Distal hereditary motor neuropathies

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Distal hereditary motor neuropathies (distal HMN, dHMN) are a genetically and clinically heterogeneous group of motor neuron diseases that result from genetic mutations in various genes and are characterized by degeneration and loss of motor neuron cells in the anterior horn of the spinal cord and subsequent muscle atrophy.

Even though they can hardly be distinguished from hereditary motor and sensory neuropathies on the clinical level, dHMNs are a considered a separate class of disorders.

Classification

In 1993, A. E. Hardnig proposed to classify hereditary motor neuropathies into seven groups based on age at onset, mode of inheritance, and presence of additional features. This initial classification has since been widely adopted and expanded and currently looks as follows:

HMN type Common/alternate names	OMIM	Gene	Locus	Inheritance	Characteristics
HMN1	182960	?	?	Autosomal dominant	Juvenile-onset
HMN2A Autosomal dominant distal spinal muscular atrophy	158590	HSPB8	12q24.23	Autosomal dominant	Adult-onset. Allelic with Charcot–Marie–Tooth disease type 2F
HMN2B	608634	HSPB1	7q11.23	Autosomal dominant	Adult-onset
HMN2C	613376	HSPB3	5q11.2	Autosomal dominant
HMN3, HMN4 Distal spinal muscular atrophy type 3	607088	?	11q13	Autosomal recessive	HMN3 – benign form: early adult onset, slowly progressive, no diaphragmatic paralysis HMN4 – severe form: juvenile-onset with diaphragmatic paralysis
HMN5A Distal spinal muscular atrophy type 5A	600794	GARS	7p14.3	Autosomal dominant	Upper limb predominance with spasticity of lower limbs Locus and phenotype overlapping with CMT-2D and SPG-17
HMN5B Distal spinal muscular atrophy type 5B	614751	REEP1	2p11.2	Autosomal dominant	Locus and phenotype overlapping with SPG-31
HMN6 Distal spinal muscular atrophy type 1 Spinal muscular atrophy with respiratory distress type 1	604320	IGHMBP2	11q13.2–q13.4	Autosomal recessive	Infant-onset, severe, with diaphragmatic failure
HMN7A Spinal muscular atrophy with vocal cord paralysis Harper–Young myopathy	158580	?	2q14.2	Autosomal dominant	Adult-onset with vocal cord paralysis
HMN7B	607641	DCTN1	2p13.1	Autosomal dominant	Adult-onset with vocal cord paralysis and facial weakness
HMN-J Distal spinal muscular atrophy type 2 Jerash type spinal muscular atrophy	605726	HMNJ	9p21.1–p12	Autosomal recessive	Juvenile-onset with pyramidal features

See also

• Charcot–Marie–Tooth disease
• Hereditary motor and sensory neuropathies
• Hereditary spastic paraplegia
• Motor neuron disease
• Spinal muscular atrophies