Facioscapulohumeral Muscular Dystrophy 2

A number sign (#) is used with this entry because facioscapulohumeral muscular dystrophy-2 (FSHD2) shows digenic inheritance. It is caused by the combination of a heterozygous mutation in the SMCHD1 gene (614982) on chromosome 18p and presence of a haplotype on chromosome 4 that is permissive for DUX4 (606009) expression.

Description

Facioscapulohumeral muscular dystrophy is a form of muscular dystrophy characterized by muscle weakness that first affects the facial muscles and upper extremities, later progressing to involve the lower extremities. The pattern of weakness is usually asymmetric (summary by Lemmers et al., 2012).

FSHD1 (158900), which is clinically indistinguishable from FSHD2, is associated with contraction of the D4Z4 macrosatellite repeat (see 606009) in the subtelomeric region of chromosome 4q35. The disease mechanisms of FSHD1 and FSHD2 converge at the level of D4Z4 chromatin relaxation and variegated expression of DUX4 in skeletal muscle (summary by Lemmers et al., 2012).

Clinical Features

De Greef et al. (2010) examined 33 patients with FSHD2, defined as having no D4Z4 repeat less than 11 units on the permissive 4A161 haplotype, low D4Z4 methylation levels on chromosomes 4q and 10q, and a clinical phenotype consistent with FSHD. The average age at onset was 26 years (range 0 to 60), almost 10 years later than in FSHD1. The most common initial symptom was scapular weakness (61%), followed by foot dorsiflexor weakness (27%), facial weakness (10%), and hip girdle weakness (3%). On examination, all had scapular weakness, 79% had foot dorsiflexor weakness, and all but 2 patients had facial weakness. Positive Beevor sign, indicating abdominal muscle weakness, was found in 67% tested. The clinical severity score on average was similar to that reported in FSHD1. Less common findings included lack of ambulation (9%) and hearing loss (18%). Evaluation of the retinal vessels was not performed, but 2 patients examined showed no retinal vasculopathy. The majority of cases (20/33) were sporadic, 11 were familial, and the inheritance pattern was uncertain in 2, suggesting a different inheritance pattern from that in FSHD1.

Sacconi et al. (2012) reported 6 patients with sporadic occurrence of FSHD2, including 1 reported by de Greef et al. (2010). All had facial muscle and scapular weakness, and 5 had humeral, abdominal, and anterior foreleg weakness in an asymmetric pattern. Three had pelvic girdle weakness. Most of them experienced pain and fatigue, and 1 had sensorineural hearing loss. Creatine kinase levels were normal to 2-fold increased, muscle biopsies showed only mild dystrophic changes, and all had a myopathic pattern on EMG. All patients carried a 45- to 95-kb 4A161 allele and showed marked hypomethylation of the D4Z4 locus, typical of FSHD2. Study of family members showed that 3 unaffected mothers and 1 unaffected father carried the same 4A161 allele as their affected offspring, but they were not hypomethylated. One patient's unaffected daughter had significant hypomethylation, but the D4Z4 repeats were on a nonpermissive chromosome 4 background, suggesting that the hypomethylation determinant segregates independently from D4Z4 at 4q35.

Inheritance

FSHD2 shows digenic inheritance, requiring the inheritance of 2 independent genetic variations: a mutation in the SMCHD1 gene on chromosome 18p that results in D4Z4 chromatin relaxation and an FSHD-permissive DUX4 allele on chromosome 4. SMCHD1 mutations segregate independently from the FSHD-permissive DUX4 allele (summary by Lemmers et al., 2012).

Mapping

Gilbert et al. (1993) found evidence for heterogeneity in FSHD. In linkage studies, 5 of 7 families gave a posterior probability of more than 95% of being of the linked type, while 2 families appeared unlinked to that region of distal 4q. Affected members of the 2 unlinked families met the clinical criteria for the diagnosis of FSHD, including facial weakness, clavicular flattening, scapula winging, proximal muscle weakness, and myopathic changes on muscle biopsy without inflammatory or mitochondrial pathology.

In FSHD1A on 4q35-qter, the disease is associated with deletion of 3.3-kb repeats from a tandem repeat located near the gene. This repeat cross-hybridizes with a telomeric region on 10q, making this cross-hybridizing region a plausible candidate gene for FSHD1B. Speer et al. (1997) tested the most telomeric marker on 10q (sAVA4) and excluded approximately 17 cM on either side of this marker as harboring the FSHD1B gene.

Molecular Genetics

Among 33 patients with FSHD2, de Greef et al. (2010) found that all carried at least 1 D4Z4 repeat at chromosome 4q on the permissive haplotype 4A161. The shortest repeat on average was 16, which is shorter than the average of 28 observed in controls. Patients with FSHD2 showed significantly decreased methylation at D4Z4 on chromosomes 4q and 10q compared to controls, whereas those with FSHD1 had decreased methylation only at 4q. Moreover, the degree of decreased methylation in FSHD2 was significantly more than that observed in FSHD1; however, this was not associated with increased severity.

