| Knowledge-base field | Charcot-Marie-Tooth disease type 2T summary |
|---|---|
| Identity / identifiers | **Disease:** Charcot-Marie-Tooth disease type 2T (CMT2T); **MONDO:** MONDO:0044640; **OMIM:** 617017 reported in disease history/literature context as the recessive MME-related axonal CMT entity; **causal gene:** **MME** (membrane metalloendopeptidase; neprilysin), **Ensembl:** ENSG00000196549. Disease-level information here is derived from aggregated literature cohorts and curated disease-gene resources, not individual EHR datasets. (pqac-00000000, pqac-00000002, pqac-00000017) |
| Definition | Late-onset inherited peripheral neuropathy within the axonal CMT spectrum caused **definitively by biallelic loss-of-function or other pathogenic MME variants**, producing a predominantly length-dependent motor greater than sensory axonal neuropathy affecting lower limbs first. Monoallelic/heterozygous MME-associated neuropathy remains a separate **uncertain** category with incomplete penetrance or risk-factor status rather than definitive CMT2T. (pqac-00000002, pqac-00000003, pqac-00000017) |
| Inheritance | **Definitive CMT2T:** autosomal recessive, supported by segregation of **biallelic** MME variants in Japanese, Spanish, Italian, and Chinese families. **Monoallelic MME:** not sufficient for definitive disease assignment in the strongest early cohorts; later European/North American/Italian data support possible age-dependent penetrance or neuropathy risk, but causality is unresolved. (pqac-00000002, pqac-00000003, pqac-00000006, pqac-00000010, pqac-00000017) |
| Onset / temporal development | Typically **adult/late onset**, usually **5th-6th decade**. Japanese report: late-onset axonal neuropathy across 10 patients. Spanish cohort: onset **35-73 years**, median **44**. Italian 2024 cohort: onset **24-75 years**, mean **54.7**, median **55**; biallelic onset earlier than monoallelic after exclusion of likely benign variants (**49.6 vs 61.4 years, p=0.0099** in study comparison groups). Course is **slowly progressive**, chronic, lifelong; biallelic cases appear more likely to worsen more rapidly after >10 years disease duration. Suggested HPO mapping: *Adult onset* / *Late onset* / *Progressive peripheral neuropathy* (suggested mappings). (pqac-00000002, pqac-00000008, pqac-00000009, pqac-00000010, pqac-00000017, pqac-00000018) |
| Core phenotype (clinical) | Typical features: distal lower-limb weakness and atrophy, gait impairment/steppage gait, foot muscle wasting, reduced/absent ankle reflexes, later distal upper-limb involvement, mild-to-moderate sensory loss, paresthesia/tingling, neuropathic pain in some patients, cramps and contractures in some cohorts. Italian 2024: motor-onset in **53%**, sensory in **25%**, mixed in **22%**; **69%** had moderate-severe lower-limb weakness; **91%** abnormal lower-limb reflexes; about **2/3** mild-moderate touch loss; **62%** reduced pinprick; **63%** reduced light touch. Suggested HPO terms: distal muscle weakness, lower-limb amyotrophy, steppage gait/abnormal gait, foot drop, areflexia/hyporeflexia, distal sensory impairment, paresthesia, neuropathic pain, muscle cramps, pes cavus if present (suggested mappings; exact IDs not asserted). (pqac-00000002, pqac-00000003, pqac-00000008, pqac-00000009, pqac-00000018) |
| Electrophysiology / pathology | Usually **axonal sensorimotor neuropathy** with upper-limb MNCV typically in the axonal/intermediate range and markedly reduced distal CMAPs, especially in legs; lower-extremity responses may be absent in advanced cases. Italian 2024: **78%** had axonal motor-sensory polyneuropathy; upper-limb MNCV **31-68 m/s**, mean **52**. Spanish cohort noted chronic neurogenic denervation and peripheral nerve hyperexcitability in a subset. Japanese sural biopsies showed marked loss of large myelinated fibers, thin myelin sheaths, and fiber clustering, without onion bulbs or inflammation; NEP immunostaining was absent or reduced. Suggested ontology mappings: axonal neuropathy, chronic denervation on EMG, reduced CMAP, absent SNAP/CMAP in distal legs, sural nerve fiber loss (suggested mappings). (pqac-00000001, pqac-00000003, pqac-00000006, pqac-00000020) |
