A genetic study of Wilson's disease in the United Kingdom

Abstract

Previous studies have failed to identify mutations in the Wilson's disease gene ATP7B in a significant number of clinically diagnosed cases. This has led to concerns about genetic heterogeneity for this condition but also suggested the presence of unusual mutational mechanisms. We now present our findings in 181 patients from the United Kingdom with clinically and biochemically confirmed Wilson's disease. A total of 116 different ATP7B mutations were detected, 32 of which are novel. The overall mutation detection frequency was 98%. The likelihood of mutations in genes other than ATP7B causing a Wilson's disease phenotype is therefore very low. We report the first cases with Wilson's disease due to segmental uniparental isodisomy as well as three patients with three ATP7B mutations and three families with Wilson's disease in two consecutive generations. We determined the genetic prevalence of Wilson's disease in the United Kingdom by sequencing the entire coding region and adjacent splice sites of ATP7B in 1000 control subjects. The frequency of all single nucleotide variants with in silico evidence of pathogenicity (Class 1 variant) was 0.056 or 0.040 if only those single nucleotide variants that had previously been reported as mutations in patients with Wilson's disease were included in the analysis (Class 2 variant). The frequency of heterozygote, putative or definite disease-associated ATP7B mutations was therefore considerably higher than the previously reported occurrence of 1:90 (or 0.011) for heterozygote ATP7B mutation carriers in the general population (P < 2.2 × 10(-16) for Class 1 variants or P < 5 × 10(-11) for Class 2 variants only). Subsequent exclusion of four Class 2 variants without additional in silico evidence of pathogenicity led to a further reduction of the mutation frequency to 0.024. Using this most conservative approach, the calculated frequency of individuals predicted to carry two mutant pathogenic ATP7B alleles is 1:7026 and thus still considerably higher than the typically reported prevalence of Wilson's disease of 1:30 000 (P = 0.00093). Our study provides strong evidence for monogenic inheritance of Wilson's disease. It also has major implications for ATP7B analysis in clinical practice, namely the need to consider unusual genetic mechanisms such as uniparental disomy or the possible presence of three ATP7B mutations. The marked discrepancy between the genetic prevalence and the number of clinically diagnosed cases of Wilson's disease may be due to both reduced penetrance of ATP7B mutations and failure to diagnose patients with this eminently treatable disorder.

Figure 1

Distribution of mutations in the ATP7B coding region in patients with Wilson’s disease. The frequency of mutations found in the cohort of 181 Wilson’s disease index cases is given per exon as a percentage of the total. Mutations were detected in all exons except for exon 21. Three exons contained >10% of all mutations, namely exons 2, 8 and 14.

Figure 2

Uniparental segmental isodisomy as a novel disease-causing mechanism for Wilson’s disease. Uniparental disomy results in the patient inheriting the identical ATP7B mutation carrying allele twice from the same parent. This was due to a duplication of the boxed region from 13q14.11 to 13q34 in Family A with maternal isodisomy and from 13q14.2 to 13q34 in Family B with paternal isodisomy. ND = not done; ‘=’ = no mutation; R1319* = p.Arg1319*, (c.3955C>T); H1069Q = p.His1069Gln, (c.3207C>A). Microsatellite markers with allele sizes as marked.

Figure 3

‘Pseudodominant’ Wilson’s disease families. Pedigrees of three families with members affected by Wilson’s disease in two consecutive generations. In Pedigree 1, both parents of the affected child were unaffected but a two paternal aunt also has Wilson’s disease. In Pedigrees 2 and 3, Wilson’s disease is present in father and daughter, or father and son, respectively. Circles = female family members; squares = male family members; half-shaded symbols = heterozygote mutation carriers; fully shaded symbols = homozygous mutation carriers.

Figure 4

Exonic frequency and distribution in the ATP7B gene of all SNVs in the 1K data set with in silico evidence of pathogenicity (blue bars). The red bars denote those SNVs that have previously been identified in patients with Wilson’s disease and classified as ‘pathogenic’ in the Wilson’s disease database, University of Alberta or by the Sheffield Diagnostic Genetics Service. Exons are numbered along the bottom axis with the numbers of SNVs per exon on the left-hand axis.

Figure 5

Exonic frequency and distribution of all ATP7B SNVs in the 5K data set with in silico evidence of pathogenicity (blue bars). The red bars denote those SNVs that have been previously identified in patients with Wilson’s disease and classified as ‘pathogenic’ in the Wilson’s disease database, University of Alberta or by the Sheffield Diagnostic Genetics Service. Exons are numbered along the bottom axis with the numbers of SNVs per exon on the left-hand axis.