Abstract
Purpose: :
Molecular genetic screening in early onset severe retinal dystrophies (EOSRD) including Leber's Congenital Amaurosis (LCA) and autosomal recessive or unspecified Retinitis pigmentosa with early onset (RP) identified eight candidate genes during the past nine years (RetGC1, RPE65, AIPL1, CRB1, RPGRIP, LRAT, TULP1, CRX). As time went by those involved in disclosing the causes of EOSRD recognized that they had found the tip of a growing iceberg. This study summarizes our research in EOSRD and conclusions drawn from it over the past seven years.
Methods: :
Two hundred twenty eight mostly sporadic index cases of LCA and 138 sporadic index cases with RP have been collected and screened for the eight candidate genes known so far. We employed screening by SSCP and subsequent direct sequencing using fluorescence labelled didesoxy nucleotides. APEX detection of known mutations has been tested in a limited set of patients.
Results: :
About one third of all patients have been screened for all eight genes, more than two thirds have been completely screened for RetGC1, RPE65, AIPL1, LRAT, and CRX. RetGC1 and RPE65 were equally causative with about 10% of LCA and about 8% of RP cases screened with both mutations among most cases could be shown. Less frequently involved were mutations in LRAT, AIPL1, RPGRIP and CRB1 with about 1 – 5% of LCA cases. We could not detect any mutation in TUPL1 in LCA as well as RP cases. CRX was not found in our set of LCA cases but only in a few cases of progressive forms of cone rod dystrophy. Screening of 34 LCA and 8 cases of other retinal dystrophies with the newly developed LCA–CHIP detected single heterozygous mutations in 27.6% and compound heterozygous or homozygous mutations in 19% of tested specimens. One specimen showed compound heterozygosity in AIPL1 and an additional mutation in CRB1. The microarray testing was in agreement with our own SSCP results which showed many novel mutations in all candidate genes tested positively.
Conclusions: :
We screened about 300 cases of LCA and RP for mutations in candidate genes for EOSRD and identified mutations in a fraction according to the literature. Application of the LCA–CHIP gave results in 27.6% of our cases but created necessity of a complete screen of the involved gene in 8% of these cases. Less than 1% of cases completed for the whole set of genes showed potential modifier mutations. Therefore, it seems questionable whether screening needs to be extended once a disease causing condition has been identified in one of the known genes.
Keywords: retinal degenerations: hereditary • gene screening • candidate gene analysis