| Entity/domain | Curated finding | Evidence type/strength | Ontology/database annotation |
|---|---|---|---|
| Disease identity | Charcot-Marie-Tooth disease type 2B2 (CMT2B2; ARCMT2B) is a very rare autosomal-recessive, predominantly axonal sensorimotor neuropathy with mild cerebellar/oculomotor involvement in the reported Costa Rican cases. | Strong for the reported kindreds; limited generalizability because evidence derives from one extended family and five additional Costa Rican subjects (pqac-00000000, pqac-00000001, pqac-00000002) | OMIM: **605589**; MONDO: **MONDO:0011570** supplied by requester—verify against the current MONDO release before ingestion; candidate class: hereditary motor and sensory neuropathy |
| Gene attribution | The best-supported cause is **biallelic PNKP loss-of-function/hypomorphic variation**. The earlier **MED25 p.Ala335Val** attribution was superseded after reanalysis found segregating biallelic PNKP variants; MED25–CMT2B2 validity is classified **Disputed** by the ClinGen CMT Gene Curation Expert Panel. | Strong human segregation/reanalysis evidence for PNKP; contradictory/insufficient evidence for MED25 (pqac-00000000, pqac-00000001) | PNKP; candidate relationship: biallelic loss of function → CMT2B2. MED25 should be retained only as a historical/disputed attribution |
| Homozygous genotype | The large consanguineous Costa Rican family carried **PNKP c.1549C>T (p.Gln517Ter; p.Gln517\*) homozygously**. The premature stop lies in the last exon and truncates the conserved C-terminal region. | Strong segregation evidence in the discovery pedigree; pathogenicity supported by predicted structural destabilization, although direct allele-specific functional testing is limited (pqac-00000000, pqac-00000001) | Variant class: germline nonsense; candidate ACMG concepts: PVS1/segregation/phenotype specificity, subject to transcript-specific curation |
| Compound-heterozygous genotype | Five unrelated Costa Rican CMT2 subjects were reported with **PNKP c.1549C>T (p.Gln517Ter)** in trans with **c.1221_1223del (p.Thr408del)**. | Strong case-level and segregation evidence across five additional subjects; p.Thr408del had also been associated with recessive ataxia (pqac-00000000, pqac-00000002) | Variant classes: germline nonsense plus in-frame deletion; verify HGVS against the designated clinical transcript before database loading |
| Peripheral neuropathy phenotype | Adult-onset, slowly progressive, length-dependent **axonal sensorimotor polyneuropathy** produced gait disturbance, falls, distal weakness and wasting, impaired/absent reflexes, and marked vibration/position-sense loss. Onset in the original family was approximately **28–42 years**; additional cases generally began in the third decade. | Strong but small human case-series evidence; severe disability was variable, including wheelchair dependence in one subject after five years (pqac-00000000, pqac-00000001, pqac-00000002) | Candidate HPO: adult onset; progressive peripheral neuropathy; axonal sensorimotor polyneuropathy; distal muscle weakness/atrophy; areflexia; impaired proprioception; gait disturbance |
| Electrophysiology | Nerve-conduction studies showed markedly reduced or absent sensory responses and reduced compound muscle action potentials, with secondary slowing of motor conduction in severe axonal disease. | Strong objective clinical evidence in reported patients (pqac-00000001, pqac-00000002) | Candidate HPO: abnormal peripheral nerve conduction; reduced CMAP amplitude; absent sensory nerve action potentials; axonal degeneration |
| Cerebellar and oculomotor signs | Slurred speech/dysarthria, wide-based or ataxic gait, postural instability, and variable oculomotor abnormalities—including oculomotor apraxia in some subjects—accompanied the neuropathy. MRI could show mild cerebellar atrophy without brainstem or white-matter abnormalities. | Moderate-to-strong case-series evidence; expression was variable (pqac-00000001, pqac-00000002) | Candidate HPO: cerebellar ataxia; dysarthria; oculomotor apraxia; abnormality of ocular movements; cerebellar atrophy |
| Features absent in reported CMT2B2 cohort | The Costa Rican CMT2B2 subjects lacked congenital microcephaly, seizures, developmental delay, cognitive impairment, and dystonia, distinguishing their presentation from classic MCSZ. | Strong for examined subjects, but absence cannot be generalized to every future PNKP-neuropathy case (pqac-00000000, pqac-00000002) | Negated candidate HPO: microcephaly; seizure; global developmental delay; cognitive impairment; dystonia |
| PNKP protein function | PNKP is a DNA-end-processing enzyme with an N-terminal FHA interaction domain and catalytic **3′-phosphatase** and **5′-kinase** activities. It converts obstructive termini into ligatable **3′-OH/5′-phosphate** ends. | Strong biochemical, structural, and cellular evidence (pqac-00000007, pqac-00000008, pqac-00000013) | Candidate GO: DNA 3′-phosphatase activity; polynucleotide 5′-hydroxyl-kinase activity; DNA repair; DNA single-strand-break repair; nonhomologous end joining |
