Genetic pathways of Lyst and exfoliation syndrome

Human eyes with exfoliation syndrome (XFS) exhibit a distinctive pattern of iris transillumination defects that are recapitulated in Lyst mutant mice carrying the beige allele. Here I present the identification and characterization of the B6-Lyst^bg-J mouse model of XFS, modifiers of Lyst mediated ocular phenotypes, mechanisms of intraocular pressure (IOP) pathology related to circadian rhythms, and mechanisms of iris transillumination defects in the B6-Lyst^bg-J mice.

Clinical and histological analysis shows that the B6-Lyst^bg-J mice have multiple similarities to human XFS including: iris transillumination defects, production of an exfoliative-like material, and pronounced pigment dispersion. Despite these insults, Lyst mutation does not cause increased IOP or optic nerve damage within the context of a C57BL/6J genetic background. However, defects in the circadian rhythm regulation of IOP were identified. Sequence analysis identifies that the beige mutation is predicted to delete a single isoleucine from the WD40 domain of the LYST protein. I identified CSNK2B as a binding partner of LYST and showed that LYST^bg-J completely disrupts the interaction. CSNK2B function in regulating E-cadherin and β-catenin binding is subsequently disrupted. These results lead to a working hypothesis that aspects of the XFS phenotype involve LYST and CSNK2B pathways, likely influencing cell-cell adherens junctions.

Epistasis experiments were used to test for genetic modifiers of Lyst, which demonstrated that albino Lyst mutant mice exhibited complete rescue of Lyst-dependent iris phenotypes. In a genetic background-driven approach, a DBA/2J strain of congenic mice was created. The DBA/2J background, which harbors multiple mutations influencing melanosomal-proteins, enhanced Lyst dependent iris phenotypes. Thus, both experimental approaches implicated melanosomes, a potential source of oxidative stress, as mechanistically contributory. Supporting a contributory role of oxidative damage, Lyst mutation resulted in genetic context sensitive changes in iris lipid hydroperoxide levels, being lowest in albino and highest in DBA/2J mice. These results identified an association between oxidative damage to lipid membranes and severity of Lyst-mediated phenotypes, uncovering a new mechanism contributing to pathophysiology involving LYST. In conclusion these results demonstrate that mutation of the Lyst gene can produce ocular features of human XFS and suggests that LYST or LYST-interacting genes may contribute to XFS.