Exclusion mapping of chromosomal regions which cross hybridise to FSHD1A associated markers in FSHD1B

Clinical Studies in Non-chromosome 4-Linked Facioscapulohumeral Muscular Dystrophy

OBJECTIVES

To characterize clinically and molecularly a large, non-chromosome 4-linked facioscapulohumeral muscular dystrophy (FSHMD) family.

METHODS

Neurological evaluations of affected (N = 55) and at-risk (N = 48) individuals were performed along with selected laboratory analyses, including creatine kinase testing, muscle biopsy, p13E-11 fragment analysis, and cytogenetic studies. Genetic analyses of the scapuloperoneal muscular dystrophy and scapuloperoneal muscular atrophy regions on chromosome 12 were performed using genetic markers flanking the intervals of interest and parametric LOD score analyses.

RESULTS

Clinically, the FSHMD in individuals in this family is indistinguishable from that observed in chromosome 4-linked FSHMD. Fragment analysis with p13E-11 showed no small fragment segregating with the family and no evidence for 4:10 translocation or deletion of the p13E-11 binding site. Linkage analysis excluded the loci for autosomal-dominant scapuloperoneal muscular dystrophy and scapuloperoneal muscular atrophy.

CONCLUSIONS

This family is clinically similar to patients with the chromosome 4-linked FSHMD. These data support our previous hypothesis of genetic heterogeneity within FSHMD.

The genes encoding for D4Z4 binding proteins HMGB2, YY1, NCL, and MYOD1 are excluded as candidate genes for FSHD1B

Facioscapulohumeral muscular dystrophy is a disease of skeletal muscle, with symptoms including both facial and shoulder girdle weakness and progression to involve the pelvic girdle and extremities in the majority of cases. For most cases of FSHD, the molecular basis of the disease can be identified as a partial deletion of the D4Z4 repeat array on the end of the long arm of chromosome 4. However, in up to 5% of FSHD families there is no linkage to 4q35. These cases are designated as FSHD1B. Proteins have been identified that bind to the D4Z4 repeats of chromosome 4q35. The genes encoding D4Z4 binding proteins YY1, HMGB2, NCL, and MYOD1 were investigated as candidate genes for FSHD1B. Coding sequences and promoter region were analyzed for HMBG2 and no sequence variations were detected. For YY1, all five exons were analyzed and a polymorphism was detected in both the unaffected and affected populations. In nucleolin (NCL), several SNPs were identified, including a SNP causing the non-synonymous change P515H; however, all polymorphisms either occurred in control samples or were previously reported. A novel polymorphism was also detected in MYOD1, but did not represent a disease-specific variation. These results suggest that HMBG2, YY1, NCL, and MYOD1 are unlikely to represent the genes responsible for FSHD in these families.

Molecular genetics of facioscapulohumeral muscular dystrophy (FSHD)

Facioscapulohumeral muscular dystrophy (FSHD; MIM 158900), is an autosomal dominant neuromuscular disorder. The disease is characterized by the weakness of the muscles of the face, upper-arm and shoulder girdle. The gene for FSHD has been mapped to 4q35 (FSHD1A) and is closely linked to D4F1O4S1, which detects two highly polymorphic loci (located at 4q35 and 10q26), with restriction enzyme EcoRI. The polymorphic EcoRI fragment detected with D4F1O4S1 is composed almost entirely of D4Z4 (3.3 kb) tandem repeats. In FSHD patients a deletion of the integral number of D4Z4 repeats generates a fragment which is usually smaller than 35 kb, whereas in normal controls, the size usually ranges from 50 to 300 kb. These 'small' EcoRI fragments segregate with FSHD in families but appear as de novo deletions in the majority of sporadic cases. Each 3.3 kb repeat contains two homeobox domains neither of which has yet been proven to encode a protein. D4Z4 is located adjacent to the 4q telomere and cross hybridizes to several different regions of the genome. Although D4Z4 probably does not encode a protein with any direct association to FSHD, a clear correlation has been shown between the deletion size at this locus and the age at onset of the disease in FSHD patients. In approximately 5-10% of FSHD families the disease locus is unlinked to 4q35 (locus designated FSHD1B), however, none of the non 4q35 loci for FSHD have yet been chromosomally located. Thus so far, only one gene, FRG1 (FSHD region gene 1) has been identified from the FSHD candidate region on 4q35. The apparent low level of expressed sequences from within this region, the integral deletions of D4Z4 repeats observed in FSHD patients and the close proximity of these repeats to the 4q telomere, all suggest that the disease may be the result of position effect variegation. To date, the molecular diagnosis of FSHD with D4F104S1 has been most secure in those families which are linked to other 4q35 markers. Recent studies based on the distinction of 4q35 fragments from those from 10q26 will facilitate molecular diagnosis. The pathophysiology and biochemical defect in FSHD still remains to be elucidated. The identification of the FSHD gene and characterization of the gene product will not only potentiate accurate diagnosis but may also unravel the complexities of the 4q35 FSHD region.