Two Cases of Facioscapulohumeral Muscular Dystrophy 2 in Korea

Abstract

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant muscular disorder characterized by weakness of facial, shoulder, abdominal, hip girdle, humeral, and anterior distal leg muscles, with descending progression from the face to the legs in an asymmetric pattern. In about 5% of patients with FSHD, no D4Z4 repeat contraction on chromosome 4q35 is observed; this disease entity is called FSHD2. FSHD2 is characterized by DNA hypomethylation on the 4q-subtelomeric macrosatellite repeat array D4Z4. In Korea, there have been no previous reports of FSHD2. We report the first two cases of FSHD2 in Korea, carrying c.3801delG and c.1580C>T mutations in the SMCHD1 gene, respectively. For rapid and accurate diagnosis of FSHD2, genetic analysis of the D4Z4 haplotype and methylation with next-generation sequencing are required.

INTRODUCTION

Facioscapulohumeral muscular dystrophy (FSHD, MIM #158900) is the most common myopathy in adults, with an overall incidence greater than 1:10000. FSHD is transmitted as an autosomal dominant inherited muscle disorder and is clinically characterized by asymmetric weakness of facial, shoulder girdle, trunk, and upper arm muscles that progresses to involve humeral, anterior lower leg, and pelvic girdle muscles, with prominent abdominal muscle weakness. It can also be accompanied by retinal telangiectasia or central nervous system involvement (mental retardation and epilepsy).¹

FSHD1, the major subtype of FSHD, is the result of chromatin relaxation by pathogenic contraction of a macrosatellite repeat, called D4Z4, on the 4q subtelomere of a 4qA allele and a functional polyadenylation signal. However, not all patients with the FSHD clinical phenotype manifest D4Z4 pathogenic contraction.² Five percent of FSHD patients have a low to normal number of repeats on chromosome 4q35. Instead, they show profound DNA hypomethylation with pathogenic variants of the SMCHD1 or DNMT3B gene and the 4qA variant with a polyadenylation signal.³ These patients are diagnosed with FSHD2 due to the similar clinical phenotype with FSHD1. While FSHD2 patients were first reported in Western countries and then in Japan, there have been no reports of FSHD2 in Korea. We report two Korean patients with FSHD2 due to a novel frameshift and a reported point mutation in the SMCHD1 gene.

CASE REPORT

Case 1

A 40-year-old male visited our clinic for asymmetric upper and lower extremity weakness. The patient reported that he had suffered bilateral shoulder weakness since the age of 10 years. When he was 13-years-old, he presented with a right winged scapula. In addition, he reported bilateral leg weakness since he was 24 years old. Physical examination revealed a transverse smile; winged scapula; “Popeye” arm appearance; facial weakness; and asymmetric, proximal dominant arm and leg weakness (right side more severe than the left). Serum creatinine kinase levels were mildly elevated at 319 unit/L. His father and elder brother had similar symptoms (Fig. 1A). His elder brother had been visiting another hospital for similar symptoms since he was 10 years old, and his muscle pathology showed severe chronic myopathic changes (marked increased size variability, hypertrophic fibers, degenerating and atrophic fibers, endomysial fibrosis and mild fatty infiltration, disruption of intermyofibrillar network, and lobulated fibers) with normal dystrophin immunoreactivity. The elder brother was diagnosed with FSHD based on clinical phenotype, but refused a genetic study. However, our patient agreed to undergo genetic analysis for diagnosis. Therefore, we performed conventional Southern blot analysis with EcoRI and BlnI enzymes, but the D4Z4 repeat size was 12, which is normal. Next, whole exome sequencing revealed a novel frameshift mutation (c.3801delG, p.Glu1267Aspfs*5) in SMCHD1 [Fig. 1B, upper (patient) and lower (mother) panels], a mutation that has not been described in previous reports or public databases (ExAC, ESP6500, 1000 genome). Finally, methylation-sensitive Southern blot analysis with FseI showed mildly reduced D4Z4 repeat size on 4q (41kb/39kb) and reduced D4Z4 methylation ratio (14.6%). Accordingly, we could report our case 1 as “pathogenic” and carrying a novel variant, as indicated in ACMG-AMP guidelines.⁴

Fig. 1. Family pedigree and validation data of case 1. (A) Pedigree. The patient's father and elder brother showed similar symptoms. Arrow: patient. (B) Validation. Upper, patient; Lower, patient's mother.

