Absence of Dystrophin Related Protein-2 disrupts Cajal bands in a patient with Charcot-Marie-Tooth disease

Abstract

Using exome sequencing in an individual with Charcot-Marie-Tooth disease (CMT) we have identified a mutation in the X-linked dystrophin-related protein 2 (DRP2) gene. A 60-year-old gentleman presented to our clinic and underwent clinical, electrophysiological and skin biopsy studies. The patient had clinical features of a length dependent sensorimotor neuropathy with an age of onset of 50 years. Neurophysiology revealed prolonged latencies with intermediate conduction velocities but no conduction block or temporal dispersion. A panel of 23 disease causing genes was sequenced and ultimately was uninformative. Whole exome sequencing revealed a stop mutation in DRP2, c.805C>T (Q269*). DRP2 interacts with periaxin and dystroglycan to form the periaxin-DRP2-dystroglycan complex which plays a role in the maintenance of the well-characterized Cajal bands of myelinating Schwann cells. Skin biopsies from our patient revealed a lack of DRP2 in myelinated dermal nerves by immunofluorescence. Furthermore electron microscopy failed to identify Cajal bands in the patient's dermal myelinated axons in keeping with ultrastructural pathology seen in the Drp2 knockout mouse. Both the electrophysiologic and dermal nerve twig pathology support the interpretation that this patient's DRP2 mutation causes characteristic morphological abnormalities recapitulating the Drp2 knockout model and potentially represents a novel genetic cause of CMT.

Keywords: Charcot–Marie–Tooth disease; Hereditary motor and sensory neuropathy; Myelin; Nerve conduction studies; Whole exome sequencing.

Figure 1. Clinical features and family pedigree

A. Wasting of anterior shin in the proband B. Wasting of calf muscles in the proband C. Pedigree structure for the reported patient with DRP2 mutation. The proband is indicated by the arrow. Individual generations are numbered with Roman numerals on the left. Open symbols represent unaffected individuals. Filled circles = CMT females. Filled squares= CMT males *= individual with DRP2 stop mutation. Semi-circles= females with hammertoes. Semi-squares=hammertoes. Individual I.3 had an abnormal gait in addition to hammertoes. Individual II.5 has a diagnosis of peripheral neuropathy.

Figure 2. Dermal nerve fibres of the proband lack DRP2

Immunofluorescence staining and electron microscopy of the patient and healthy controls. Panels A-F show immunofluorescent images from a healthy control (A-C) and the patient (D-F) illustrating neurofilament staining the axon in green, myelin basic protein revealing the overlying myelin sheath in blue and DRP2 located at the abaxonal surface of the myelin sheath in red. No discernible DRP2 expression was demonstrated in our patient. G & H, electron micrographs of transverse sections of dermal axons in control (G) and patient (F). Cajal bands were easily demonstrated in the control patient and are marked with asterisks in red. No distinct Cajal bands were observed in the patient with most myelinated axons demonstrating a circumferential band of Schwann cell cytoplasm surrounding the abaxonal myelin. In addition axonal inclusions are visible in the patient. I= representative diagram of a myelinated axon and the periaxin-DRP2-dystroglycan (PDG) complex.