G.P.96 The point mutations spectrum of survival motor neuron gene 1 (SMN1) in Polish SMA patients lacking a homozygous SMN1 deletion

Abstract

Abstract About 96% of SMA patients display biallelic loss (deletion or conversion) of the SMN1 gene, while remaining patients combine the deletion on one allele with an intragenic mutation on the other. The aim of the study described here was thus to identify point mutations in a group of 606 patients diagnosed for SMA with excluded biallelic loss of the SMN1 gene. First, SMN1 dosage analysis was performed using the MLPA method (SALSA MLPA KIT P060 SMA Carrier) or the real-time quantitative PCR technique. The identification of one copy of SMN1 suggested the presence of a deletion in combination with a point mutation and such patients (32 cases) were subjected to SMN1 sequence analysis. The analysis was performed using long-range PCR (amplification of exons 2–7) for template preparation, as well as direct sequencing of all SMN1 exons. Finally, point missense mutations or small deletions in the SMN1 gene were identified in 15 patients. The most frequent missense mutation (p.Thr274Ile) was identified in seven patients presenting a mild phenotype (SMA type 3). Three other missense mutations, i.e. p.Ser230Leu, p.Ala11Gly and p.Pro244Leu, were identified in a further three spinal muscular atrophy patients. In five patients, small deletions were found, i.e. the c.429_435del7 mutation in three cases, and the c.431delC mutation in two. These mutations not described previously were characteristic of patients presenting a severe phenotype. Our results suggest a specific point mutations spectrum in the Polish population. Small deletions not identified thus far could suggest a possible founder effect. In patients without exon 7 deletion, presenting mild form of SMA, the analysis of p.Thr274Ile mutation should be considered.