye by the glial cells in the lamina cribrosa, whose health would aVect the optic nerve at what is called the optic nerve head (ONH), site of the cupping in glaucoma.