Welander distal myopathy : gene mapping and analysis of candidate genes

Abstract

Distal myopathies constitute of a very heterogeneous group of muscular disorders. Distal myopathies have previously been classified according to features such as; clinical symptoms, age of onset and histopathological changes. As an increasing knowledge of the genetic mechanism behind these disorders is being revealed, it is likely that this will lead to changes in the classification of distal myopathies. Welander distal myopathy (WDM) was first described in 1951 by Lisa Welander. WDM is characterized by autosomal dominant mode of inheritance, late onset, and distal distribution of muscular weakness. The hands are first affected with weakness of the finger extensor muscles. Homozygous cases have been observed and these are more severely affected displaying proximal muscle involvement. The disorder has been reported in both Sweden and in Finland. Most cases originate from the middle parts of Sweden. The progression rate is slow and the life span is not shortened. Histopathological changes seen in muscle biopsies are mainly of myopathic type and include rimmed vacuoles.

This thesis is based on the genetic studies of WDM. In order to establish the chromosomal location of the disease locus a whole genome scan was performed in Swedish family material. The Swedish WDM material consists of 305 subjects from thirteen different families, with a total of 124 clinically affected WDM patients. Strong genetic linkage was demonstrated to chromosome 2p13 with a maximum lod score of 17.97 for marker D2S2113 at recombination fraction 0.0. Critical recombinations narrowed down the candidate region to approximately 2.4 cM. A common shared haplotype was observed among all affected subjects consisting of five microsatellite markers. This common shared haplotype makes it possible that one single ancestor mutation is responsible for the gene disruption causing WDM in all thirteen Swedish families.

Patients with WDM have also been reported among the Finnish population. Thirty-one Finnish patients with WDM were screened for histopathological changes in muscle biopsies. They were also genotyped with the five markers of the common shared haplotype. Clinical examination of these patients showed distal myopathy with onset in the long extensor muscle of the hands and fingers, also seen in Swedish Welander patients. Muscle biopsies showed characteristic myopathic changes. Haplotype analysis revealed that the same common shared haplotype is also cosegregating among the Finnish patients and a common ancestry is therefore further supported for all WDM patients.

In order to further narrow down the WDM candidate region, refined mapping was performed using an extended set of microsatellite markers and SNPs. The WDM candidate region was narrowed down to an interval spanning approximately 2.2 Mb flanked by markers D2S358 as the telomeric border and PAC3-H52 as the new centromeric border. We were also able to formally exclude the dysferlin gene, as it is located outside the WDM candidate region. Dysferlin was considered an important candidate for WDM as mutations in this gene are responsible for two other distal myopathies also linked to the 2p13 region, Limb-girdle muscular dystrophy type 2B and Miyoshi myopathy.

Intensified direct candidate analysis was performed on ten genes from the WDM region for mutation analysis. Candidate genes were primarily chosen for their location but possible muscle phenotype involvement was also considered. Fourteen WDM patients of which two were homozygous for possible disease causing mutations were tested and compared to two healthy control subjects as well as to the published gene sequence for each candidate gene. Genomic DNA was used and direct sequencing was performed to screen all exons and exon-intron borders of the ten candidate genes. Seven of the genes were located within the common shared haplotype region (PCBP1, FLJ20558, TIA1, PCYOX1, SNRPG, FLJ14668 and TGFA). Three of the screened candidate genes were within the WDM candidate region but outside the common shared haplotype (ADD2, RY, and ANXA4). No disease causing mutations were detected in any of these genes. The genes that have not yet been screened within this region represent a broad range of proteins with many diverse possible functions. No obvious candidate stands out among them based on present knowledge and information. The cause of WDM still remains to be identified.