[A family with nonsynonymous mutations of filamin C actin-binding domain]

We report a sibling case of progressive muscle weakness beginning in their 40s. The younger brother was initially suspected to have spinal muscular atrophy based on electromyography findings, but no SMN1 gene deletion was identified. Whole-exome sequencing in the siblings detected a heterozygous nonsynonymous mutation, c.577G>A (p.Ala193Thr), located in the actin-binding domain (ABD) of the filamin C (FLNC) gene. Muscle weakness of a sibling started in the proximal lower limbs and gradually progressed to the distal lower limbs, trunk, and upper limbs. Muscle MRI showed marked involvement of the medial and posterior thigh muscles, gastrocnemius, and soleus muscles, with relative sparing of the muscles of rectus femoris, gracilis, and tibialis anterior. These findings suggest a characteristic pattern of muscle involvement associated with this FLNC-ABD mutation.