BICD2 and autosomal dominant childhood-onset proximal spinal muscular atrophy with contractures: Interestingly, heterozygous missense variants in human BICD2 cause autosomal dominant lower extremity-predominant spinal muscular atrophy 2 (SMALED2; MIM # 615299), which presents a loss of spinal motor neurons, muscle weakness, and atrophy predominantly of the lower limbs [31, 34, 35].