Missing genetic variations in GNE myopathy: rearrangement hotspots encompassing 5′ UTR and founder allele

GNE myopathy is an autosomal recessive distal myopathy caused by loss-of-function mutations in the GNE gene, which encodes UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE), a key enzyme in sialic-acid biosynthesis. By comprehensive screening of manifesting patients using a fine-mapped targeted next-generation sequencing (NGS), we identified copy number variations (CNVs) in 13 patients from 11 unrelated families. The nine unique CNVs largely vary in size from 0.3 to 72 kb. Over half of the cases carry different deletions spanning merely exon 2, which contains the 5' untranslated region (5' UTR) of the muscle major transcript hGNE1. Of most unique CNVs, either the telomeric or the centromeric breakpoint locates within intron 2, indicating rearrangement hotspots. Haplotype analysis suggested the existence of a founder allele with exon 2 deletion. The breakpoints for all CNVs were determined by long-range PCR and sequencing. All of the breakpoints of gross deletion/duplications reside within directly oriented pairs of Alu repeats. The results of this study firstly widen the spectra of mutations to CNVs encompassing 5' UTR, underscoring the pivotal role of the hGNE1 transcript. Alu-mediated non-recurrent CNVs may have been overlooked in a wide variety of recessive phenotypes, especially in those associated with genomic Alu-rich genes such as GNE.