| Genetics / pathogenic variants | Disease gene is **MME**. Reported pathogenic variant classes include nonsense, frameshift, canonical splice, in-frame deletion, and missense. Foundational Japanese examples include **c.661C>T (p.Gln221*)**, **c.1231_1233delTGT (p.Cys411del)**, **c.1861T>C (p.Cys621Arg)**, splice variants including **c.654+1G>A** and **c.439-2T>A**. Chinese 2024 report: **c.2122A>T (p.K708*)** homozygous pathogenic variant; and **c.1342C>T (p.R448*)** plus **c.2071_2072delinsTT (p.A691L)** compound heterozygous genotype. Italian 2024 classified variants by ACMG as **8 pathogenic, 13 likely pathogenic, 10 VUS, 1 likely benign** and provided non-Finnish European frequencies for several alleles, with many loss-of-function alleles absent or ultra-rare in gnomAD NFE. Germline origin is implied; no somatic role established. (pqac-00000001, pqac-00000005, pqac-00000015, pqac-00000016, pqac-00000022, pqac-00000023) |
| Mechanism / pathophysiology | Best-supported mechanism is **MME/neprilysin loss of function** in the peripheral nervous system, probably disrupting neuropeptide homeostasis and axon-Schwann cell biology, leading to a length-dependent axonopathy. Japanese study showed absent or markedly reduced NEP protein in sural nerve and higher expression in myelin sheaths than axons, supporting relevance to Schwann-cell/myelin-associated compartments despite primarily axonal clinical disease. Italian 2024 concluded many variants are predicted to trigger **nonsense-mediated decay (NMD)**; missense effects may alter local H-bonds/steric interactions. Mechanistic certainty remains moderate because no disease-specific transcriptomic/proteomic/metabolomic signature has been established. Suggested GO terms: peptide catabolic process, metallopeptidase activity, neuron projection maintenance, axon ensheathment/axon-Schwann cell interaction, peripheral nervous system development (suggested mappings). (pqac-00000001, pqac-00000004, pqac-00000007, pqac-00000024) |
| Anatomy / cell types / subcellular localization | **Primary structures:** peripheral nerves, especially long nerves to lower limbs; sural nerve is pathologically documented. **Tissues/cells:** peripheral axons and myelinating Schwann cells are the main implicated cell populations. **Subcellular/protein localization:** NEP is a cell-surface zinc-dependent metalloprotease and exists predominantly as a homodimer on the cell surface. Suggested anatomical/cell ontology mappings: peripheral nerve, sural nerve, lower limb musculature, axon, myelinating Schwann cell; suggested GO cellular component: plasma membrane/cell surface. (pqac-00000001, pqac-00000016, pqac-00000017) |
| Diagnosis | Real-world diagnosis relies on clinical pattern recognition of late-onset length-dependent axonal neuropathy, **NCS/EMG**, exclusion of acquired neuropathy causes, and **molecular confirmation of MME variants** by targeted neuropathy panel, Sanger confirmation, or broader NGS approaches. Italian cohort used referral-center routine CMT diagnostics plus laboratory exclusion workup; Chinese 2024 used targeted neuromuscular NGS. Biopsy is supportive but not required when genetics is definitive. Suggested differential diagnoses: other late-onset hereditary axonal neuropathies, distal hereditary motor neuropathy, RFC1-related CANVAS/sensory neuropathy, acquired axonal neuropathies, and CIDP mimics when electrophysiology is atypical. Broader CMT evidence supports targeted NGS panels first, with WES/WGS for unsolved cases. (pqac-00000016, pqac-00000018, pqac-00000013, pqac-00000014) |
| Management | **No MME-specific approved therapy.** Current care is symptomatic/supportive as for CMT: physical therapy, occupational therapy, exercise/strength and balance work, orthoses, fall prevention, pain/cramp management, and orthopedic/foot interventions when indicated. Evidence for these interventions is largely **CMT-general**, not CMT2T-specific. No dedicated CMT2T interventional trial was identified in the provided evidence. (pqac-00000012, pqac-00000013, pqac-00000014) |
