Molecular characterization of Portuguese patients with mucopolysaccharidosis type II shows evidence that the IDS gene is prone to splicing mutations

Abstract

Mucopolysaccharidosis type II (MPS II) is an X-linked recessive lysosomal storage disease caused by a defect in the iduronate-2-sulfatase gene (IDS). Alternative splicing of the IDS gene can occur and the underlying regulatory mechanism may be rather complex. Nevertheless, little information is available on the role of variations at the IDS locus in the splicing process. Here we report that splice mutations at the IDS locus are an important source of MPS II pathogenicity, accounting for almost 56% of Portuguese cases. Among 16 unrelated Portuguese MPS II patients, 15 different mutations were identified: six intronic splice mutations (c.104-2AG, c.241-2A>G, c.241-1G>A, c.418+1G>A, c.880-8AG and c.1181-1G>C); two exonic splice mutations (c.1006G>lC and c.1122C>T); five missense mutations (D269V, D69V, D148N, R88C and P86L); one nonsense mutation (Q465Ter); one total IDS gene deletion; and one rearrangement involving a IDS gene inversion. Furthermore, nine of the 15 detected mutations affected the usual splicing pattern at the locus. Some of them are responsible for dramatic changes in the splicing mechanism. For example, the substitution mutation, c.418+1G>A, revealed the presence of an exonic sequence inside intron 3. Our study provides evidence that the IDS locus is prone to splicing mutations and that such susceptibility is particularly high in exon 3 and neighbouring regions. Consequently, mutation screening of the IDS gene cannot be restricted to gDNA examination. Unless cDNA analysis is also conducted, misclassifications as silent or missense mutations can be produced and even uncharacteristic splice-site mutations can be misinterpreted as classic splicing defects that may generate severe, unconventional splicing alterations.

Molecular analysis of the iduronate‐2‐sulfatase gene in Thai patients with Hunter syndrome

“…1C). This pattern of alternative transcripts is similar to that seen for the c.418+1G>C (Lualdi et al 2006) and c.418+1G>A (Alves et al 2006) mutations at the first nucleotide of intron 3, where four alternative transcripts were observed. Alves and colleagues (2006) also observed the cryptic exon 3a at low abundance in control transcripts.…”

“…The cryptic exon generated in some transcripts in (C) is denoted 3a, as described by Alves and colleagues (2006). The Shapiro-Senapathy scores for the splice sites are shown in italic at the splice sites, as described by Lualdi and colleagues (2006) (Alves et al 2006;Vafiadaki et al 1998), it resulted in a transcript lacking the first 44 nucleotides of exon 3 (transcript 1) in addition to the correctly spliced transcript (transcript 2) for p.P86L (Fig. 1B).…”

“…In a few cases, these mutations were previously reported in intermediate Hunter cases. Given the range of complexity of transcript splicing for the MPS II gene and its disruption by point mutations (Alves et al 2006;Lualdi et al 2006), splicing factors may play a role in this. The identification of eight novel mutations and characterization of the c.418G>A transcripts here contributes to the understanding of the molecular aetiology of this gene, which may facilitate better genotype-phenotype correlations.…”

“…Approximately 80% of the mutations identified are point mutations and small structural aberrations, while large alterations cause about 20% of MPS II cases (Hopwood et al 1993). In addition, transcript splicing errors have been reported to be involved in up to 50% of mutations in some populations (Alves et al 2006). Although many IDS gene mutations have been identified in Asia, including in China, Taiwan, Japan and Korea (Chang et al 2005;Dou et al 2007;Kato et al 2005;Kim et al 2003), none have previously been reported from South-East Asia.…”

A new mutation (1062 del 16) of iduronate-2-sulfatase gene from a Chinese patient with hunter syndrome

“…The detection of this mutation is helpful for demonstrating that IDS gene has a high new mutation rate and remarkable hot spots (Rathmann et al, 1996). Recently, Alves et al(2006) confirmed that IDS gene has a high mutation rate of splicing site. The study of Lualdi et al(2006) revealed that point mutation of IDS gene can activate multiple larvaceous splicing sites, leading to mis-splicing RNA.…”