Identification of a GNE homozygous mutation in a Han‐Chinese family with GNE myopathy

Abstract GNE myopathy is a rare, recessively inherited, early adult‐onset myopathy, characterized by distal and proximal muscle degeneration which often spares the quadriceps. It is caused by mutations in the UDP‐N‐acetylglucosamine 2‐epimerase/N‐acetylmannosamine kinase gene (GNE). This study aimed to identify the disease‐causing mutation in a three‐generation Han‐Chinese family with members who have been diagnosed with myopathy. A homozygous missense mutation, c.1627G>A (p.V543M) in the GNE gene co‐segregates with the myopathy present in this family. A GNE myopathy diagnosis is evidenced by characteristic clinical manifestations, rimmed vacuoles in muscle biopsies and the presence of biallelic GNE mutations. This finding broadens the GNE gene mutation spectrum and extends the GNE myopathy phenotype spectrum.


| INTRODUCTION
GNE myopathy, also referred to as Nonoaka myopathy, was first described as distal myopathy having rimmed vacuoles (DMRV) in 1981 in Japanese patients, later called "hereditary inclusion body myopathy (HIBM)" or "inclusion body myopathy 2 (IBM2)". It is a rare, recessively inherited, early adult-onset myopathy. 1 It is typically characterized by slow progression that preferentially affects the tibialis anterior muscles, commonly spares the quadriceps femoris muscles and gradually spreads to other muscles. 2,3 GNE myopathy occurs worldwide. The prevalence ranges from 1/1 000 000 to 21/ 1 000 000, although it may be underestimated as many patients escape diagnosis. 1,4 It has a higher prevalence in Middle Eastern Jews and Japanese. GNE myopathy is caused by the mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase gene (GNE), which encodes the bifunctional enzyme catalysing the first two rate-limiting steps in sialic acid biosynthesis. 5 In this study, targeted exome capture and high-throughput sequencing of 411 known genes involved in neuromuscular disorders were performed to identify genetic causes of an autosomal recessive GNE myopathy in a three-generation Han-Chinese pedigree. 6 A homozygous missense mutation, c.1627G>A (p.V543M), in the GNE gene was found to co-segregate with the GNE myopathy present in this family.

| Variant analysis and direct Sanger sequencing
Genomic DNA (gDNA) was isolated from peripheral blood using the phenol-chloroform extraction method. 7 Table 1.

| GNE mutation screening
A prioritization scheme similar to those described in recent studies was used to identify proband pathogenic variants. 13  and MutationTaster, c.1627G>A (p.V543M) in the GNE gene is deemed "severe", that is protein damaging. The predicted scores and results of functional prediction software programs appear in Table 2.

| DISCUSSION
The GNE gene, mapped to chromosome 9p13.3, spans 62.6 kb and contains 13 exons. 1  Non-allosteric, homozygous or compound heterozygous, primarily missense mutations in the GNE gene result in reduced GNE epimerase and kinase enzymatic activity. This leads to decreased sialic acid production, which appears to be the main contributor to this still elusive disease pathology. 1  In mice, GNE protein expresses at an early embryonic stage and Gne −/− mice is lethal. 4,24 This is consistent with the lack of biallelic null mutations and only "mildly deleterious" mutations reported in GNE myopathy patients. 4 Given that Gne-deficient hyposialylation is the core pathogenic factor, experimental prophylactic treatments with N-acetylmannosamine or sialic acid metabolites were performed, and effectively avoiding muscle weakness and atrophy in Gne-deficient mice models were found. [25][26][27][28]  parents are from the same geographical region.

| CONCLUSIONS
In summary, a homozygous missense mutation c.1627G>A

ACKNOWLEDG EMENTS
We thank the participating members and investigators for their cooperation and efforts in collecting clinical and genetic information and DNA specimens.

COMPLI ANCE WI TH ETHICAL S TANDARDS
Informed consent was obtained from the individuals. The study received approval from the Ethics Committee of the Third Xiangya Hospital, Central South University, Changsha, Hunan, China.

CONFLI CT OF INTEREST
The authors declare that they have no conflict of interest.