Novel BICD2 mutation in a Japanese family with autosomal dominant lower extremity-predominant spinal muscular atrophy-2

Introduction

The most common form of spinal muscular atrophy (SMA) is a recessive disorder caused by SMN1 mutations in 5q13, whereas the genetic etiologies of non-5q SMA are very heterogenous and largely remain to be elucidated. We present a father and son with atrophy and weakness of the lower leg muscles since infancy. Genetic studies in this family revealed a novel BICD2 mutation causing autosomal dominant lower extremity-predominant SMA type 2.

Patients

The proband was the father, aged 30, and the son was aged 3. Both of them were born uneventfully to nonconsanguineous parents. While the father first walked at the age of 19?months, the son was unable to walk at age 3?years. In both, knee and ankle reflexes were absent and sensation was intact. Serum creatine kinase levels were normal. The son showed congenital arthrogryposis and underwent orthopedic corrections for talipes calcaneovalgus. Investigation of the father at the age of 5?years revealed normal results on nerve conduction studies and sural nerve biopsy. Electromyography showed chronic neurogenic change, and muscle biopsy showed features suggestive of denervation. The father was diagnosed clinically with a sporadic distal SMA. Follow-up studies showed very slow progression.

Investigations and results

Next-generation and Sanger sequencing revealed a deleterious mutation in BICD2: c.1667A>G, p.Tyr556Cys, in this family.

Discussion

BICD2 is a cytoplasmic conserved motor-adaptor protein involved in anterograde and retrograde transport along the microtubules. Next-generation sequencing will further clarify the genetic basis of non-5q SMA.