OCRL1 mutation analysis in French Lowe syndrome patients: implications for molecular diagnosis strategy and genetic counseling

The oculocerebrorenal syndrome of Lowe (OCRL) is a rare X-linked recessively inherited disease characterized by a severe pleiotropic phenotype including mental retardation, bilateral congenital cataract, and renal Fanconi syndrome. The gene responsible for OCRL encodes an inositol polyphosphate-5-phosphatase. We performed mutation analysis in 36 families and characterized 27 new mutations with two of them being recurrent mutations. The panel of mutations consisted of 27 truncating mutations (frameshift, nonsense, splice site mutations, and large genomic deletions), one in-frame deletion, and six missense mutations. The four large genomic deletions occurred in the first half of the gene, whereas all the remaining mutations took place in the second part of the gene and were concentrated in a few exons. This distribution may be of interest in terms of screening strategy when looking for unknown mutations. Haplotyping of the families was performed to analyze segregation of the mutated loci, and revealed a somatic mosaicism in one family. This is the second case of mosaicism we characterized in a total panel of 44 unrelated families affected by Lowe's syndrome. Considering the low number of families investigated, it appeared that somatic and germinal mosaicisms are quite common in this disease and must be taken into account for genetic counseling.