Clinical features of facioscapulohumeral muscular dystrophy 2

Abstract

Objective: In some 5% of patients with facioscapulohumeral muscular dystrophy (FSHD), no D4Z4 repeat contraction on chromosome 4q35 is observed. Such patients, termed patients with FSHD2, show loss of DNA methylation and heterochromatin markers at the D4Z4 repeat that are similar to patients with D4Z4 contractions (FSHD1). This commonality suggests that a change in D4Z4 chromatin structure unifies FSHD1 and FSHD2. The aim of our study was to critically evaluate the clinical features in patients with FSHD2 in order to establish whether these patients are phenotypically identical to FSHD1 and to establish the effects of the (epi-) genotype on the phenotype.

Methods: This cross-sectional study studied 33 patients with FSHD2 from 27 families, the largest cohort described to date. All patients were clinically assessed using a standardized clinical evaluation form. Genotype analysis was performed by pulsed field gel electrophoresis and PCR; D4Z4 methylation was studied by methylation-sensitive Southern blot analysis.

Results: FSHD2 is identical to FSHD1 in its clinical presentation. Notable differences include a higher incidence (67%) of sporadic cases and the absence of gender differences in disease severity in FSHD2. Overall, average disease severity in FSHD2 was similar to that reported in FSHD1 and was not influenced by D4Z4 repeat size. In FSHD2, a small effect of the degree of hypomethylation on disease severity was observed.

Conclusions: Clinically, patients with FSHD2 are indistinguishable from patients with FSHD1. The present data suggest that FSHD1 and FSHD2 are the result of the same pathophysiologic process.

Figure 1 Two conditions predispose to facioscapulohumeral muscular dystrophy (FSHD) Both contraction-dependent and contraction-independent D4Z4 chromatin changes on the permissive 4A161 haplotype result in FSHD development. Black triangles and black dots represent a more closed chromatin structure as found in control individuals. White triangles and white dots correspond to a more open chromatin structure as found in patients with FSHD1 and patients with FSHD2.

Figure 2 D4Z4 methylation levels in control individuals, patients with facioscapulohumeral muscular dystrophy (FSHD) 1, and patients with FSHD2 D4Z4 methylation levels were determined using methylation-sensitive restriction enzymes in combination with Southern blot analysis. Results are shown for the BsaAI, FseI, and CpoI restriction sites in the proximal D4Z4 repeat unit on chromosome 4q and for the CpoI restriction site in the proximal D4Z4 repeat unit on chromosome 10q. *p < 0.01 vs control individuals; **p < 0.01 vs patients with FSHD1.

Figure 3 Two facioscapulohumeral muscular dystrophy (FSHD) 2 families (A) Family 2, a Canadian family, consists of parents with 3 sons. Two of them, the oldest and youngest, are affected by FSHD2. Both show D4Z4 hypomethylation and carry the permissive 4A161 haplotype. However, while the oldest brother carries a maternally inherited 4A161 repeat of 21 units and a paternally inherited 4A161 repeat of 22 units, the youngest brother carries a maternal repeat of 13 units on a 4A161 chromosome. Thus, for the development of FSHD2 D4Z4 hypomethylation and a 4A161 chromosome, but not a specific 4A161 chromosome, are necessary. (B) Family 6, a French family, consists of 4 siblings. Only the oldest brother is affected; on both chromosomes 4q the permissive 4A161 haplotype is present and D4Z4 hypomethylation is observed at 3 of the 4 sites tested. Importantly, one of his unaffected brothers carries similar D4Z4 chromosomes but does not present with D4Z4 hypomethylation, while his other unaffected brother has D4Z4 hypomethylation but lacks the permissive 4A161 haplotype present in all patients with FSHD.