Molecular Analysis of the ABCA4 Gene in French-Canadian Stargardt Patients: Identification of Novel Mutations and Absence of Founder Effects

Abstract

Purpose: : Stargardt disease (STGD) is inherited as an autosomal recessive (AR) or dominant trait, with ABCA4 causing AR inheritance. To determine the nature and prevalence of mutations causing AR Stargardt in the French-Canadian population, we screened the ABCA4 gene in 23 families.

Methods: : STGD was confirmed in 30 patients. Disease haplotypes and AR inheritance were confirmed by genotyping 6 microsatellite markers within the ABCA4 region. Mutations were screened by sequencing all of ABCA4 50 exons, splicing junctions and promoter region.

Results: : The 30 patients harbored a total of 45 distinct disease haplotypes. Our screening revealed 16 non-synonymous amino-acid (AA) changes, 2 splicing mutations and 1 variation in the promoter. 7 of these AA changes, N96H, R537C, W821R, G863A, L1250P, G1961E and L2027F, have been reported as mutations. Four families, harboring the same disease haplotype, carried G1961E, the most frequent mutation worldwide. The other 5 AA changes, R212H, H423R, R943Q, N1861I and S2255I, have been previously categorized either disease-causing mutations or non-disease associated polymorphisms. We also found 7 novel mutations; one in the promoter at -439 A>G; the F56S, I73T, A595V and A1773E AA changes and the intron IVS19+1 G>C and IVS37-2ΔA splicing mutations. Two of them were confirmed as mutations using specific assays. The other novel mutations are still to be confirmed. Mutations in the remaining disease haplotypes are still to be identified.

Conclusions: : Our discovery rate was measured at 63% (19/30). Indeed, both disease-alleles were identified in 8 of our 30 patients, while 11 patients showed only 1 mutation. We also report 7 novel mutations. Founder effects, frequent in the French-Canadian population, were not detected in ABCA4, probably due to the vast number of exons favoring DNA rearrangements in the gene.

Keywords: macula/fovea • mutations • gene/expression