First Identification of Compound Heterozygous FKRP Mutations in a Korean Patient with Limb-Girdle Muscular Dystrophy.

Abstract

Dystroglycanopathies are a genetically and clinically heterogeneous group of muscular dystrophies that are characterized by reduced glycosylation of α-dystroglycan. Among 18 causative genes, the fukutin-related protein (FKRP; MIM 606596) gene is the most common cause in Caucasians.1 However, FKRP mutations have rarely been found in the Asian population,2 and have not been reported previously in Korea. Herein we report novel compound FKRP mutations in a patient with myopathy, which represents the first reported case in Korea. A 43-year-old female (II-4) (Fig. 1A) was the fourth child of nonconsanguineous healthy Korean parents who achieved normal motor milestones after birth. She first noticed difficulty in climbing stairs at age 7 years, but muscle weakness did not progress. At the age of 23 years the patient perceived the progression of muscle weakness and was admitted to our institute. She displayed proximal weakness, especially in the pelvic girdle muscles. Mild winged scapulae and lordosis were also observed. No sensory deficits, joint contracture, tongue hypertrophy, or mental retardation were apparent. Her serum creatine kinase level was markedly elevated, at 2,439 IU/L (reference value: <185 IU/L). An electrophysiological study revealed a generalized myogenic process. No cardiac abnormality was observed. At the last examination, when she was 43 years old, she could walk with assistance. Fig. 1 Pedigree, sequencing chromatograms, and conservation profile of the patient. A: Pedigree of a Korean patient with compound heterozygous FKRP mutations. The filled symbol indicates the affected member; open symbols indicate unaffected members. B: Sequencing ... The genetic cause of this patient's myopathy was evaluated by performing targeted sequencing of 69 myopathy-causative genes including FKRP (Supplementary Information, Supplementary Table 1, 2, and 3 in the online-only Data Supplement). DNA fragments in the target regions were enriched by solution-based hybridization capture, followed by sequencing with an Illumina Hiseq2000 platform. The sequencing data were analyzed using a standard pipeline. Variants were then filtered further based on the patient's phenotype. Compound heterozygous FKRP mutations of c.857G>C and c.1170_1171delGC were identified (Fig. 1B). The c.1170_1171delGC mutation has been reported previously in a Japanese patient with congenital muscular dystrophy.2 However, the c.857G>C mutation was novel, and was not detected in 352 healthy controls, dbSNP138, or the 1000 Genomes Database (September 2014 release). In silico analysis using SIFT and PolyPhen2 predicted this mutation to be disease-causing. Genomic evolutionary rate profiling (GERP) indicated that the affected nucleotide is highly conserved (score=4.16), as became apparent upon inspection of the amino acid sequence in the altered region of the protein (Fig. 1C). Thus, the compound heterozygous FKRP mutations of c.1170_1171delGC and c.857G>C were determined to be the underlying cause of the myopathy in this patient. FKRP mutations result in various clinical presentations from severe brain abnormality to mild limb-girdle muscular dystrophy.1 This variability makes it difficult to diagnose FKRP-related muscular dystrophy based on clinical findings alone. Therefore, a step-by-step, classical approach with assessment of muscle pathology, muscle immunoanalysis, and mutational analysis is needed for the diagnosis of FKRP-related muscular dystrophy. The findings of a recent study in Taiwan that employed this approach suggested that FKRP-related muscular dystrophy is underdiagnosed, and not rare in the Asian population.3 However, this classical approach often fails to identify the causative gene due to pathological variability and the limited power of muscle immunoanalysis. Advances in next-generation sequencing, including targeted sequencing, have enabled the rapid and cost-effective analysis of many causative genes in uncategorized muscular dystrophy.4 In the present case, compound heterozygous FKRP mutations were identified using such next-generation sequencing. In conclusion, novel compound heterozygous FKRP mutations were identified in a patient with limb-girdle muscular dystrophy; this is the first reported case in Korea.