Founder Mutation c.1516A>C in KLHL40 Is a Frequent Cause of Nemaline Myopathy With Hyponatremia in Ethnic Chinese

KLHL40-related nemaline myopathy is a severe autosomal recessive muscle disorder. The current study describes 4 cases of KLHL40-related nemaline myopathy in Hong Kong ethnic Chinese presenting within 3 years, which are confirmed with clinicopathologic features and genetic studies. The incidence is estimated to be at least 1 in 45 226 livebirths (at least 1 in 41 608 among ethnic Chinese livebirths) in Hong Kong. Hyponatremia appears to be another common feature in these patients. Salient histological features include nemaline bodies ranging from 200 to 500 nm in diameters on ultrastructural examination as well as negative KLHL40 immunohistochemistry; type II fiber predominance is obvious in 2 cases. We demonstrate the founder effect associated with genetic variant c.1516A>C (p.Thr506Pro) by polymorphic marker analysis, which revealed a 0.56-0.75-Mb or 0.41-0.78-cM shared haplotype encompassing the disease allele. The mutation is believed to have occurred around 412 generations ago or 6220 BCE, as estimated using DMLE+ and a formula described by Boehnke. We believe the founder variant might possibly underlie a sizable portion of nemaline myopathy in ethnic Chinese. Analysis of the KLHL40 gene may be considered as the first-tier testing of congenital myopathy in this ethnic group.