Dutch patients with glycogen storage disease type II show common ancestry for the 525delT and del exon 18 mutations.

EDITOR—Glycogen storage disease type II (GSD II) is an autosomal recessive lysosomal storage disorder caused by deficiency of acid á-glucosidase. The enzyme deficiency results in intralysosomal accumulation of glycogen in skeletal muscle and in other tissues. There are early and late onset phenotypes which diVer with respect to age at onset, extent of organ involvement, and clinical course of the disease. The genotype frequency of GSD II was recently shown to be 1 in 40 000 by mutation screening in the general population, which is higher than previously estimated. 3 Over 40 diVerent mutations in the acid á-glucosidase (GAA) gene have been reported. Most mutations are rare and have been found in only a few patients. However, some mutations have been reported in several unrelated patients with defined ethnic origins. The C1935A transversion, frequently found in Chinese patients with infantile GSD II, appears to originate from a common founder. Other frequent mutations include the R854X mutation in Afro-Americans, the 2741AG→CAGG insertion in Turkish patients, and the G925A mutation in European patients. It remains to be determined whether these frequent mutations represent common descent or result from independent recurrence. The IVS1(−13T→G) mutation is the most frequent mutation in late onset GSD II patients from diVerent ethnic origins. In The Netherlands, most late onset GSD II patients carry the IVS1(−13T→G) mutation in combination with either the 525delT or the del exon 18 mutation, whereas infantile GSD II patients often show homozygosity or compound heterozygosity for the 525delT and the del exon 18 mutations. The latter mutations are fully deleterious and are associated with complete loss of enzyme activity. 13 The deletion of exon 18 extends from IVS17 to IVS18 and includes the coding sequence of exon 18. Analysis of the deletion junction showed a direct eight nucleotide repeat sequence flanking the deletion, with one direct repeat included in the deletion and the second direct repeat at the deletion junction. 15 This repeat sequence could be instrumental in the mutation event. So far, the mutation has not been reported in patients of non-white origin. The 525delT mutation has also not been reported in non-white patients, and is relatively rare in “non-Dutch” patients. In order to determine whether the 525delT and del exon 18 mutations represent founder events or independent, de novo mutations, we constructed haplotypes using four single nucleotide polymorphisms (SNPs) in the GAA gene. We used a set of 28 unrelated GSD II patients to determine the extent of haplotype sharing between the individual patients carrying identical mutations. The patient population included 26 white Dutch patients and their parents from 26 families with infantile GSD II, and three white Dutch patients and their parents from two families with adult GSD II. All patients carried at least one frequent mutation (525delT or del exon 18) and had deficient GAA activity, measured in fibroblasts and leucocytes. Genomic DNA was extracted from cultured skin fibroblasts and from peripheral blood cells using standard procedures. Mutation analysis was performed as described previously. We analysed four intragenic single nucleotide polymorphisms (SNPs) by PCR amplification, followed by digestion of the PCR product with the appropriate restriction enzyme (table 1). To amplify exons 3, 8, 11, and 17, information was obtained from Martiniuk et al. Fragments were electrophoresed on a 2% agarose gel. Genotyping parents of GSD II patients assigned the phase of the alleles. The order of SNPs and mutations was as follows: 525delT exon 3 SNP exon 8 SNP exon 11 SNP exon 17 SNP del exon 18. We estimated allele frequencies by direct counting of chromosomes. It has been shown in several studies that inclusion of genotypes from incomplete families, or the inclusion of reconstructed genotypes, may introduce serious bias into the estimation of allele and haplotype frequencies. Therefore, in the analysis of linkage disequilibrium and the estimation of Table 1 Analysis of SNPs within the GAA gene