Functional Characterization of Splice Variants in the Diagnosis of Albinism.

Albinism is a genetically heterogeneous disease in which 21 genes are known so far. Its inheritance mode is autosomal recessive except for one X-linked form. The molecular analysis of exonic sequences of these genes allows for about a 70% diagnostic rate. About half (15%) of the unsolved cases are heterozygous for one pathogenic or probably pathogenic variant. Assuming that the missing variant may be located in non-coding regions, we performed sequencing for 122 such heterozygous patients of either the whole genome (27 patients) or our NGS panel (95 patients) that includes, in addition to all exons of the 21 genes, the introns and flanking sequences of five genes, TYR, OCA2, SLC45A2, GPR143 and HPS1. Rare variants (MAF < 0.01) in trans to the first variant were tested by RT-PCR and/or minigene assay. Of the 14 variants tested, nine caused either exon skipping or the inclusion of a pseudoexon, allowing for the diagnosis of 11 patients. This represents 9.8% (12/122) supplementary diagnosis for formerly unsolved patients and 75% (12/16) of those in whom the candidate variant was in trans to the first variant. Of note, one missense variant was demonstrated to cause skipping of the exon in which it is located, thus shedding new light on its pathogenic mechanism. Searching for non-coding variants and testing them for an effect on RNA splicing is warranted in order to increase the diagnostic rate.

Genotypic spectrum of albinism in Mali.

Albinism is a phenotypically and genetically heterogeneous condition characterized by a variable degree of hypopigmentation and by ocular features leading to reduced visual acuity. Whereas numerous genotypic studies have been conducted throughout the world, very little is known about the genotypic spectrum of albinism in Africa and especially in sub-Saharan Western Africa. Here we report the analysis of all known albinism genes in a series a 23 patients originating from Mali. Four were diagnosed with OCA 1 (oculocutaneous albinism type 1), 17 with OCA 2, and two with OCA 4. OCA2 variant NM_000275.3:c.819_822delinsGGTC was most frequently encountered. Four novel variants were identified (two in TYR, two in OCA2). A deep intronic variant was found to alter splicing of the OCA2 RNA by inclusion of a pseudo exon. Of note, the OCA2 exon 7 deletion commonly found in eastern, central, and southern Africa was absent from this series. African patients with OCA 1 and OCA 4 had only been reported twice and once, respectively, in previous publications. This study constitutes the first report of the genotypic spectrum of albinism in a western sub-Saharan country.