MFN2 Hereditary Motor and Sensory Neuropathy

Clinical characteristics: MFN2 hereditary motor and sensory neuropathy (MFN2-HMSN) is a classic axonal peripheral sensorimotor neuropathy, inherited in either an autosomal dominant (AD) manner (~90%) or an autosomal recessive (AR) manner (~10%). MFN2-HMSN is characterized by more severe involvement of the lower extremities than the upper extremities, distal upper-extremity involvement as the neuropathy progresses, more prominent motor deficits than sensory deficits, and normal (>42 m/s) or only slightly decreased nerve conduction velocities (NCVs). Postural tremor is common. Median onset is age 12 years in the AD form and age eight years in the AR form. The prevalence of optic atrophy is approximately 7% in the AD form and approximately 20% in the AR form.

Diagnosis/testing: Molecular genetic testing establishes the diagnosis of MFN2-HMSN in 90% of probands with suggestive findings by identifying a heterozygous MFN2 pathogenic variant and in 10% of probands with suggestive findings by identifying biallelic MFN2 pathogenic variants.

Management: Treatment of manifestations: Neuropathy is often managed by a multidisciplinary team that includes a neurologist, a physiatrist, an orthopedic surgeon, and physical and occupational therapists. Symptomatic treatment relies on special shoes and/or ankle/foot orthoses to correct foot drop and aid walking; surgery as needed for severe pes cavus; forearm crutches, canes, wheelchairs as needed for mobility; exercise as tolerated; acetaminophen or nonsteroidal anti-inflammatory agents for musculoskeletal pain; treatment of neuropathic pain with tricyclic antidepressants or drugs such as carbamazepine or gabapentin. Optic atrophy is managed with low vision aids as per a low vision clinic, consultation with community vision services, and career/employment counseling.

Surveillance: Routine evaluation by: a neurologist to assess disease progression; physical therapy to assess gross motor skills including gait and strength; occupational therapy to assess fine motor skills and coping strategies; and ophthalmologist and low vision clinic to assess visual acuity and need for modification of low vision aids, respectively.

Agents/circumstances to avoid: Obesity (which makes ambulation more difficult); medications (e.g., vincristine, isoniazid, nitrofurantoin) known to cause nerve damage; alcohol and malnutrition (which can cause or exacerbate neuropathy).

Genetic counseling: Approximately 90% of MFN2-HMSN is inherited an autosomal dominant (AD) manner, and approximately 10% is inherited in an autosomal recessive (AR) manner. Semi-dominant inheritance (i.e., an MFN2 pathogenic variant is associated with mild disease in the heterozygous state and more severe disease in the homozygous or compound heterozygous state) has been reported in two families.

1. AD MFN2-HMSN. Most affected individuals have an affected parent; the proportion of individuals with a de novo MFN2 pathogenic variant is unknown. Each child of an affected individual has a 50% chance of inheriting the MFN2 pathogenic variant.

2. AR MFN2-HMSN. At conception, each sib of an individual with autosomal recessive MFN2-HMSN has a 25% chance of being affected, a 50% chance of being an asymptomatic heterozygote (i.e., carrier), and a 25% chance of being unaffected and not a carrier.

Once the MFN2 pathogenic variant(s) have been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing for MFN2-HMSN are possible.