Van Overveld et al. (2003) showed that contraction of the D4Z4 repeat array in cases of FSHD1A causes marked hypomethylation of the contracted D4Z4 allele. Individuals with FSHD clinically identical to other cases but with an unaltered D4Z4 also have hypomethylation of D4Z4. These results strongly suggested that hypomethylation of D4Z4 is a key event in the cascade of epigenetic events causing FSHD1.

Using chromatin immunoprecipitation (ChIP) in HeLa cells, Zeng et al. (2009) found SUV39H1 (300254)-mediated trimethylation of histone H3 (see 602810) at lysine-9 (H3K9), as well as trimethylation at H3 at lysine-27 (H3K27), both at D4Z4, representing transcriptionally repressive heterochromatin. There was also H3K4 dimethylation and H3 acetylation at proximal D4Z4 repeat regions, marking transcriptionally permissive euchromatin. The methylation signal at H3K9, at both the 4q and the 10q locus, was significantly decreased in cell lines derived from patients with FSHD1 (myoblasts and fibroblasts) and FSHD2 (fibroblasts) compared to controls. Contraction of D4Z4 at 1 allele showed a dominant effect on methylation of H3K9 at the other allele, as well as at the 10q locus, suggesting a spreading effect of histone modification. DNA hypomethylation was not observed in FSHD cells, and the decrease in H3K9 methylation was not observed in cells from patients with other forms of muscular dystrophy. Immunoprecipitation studies showed that loss of methylation at H3K9 interrupted binding of CBX3 (604477) and the cohesin complex (see, e.g., SCC1, 606462) at this region. Zeng et al. (2009) hypothesized that loss of H3K9 methylation, and thus loss of CBX3 and cohesion, results in the disruption of chromatin regulation, thereby causing abnormal derepression of distant target genes that leads to the dystrophic phenotype specific to muscle tissue.

Mutation in the SMCHD1 Gene

In affected members of 15 (79%) of 19 families with facioscapulohumeral muscular dystrophy 1B, Lemmers et al. (2012) identified heterozygous loss-of-function mutations in the SMCHD1 gene (see, e.g., 614982.0001-614982.0005). The mutations in 7 families were initially identified by exome sequencing and confirmed by Sanger sequencing. The mutational spectrum included small deletions, splice site mutations, and missense mutations, resulting in haploinsufficiency. Patients showed D4Z4 hypomethylation to levels less than 25% (normal being about 50%), and protein blot analysis in several patients showed decreased SMCHD1 protein in fibroblasts. Affected individuals were also heterozygous or homozygous for an FSHD1-permissive D4Z4 haplotype that contains a polyadenylation signal to stabilize DUX4 mRNA in skeletal muscle. Primary myotubes from a normal individual with a normal-sized and methylated D4Z4 array on a permissive haplotype showed no DUX4 mRNA. However, decreasing SMCHD1 expression to about 50% using RNA interference resulted in transcriptional activation of DUX4 and a variegated pattern of DUX4 protein expression in the myotubes. The pattern of variegated DUX4 expression that resulted was similar to that observed in FSHD1 and FSMD2 myotube cultures. The findings indicated that SMCHD1 activity is necessary for D4Z4 hypermethylation and somatic repression of DUX4, and that reduction of SMCHD1 results in D4Z4 arrays that express DUX4 when a permissive haplotype is present. The SMCHD1 mutation and the permissive D4Z4 haplotype segregated independently in the families, indicating digenic inheritance. Of the 26 individuals with hypomethylation at D4Z4, a SMCHD1 mutation, and a permissive D4Z4 haplotype, 5 (19%) were asymptomatic, indicating incomplete penetrance.

Genotype/Phenotype Correlations

Sacconi et al. (2013) found that mutation in the SMCHD1 gene is a modifier of disease severity in families affected by FSHD1. Three unrelated families with intrafamilial clinical variability of the disorder were studied. In 1 family, a mildly affected man with FSHD1 carried a 9-unit D4Z4 repeat on a 4A allele with no SMCHD1 mutations, whereas his mildly affected wife carried a SMCHD1 mutation (T527M; 614982.0006) on a normal-sized 4A allele, consistent with FSHD2. Their more severely affected son and grandson each carried the 9-unit D4Z4 repeat on a 4A allele as well as the T527M SMCHD1 mutation, consistent with having both FSHD1 and FSHD2. In a second family, a man with a severe early-onset phenotype had both a 9-unit D4Z4 repeat on a 4A permissive allele and a mutation in the SMCHD1 gene. Each of his children, who had milder symptoms, inherited 1 of the genetic defects. In a third family, a man with a severe phenotype was also found to carry a 9-unit D4Z4 repeat on a 4A permissive allele with a SMCHD1 mutation. No information from his parents was available. Transduction of SMCHD1 shRNA into FSHD1 myotubes caused increased levels of DUX4 mRNA as well as transcriptional activation of known DUX4 target genes. These findings were consistent with further chromatin relaxation of the contracted FSHD1 repeat upon knockdown of SMCHD1. Sacconi et al. (2013) concluded that FSHD1 and FSHD2 share a common pathophysiologic pathway converging on transcriptional derepression of DUX4 in skeletal muscle.