Alu-Alu recombination underlying the first large genomic deletion in GlcNAc-phosphotransferase α/β (GNPTAB) gene in a MLII α/β patient

Abstract

Mucolipidosis type II alpha/beta is a severe, autosomal recessive lysosomal storage disorder, caused by a defect in the GNPTAB gene that codes for the alpha/beta subunits of the GlcNAc-phosphotransferase. To date, over 100 different mutations have been identified in MLII alpha/beta patients but no large deletions have been reported. Here we present the first case of a large homozygous intragenic GNPTAB gene deletion (c.3435-386­_3602+343del897) encompassing exon 19, identified in a ML II alpha/beta patient. Long range PCR and sequencing methodologies were used to refine the characterization of this rearrangement, leading to the identification of a 21bp repetitive motif in introns 18 and 19. Further analysis revealed that both the 5’ and 3’ breakpoints were located within highly homologous Alu elements (Alu-Sz in intron 18 and Alu-Sq2, in intron 19), suggesting that this deletion has probably resulted from Alu-Alu unequal homologous recombination. RT-PCR methods were used to further evaluate the consequences of the alteration for the processing of the mutant pre mRNA GNPTAB, revealing the production of three abnormal transcripts: one without exon 19 (p.Lys1146_Trp1201del); another with an additional loss of exon 20 (p.Arg1145Serfs*2), and a third in which exon 19 was substituted by a pseudoexon inclusion consisting of a 62 bp fragment from intron 18 (p.Arg1145Serfs*16). Interestingly, this 62 bp fragment corresponds to the Alu-Sz element integrated in intron 18. This represents the first description of a large deletion identified in the GNPTAB gene and contributes to enrich the knowledge on the molecular mechanisms underlying causative mutations in ML II.