Inactivation of the Fas gene by Alu insertion: retrotransposition in an intron causing splicing variation and autoimmune lymphoproliferative syndrome.

Mutations in the Fas (apo-1, CD95) gene result in autoimmune lymphoproliferative syndrome (ALPS). These mutations are dominated by small deletions and point mutations that result in splicing errors or missense changes. We report here a novel mutation caused by retrotransposon insertion, which results in loss of exon 8 and ALPS. A father and son suffering from recurrent lymphadenopathy were examined for resistance to Fas-mediated apoptosis. A functional defect was detected and RT-PCR analysis revealed two different copies of Fas mRNA, one normal and a second shorter version lacking exon 8. DNA analysis of the genomic region between exons seven and nine in the longer copy revealed two PCR products, one being 331 base pairs (bp) longer than expected. Sequencing revealed that intron 7 had undergone an insertion event with an Alu element (99.31% homology with Alu-Sb1) of 331 bp. This element included a 34-bp Poly A tract that was flanked on each side by a perfect 17 bp direct duplication of the target site. Both patients were heterozygous for the mutated allele that produced Fas mRNA lacking exon 8, although not due to loss of a splice junction. The structure of the insertion suggests that the Alu element may have integrated by retrotransposition, and represents the first report of a retrotransposon causing ALPS.