| Single-strand-break repair mechanism | Oxidative or TOP1-associated damage generates noncanonical DNA ends. PARP1/PARP2 and the XRCC1 scaffold organize repair; PNKP processes 3′-phosphate and 5′-OH termini, POLB fills gaps, and LIG3 seals the strand. XRCC1 interaction stimulates PNKP recruitment and catalytic turnover. | Strong pathway-level biochemical/cellular evidence; exact recruitment order may vary by lesion and remains partly unresolved (pqac-00000008, pqac-00000009, pqac-00000013) | Candidate GO: base-excision repair; DNA single-strand-break repair; response to oxidative stress. Candidate compartments: nucleus, chromatin, DNA-repair complex |
| Downstream pathophysiology | Reduced PNKP function is expected to cause persistent strand-break termini, impaired repair, transcriptional/genomic stress, and preferential dysfunction or loss of long-lived peripheral and cerebellar neurons. The final link from the two Costa Rican alleles to selective axonal degeneration remains **inferred**, not directly demonstrated in patient neurons. | Strong general PNKP/DNA-repair evidence but moderate, inferential disease-specific chain (pqac-00000001, pqac-00000010, pqac-00000013) | Candidate GO: cellular response to DNA damage stimulus; maintenance of genome stability; neuron death. Candidate CL: peripheral sensory neuron, lower motor neuron, cerebellar Purkinje cell |
| Allelic relationship to AOA4 | **AOA4 (OMIM 616267)** is another biallelic PNKP disorder, typically emphasizing progressive cerebellar ataxia, oculomotor apraxia, and axonal polyneuropathy. CMT2B2 occupies the neuropathy-predominant, adult-onset end of an overlapping spectrum. | Strong human allelic-spectrum evidence; strict genotype–phenotype boundaries are not established (pqac-00000000, pqac-00000003, pqac-00000005) | Candidate disease relationship: allelic disorder; candidate HPO: cerebellar ataxia, oculomotor apraxia, axonal neuropathy |
| Allelic relationship to MCSZ | **MCSZ (OMIM 613402)** usually presents with congenital microcephaly, early seizures, developmental delay, and progressive cerebellar atrophy; later neuropathy/ataxia may occur. Published cohorts support a continuum from severe neurodevelopmental disease to adult neurodegeneration. | Strong human evidence for a phenotypic continuum; simple residual-activity rules remain incomplete (pqac-00000003, pqac-00000004, pqac-00000005) | Candidate disease relationship: allelic disorder; candidate HPO: congenital microcephaly, early-onset seizure, global developmental delay, cerebellar atrophy |
| Diagnostics | Diagnosis requires neurological examination, nerve-conduction studies/EMG, assessment for ataxia and eye-movement abnormalities, brain MRI when central signs are present, and molecular confirmation of **biallelic PNKP variants** by neuropathy/ataxia panel, exome, or genome sequencing with segregation analysis. | Clinically well supported, but no CMT2B2-specific consensus criteria or validated biochemical diagnostic assay exist (pqac-00000000, pqac-00000001, pqac-00000002) | Candidate NCIT interventions: genetic testing, whole-exome sequencing, whole-genome sequencing, electromyography, nerve-conduction study, magnetic-resonance imaging |
| Treatment status | No approved disease-modifying, gene, RNA, or PNKP-replacement therapy and no relevant CMT2B2-specific interventional trial were identified. Current care is supportive: physical/occupational therapy, orthoses and mobility aids, fall prevention, pain management, speech support, and orthopedic evaluation for deformity. | Supportive practice extrapolated from general CMT/ataxia management; no CMT2B2 treatment-response rates | Candidate NCIT interventions: physical therapy, occupational therapy, orthotic device, assistive device, pain management, speech therapy, orthopedic surgery |
| Epidemiology and evidence gaps | Disease-specific prevalence, incidence, carrier frequency, sex ratio, penetrance, life expectancy, quantitative natural history, modifiers, protective factors, pharmacogenomics, biomarkers, and quality-of-life statistics are unavailable. No exact p.Gln517Ter/p.Thr408del neuronal model or disease-specific single-cell, spatial, or multi-omics dataset was identified. | Major limitation: extremely small, geographically concentrated ascertainment; broader PNKP cohorts cannot substitute for CMT2B2-specific estimates (pqac-00000000, pqac-00000003, pqac-00000005, pqac-00000006) | Mark as unknown/not established; avoid assigning population frequencies or phenotype percentages beyond the published cohort |


*Table: Compact knowledge-base table distinguishing PNKP-related CMT2B2 from the disputed MED25 attribution and summarizing its variants, phenotype, mechanism, allelic spectrum, diagnostics, treatment status, and evidence gaps.*