Case 2

A 38-year-old male visited our clinic for asymmetric upper extremity weakness. The patient reported that, at the age of nine, he had difficulty performing iron bar gymnastics. Since his third decade, he experienced prominent right shoulder and arm weakness, and he presented with facial asymmetry and weakness. Physical examination showed a transverse smile, positive Beevor's sign, and winged scapula (Fig. 2B). In addition, he showed facial and bilateral arm and leg weakness with proximal dominance and asymmetricity (right side more severe than the left) on neurological examination. Serum creatinine kinase levels were mildly elevated at 224 unit/L. His mother and elder sister had similar symptoms (Fig. 2A). We performed genetic analysis for FSHD. Southern blotting analysis by the EcoRI/BlnI digestion method using genomic DNA revealed that a fragment size of 37 kb, and the number of D4Z4 repeats was nine. Bisulfite sequencing to determine the haplotype of D4Z4 using digestion with HindIII and hybridized with 4qA probe and target sequencing analysis to screen for mutations in the SMCHD1 gene by a next-generation sequencer (Ion PGMTM) were performed for FSHD2 diagnosis. We found reduced DNA methylation ratio (16.8%), D4Z4 haplotype 4qA (homozygous), and c.1580C>T (p.T527M) SMCHD1 mutation, which was previously reported to be a pathogenic allele.⁵

Fig. 2. Family pedigree and winged scapula of case 2. (A) Pedigree. The patient's mother and elder sister showed similar symptoms. Arrow: patient. (B) Winged scapula.

Informed consent was obtained from the two patients regarding the reporting and publication of this case report.

DISCUSSION

We report the first two cases of FSHD2 in Korea. Since 2008, there have been some reports in patients with FSHD1. In Korea, all studies of FSHD have been performed in patients with confirmation of D4Z4 contraction.⁶,⁷,⁸ Those studies reported the following; 1) facial muscles are initially affected, and an asymmetric face and shoulder girdle weakness, winging scapular, transverse smile, and Beevor's sign are important findings;⁷ 2) most Korean FSHD1 patients (102/108 patients, 94.4%) carry one to six units of D4Z4 repeat;⁶,⁸ 3) female are more severely affected than male, even though there are no differences in age at examination or in D4Z4 copy number between the two sexes.⁸

In case 1, the patient and his brother showed the typical FSHD phenotype; however, a conventional Southern blot study showed low to normal D4Z4 repeat size. Re-analysis of the D4Z4 repeat size showed mild contraction (nine repeats). In case 2, Southern blot analysis showed nine D4Z4 repeats. Previous Korean and Japanese reports of patients with FSHD,⁸,⁹ revealed that most East Asian FSHD1 patients have one to six D4Z4 repeats, although four of 13 FSHD2 patients in Japan had nine to 10 D4Z4 repeats.⁹ The two present cases had nine homozygous repeats or 9/10 repeats of D4Z4. Considering previously reported genetic profiles of East Asian FSHD1 and FSHD2 cases, FSHD2 is a more likely diagnosis than FSHD1+2 or FSHD1 for diagnosis in our cases. SMCHD1 mutation has been reported as modifier of disease severity in FSHD families,⁵ although through some case series and studies, researchers have suggested that mildly contracted D4Z4 repeats with SMCHD1 mutation may be important for the development.⁹

The mutation in case 1 was a frameshift mutation causing a truncated protein, and we confirmed that DNA methylation was decreased. Lemmers, et al.² previously reported 45 mutations in 51 of 60 families with FSHD2. Two families from the USA and South Korea showed a splicing site mutation (c.3801+1G>A, exon 29 skipping) near the site of the mutations, similar to the present cases, but a different type of mutation.

Mild dysphagia in early stages was observed in some patients (3 of the 13 patients) with SMCHD1 mutations in a Japanese study,⁹ and they did not find any other clinical findings of FSHD2. Dysphagia is commonly presented in FSHD at late advanced stages.¹ Our patients did not have any subjective swallowing difficulty. Unfortunately, we did not perform objective evaluation for dysphagia, and thus we do not know whether they had “silent aspiration” without any cough or outward signs of aspiration. Dysphagia could be a distinctive feature of FSHD2, but needs additional study.

We report the first confirmed cases of FSHD2 due to novel frameshift mutation and reported point mutation in Korea with detailed clinical manifestation. We suggest genetic analysis of the D4Z4 haplotype and methylation as important diagnostic steps for accurate diagnosis of FSHD, and whole exome sequencing is another option in Korea.