| Prognosis / outcomes | Available data suggest **mild-to-moderate but progressive disability**, predominantly affecting ambulation and lower-limb function. Italian 2024 mean **CMTES 10** (range **1-21**); disease severity correlates more with duration, and biallelic disease appears more severe after >10 years. CNS involvement appears minimal: no obvious cognitive impairment in major cohorts, and Japanese patients lacked clear Alzheimer/dementia evidence. Life expectancy, disease-specific mortality, and robust survival statistics are **not established** for CMT2T. (pqac-00000004, pqac-00000009, pqac-00000020) |
| Epidemiology / population | CMT2T is **rare** and no population prevalence/incidence specific to CMT2T was established in the provided evidence. Foundational evidence suggests MME was a relatively frequent cause of **adult-onset autosomal recessive CMT2 in Japan**; Spanish authors also found biallelic MME among their more common late-onset recessive axonal neuropathy diagnoses. Italian 2024 assembled **32 patients** from four centers, supporting recognition across Europe. Sex distribution in the Italian cohort was near-even (**18 male, 14 female**). No validated carrier-frequency estimate for disease-causing biallelic CMT2T was identified here. (pqac-00000002, pqac-00000003, pqac-00000009, pqac-00000018) |
| Recent 2024 evidence | **Italian cohort (2024):** largest disease-focused update in provided evidence, 32 patients; refined phenotype, ACMG variant classes, onset distribution, and stronger evidence that biallelic cases progress faster with longer duration. **Chinese case report (2024):** added novel homozygous **p.K708*** and further support for late-onset recessive MME-related dHMN/CMT2 overlap with prominent motor axonal impairment and slight sensory involvement. These are the most relevant 2024 disease-specific developments in the supplied sources. (pqac-00000009, pqac-00000010, pqac-00000015, pqac-00000016) |
| Monoallelic MME-associated neuropathy status | **Not equivalent to definitive CMT2T.** Strong earlier cohorts found healthy heterozygous relatives older than affected recessive cases and concluded heterozygous variants did not segregate convincingly with disease. Later studies propose heterozygous variants may be incompletely penetrant or act as risk factors, possibly requiring a second genetic/environmental hit. Knowledge-base curation should therefore separate: **(1) definitive biallelic recessive CMT2T** from **(2) uncertain monoallelic MME-associated late-onset axonal neuropathy/risk state**. (pqac-00000003, pqac-00000007, pqac-00000010, pqac-00000017) |
| Prevention / counseling | No primary prevention exists for the neuropathy itself. Recommended measures are **genetic counseling**, family segregation testing, carrier testing in relatives when a familial pathogenic variant is known, and reproductive counseling. Accurate distinction between biallelic disease and heterozygous uncertain findings is especially important for counseling. (pqac-00000003, pqac-00000012) |
| Evidence gaps | Disease-specific gaps include: uncertain **OMIM/MONDO cross-mapping verification beyond provided contexts**; limited prevalence/incidence data; no established penetrance estimates; no validated disease-specific biomarkers; no transcriptomic/proteomic/metabolomic signature; no proven modifier genes or gene-environment interactions; no established renal/CNS complication frequency; no MME-targeted therapy or CMT2T-specific trial; limited animal/model-organism evidence that recapitulates peripheral neuropathy despite NEP biology. (pqac-00000004, pqac-00000007, pqac-00000010, pqac-00000024) |


*Table: This compact table summarizes the current evidence base for Charcot-Marie-Tooth disease type 2T, emphasizing definitive biallelic MME-related recessive disease versus uncertain monoallelic MME-associated neuropathy. It is designed to support structured curation of identifiers, phenotype, mechanism, diagnosis, management, prognosis, and evidence gaps.*