POMK mutation in a family with congenital muscular dystrophy with merosin deficiency, hypomyelination, mild hearing deficit and intellectual disability

Background: Congenital muscular dystrophies (CMD) with hypoglycosylation of α-dystroglycan are clinically and genetically heterogeneous disorders that are often associated with brain malformations and eye defects. Presently, 16 proteins are known whose dysfunction impedes glycosylation of α-dystroglycan and leads to secondary dystroglycanopathy.

Objective: To identify the cause of CMD with secondary merosin deficiency, hypomyelination and intellectual disability in two siblings from a consanguineous family.

Methods: Autozygosity mapping followed by whole exome sequencing and immunochemistry were used to discover and verify a new genetic defect in two siblings with CMD.

Results: We identified a homozygous missense mutation (c.325C>T, p.Q109*) in protein O-mannosyl kinase (POMK) that encodes a glycosylation-specific kinase (SGK196) required for function of the dystroglycan complex. The protein was absent from skeletal muscle and skin fibroblasts of the patients. In patient muscle, β-dystroglycan was normally expressed at the sarcolemma, while α-dystroglycan failed to do so. Further, we detected co-localisation of POMK with desmin at the costameres in healthy muscle, and a substantial loss of desmin from the patient muscle.

Conclusions: Homozygous truncating mutations in POMK lead to CMD with secondary merosin deficiency, hypomyelination and intellectual disability. Loss of desmin suggests that failure of proper α-dystroglycan glycosylation impedes the binding to extracellular matrix proteins and also affects the Cytoskeleton.

Clinical genetics; Genome-wide; Molecular genetics; Muscle disease.