Modifier genes in the phenotypic manifestation of primary disease-causing mutations

Primary disease-causing mutations have been identified in many inherited eye diseases. However, there is a wide range of variation in the penetrance and phenotypic expression of these mutations, making identification of the mutation alone insufficient in providing accurate prognosis and treatment. We studied two genetic eye diseases in which the primary disease-causing mutations have been identified: Leber hereditary optic neuropathy (LHON), a maternally transmitted disorder caused by mitochondrial mutations, and autosomal dominant retinal dystrophies (adRD), caused by a splice site mutation (IVS2+3A→T) in the peripherin/RDS gene. Incomplete penetrance in LHON and phenotypic variation in adRD suggest a role for modifier genes, identification of which would contribute towards the understanding of the pathophysiology of these diseases.

We show evidence suggestive of linkage to the X-chromosome at two regions (Xq21.1 and Xq26-28) by chromosomal linkage analysis in an LHON family. Screening of coding regions in X-linked candidate genes, MAOB, TIMM8A, CDR1, LDOC1 and NDUFA excluded them as potential modifiers.

We established founder effect for the RDS splice site mutation in nineteen putatively unrelated families with adRD by intragenic and STRP haplotype analysis. We identified an aberrant transcript among affected individuals harboring RDS splice site mutation by RT-PCR. We demonstrated three clinical phenotypes resulting from this mutation: pattern dystrophies, central areolar choroidal dystrophy and retinitis pigmentosa. We screened the coding region, promoter, and 3'UTR of RDS, and the coding regions of ROM1, rdCVF and GCAP1 as potential modifiers. Additional variants in RDS gene and ROM1 were excluded as modifiers. Sequence variations in GCAP1 and rdCVF were identified as potential modifiers in a few individuals harboring the splice site mutation.

Our results suggest that the phenotypic manifestation of both mitochondrial and RDS splice site mutation is complex and is due to Oligogenic effects. Further studies with additional family members and functional studies are required to establish the linkage to X-chromosome in LHON and to determine the significance of GCAP1 and RdCVF sequence variants that were identified in adRD.

In summary, a cumulative insight from the study of environmental, stochastic and genetic factors is required for the ultimate understanding and cure of these diseases.