Overview
Distal hereditary motor neuropathy type 5 (dHMN5) is a rare genetic disorder characterized by progressive muscle weakness and atrophy primarily affecting the distal muscles of the limbs, particularly the lower extremities. The clinical presentation often overlaps with other neuromuscular disorders such as axonal Charcot–Marie–Tooth disease (CMT2), juvenile-onset amyotrophic lateral sclerosis (ALS), hereditary spastic paraplegia (HSP), and spinocerebellar ataxias (SCA), making accurate diagnosis challenging. Over 30 genes have been implicated in various forms of hereditary motor neuropathies, yet genetic diagnosis remains incomplete, with causative variants identified in only 20.0–47.8% of affected patients. Understanding the molecular mechanisms underlying dHMN5, particularly through genetic analysis, is crucial for advancing diagnostic approaches and potentially developing targeted therapies.
Pathophysiology
The pathophysiology of dHMN5 involves complex genetic mutations that disrupt normal neuromuscular function. One notable example is the identification of a de novo variant c.1112T>A (p.Ile371Lys) within the C2B domain of the SYT2 gene, which plays a critical role in neurotransmitter release at the neuromuscular junction [PMID:33105646]. This mutation leads to impaired neurotransmitter release due to mixed oligomer formation with wild-type synaptotagmin, highlighting the importance of synaptotagmin function in maintaining proper synaptic transmission. Such disruptions can result in the characteristic distal muscle weakness observed in dHMN5 patients. This molecular insight aligns with broader findings that genetic mutations affecting synaptic proteins contribute significantly to the pathogenesis of hereditary motor neuropathies [PMID:26786006]. Genetic studies continue to elucidate these mechanisms, offering potential avenues for therapeutic interventions targeting specific molecular pathways.
Epidemiology
The epidemiology of dHMN5 reflects its rarity and genetic heterogeneity. Over 30 genes have been associated with various forms of distal hereditary motor neuropathies, yet the identification of causative variants remains elusive in a significant proportion of cases (20.0–47.8%) [PMID:35297556]. This underscores the complexity and challenges in genetic diagnosis. Despite this, sporadic reports from diverse populations, such as the identification of dHMN in two unrelated families in Wales, suggest a broader genetic distribution and potential underdiagnosis [PMID:26786006]. These findings highlight the need for increased awareness and comprehensive genetic testing in clinical practice to capture the full spectrum of genetic variability associated with dHMN5.
Clinical Presentation
The clinical presentation of dHMN5 typically manifests as a slowly progressive weakness predominantly affecting the distal muscles of the lower limbs, which can gradually extend to involve proximal leg muscles and even the intrinsic hand muscles [PMID:35297556]. Characteristic features include foot deformities, such as pes cavus or hammertoes, and fatigable weakness that may also impact ocular muscles, leading to symptoms like ptosis or diplopia. The variability in clinical presentation underscores the importance of recognizing atypical manifestations. For instance, a mother and daughter with dHMN5 initially presented with vocal cord dysfunction and singing difficulties, illustrating the potential for non-motor symptoms that can precede or accompany more typical motor deficits [PMID:26786006]. These diverse presentations emphasize the necessity for a thorough clinical evaluation to capture the full range of symptoms associated with dHMN5.
Diagnosis
Diagnosing dHMN5 accurately requires a multifaceted approach due to its clinical overlap with other neuromuscular disorders such as axonal CMT2, juvenile-onset ALS, and HSP [PMID:35297556]. Genetic studies are indispensable in distinguishing dHMN5 from these conditions. Advanced genetic techniques, including clinical exome sequencing, have proven particularly valuable. For example, exome sequencing successfully identified a novel de novo variant in the SYT2 gene, demonstrating its superiority over traditional gene panels in diagnosing rare genetic causes [PMID:33105646]. Electrophysiological studies, such as electromyography (EMG), play a crucial role in confirming neuromuscular junction dysfunction and supporting the clinical diagnosis by revealing characteristic patterns of denervation and muscle fiber recruitment abnormalities [PMID:33105646]. Additionally, techniques like triple-primed polymerase chain reaction (TP-PCR) have been employed to investigate STR variants in genes like NOTCH2NLC and those associated with SCA, particularly in patients who remain genetically negative through conventional panels [PMID:35297556]. Genetic testing, particularly focusing on genes like SLC5A7, where common truncating mutations have been identified, further solidifies the diagnosis and aids in distinguishing dHMN5 from other neuropathies [PMID:26786006].
Management
Currently, there are no specific curative treatments for dHMN5, and management primarily focuses on supportive care to alleviate symptoms and improve quality of life. Physical therapy is essential for maintaining muscle strength and mobility, addressing gait abnormalities, and managing foot deformities. Orthotic devices, such as ankle-foot orthoses (AFOs), can help compensate for foot drop and improve walking ability. Occupational therapy may be beneficial in adapting daily activities to accommodate motor impairments. Pain management strategies, including pharmacological interventions, are crucial for patients experiencing neuropathic pain. Multidisciplinary care involving neurologists, physiatrists, physical therapists, and orthopedic specialists is recommended to tailor interventions to individual patient needs. While symptomatic relief is achievable, ongoing research into targeted therapies based on genetic insights holds promise for future treatment advancements.
Key Recommendations
References
1 Wu C, Xiang H, Chen R, Zheng Y, Zhu M, Chen S et al.. Genetic spectrum in a cohort of patients with distal hereditary motor neuropathy. Annals of clinical and translational neurology 2022. link 2 Mironovich O, Dadali E, Malmberg S, Markova T, Ryzhkova O, Poliakov A. Identification of a Novel de Novo Variant in the SYT2 Gene Causing a Rare Type of Distal Hereditary Motor Neuropathy. Genes 2020. link 3 Ingram G, Barwick KE, Hartley L, McEntagart M, Crosby AH, Llewelyn G et al.. Distal hereditary motor neuropathy with vocal cord paresis: from difficulty in choral singing to a molecular genetic diagnosis. Practical neurology 2016. link