Split-Foot Malformation with Mesoaxial Polydactyly: disease-characteristics report

Split-foot malformation with mesoaxial polydactyly (SFMMP) is an exceptionally rare, congenital, autosomal-recessive disorder caused by biallelic variants affecting the sterile alpha motif (SAM) domain of the long MAP3K20/ZAK protein isoform. Its defining clinical combination is variable split-foot malformation, abnormal fingernails, and—in one reported family—bilateral sensorineural hearing loss. The original molecular study described five affected people in two families; consequently, most apparent phenotype percentages are descriptions of those five people, not reliable population estimates. PMID: 26755636.[46][101]

The original abstract states: “Here we describe an autosomal recessive human disease in two unrelated families characterized by a split-foot defect, nail abnormalities of the hands, and hearing loss, due to mutations disrupting the SAM domain of the protein kinase ZAK.” PMID: 26755636; published online January 11, 2016.[46]

1. Disease information and identifiers

SFMMP is a developmental limb-patterning disorder, not an acquired or progressive degeneration of an initially normal foot. “Split foot” describes a cleft appearance caused by deficiency of central foot structures; “mesoaxial polydactyly” refers to an extra, centrally positioned toe. Neither finding alone establishes this molecular diagnosis. The 2023 international skeletal-disorder nosology places SFMMP in its split-hand/foot group as NOS 39-0100, with recessive ZAK inheritance. PMIDs: 26755636, 36779427.[90][196][272][278]

Evidence provenance: This is an aggregated disease-level entry derived principally from published, individually described patients and family studies, supplemented by curated OMIM, Orphanet, HPO, and ClinVar records. It is not an EHR-derived population cohort. ClinVar additionally contains submitted individual-variant observations and literature-derived assertions; those should not be mistaken for independent disease cohorts.[46][106][113][web:EX7zcFJZkZRVQY5N1HGJM6Hx]

2. Etiology, risks, protection, and gene–environment interaction

Established cause: Homozygosity for either MAP3K20 c.1103T>G, p.(Phe368Cys) or an approximately 14.7-kb intragenic deletion removing exons 12–16 was identified in the two original families. Both lesions affect the SAM-containing long isoform, ZAKα. The missense allele segregated with disease in the Pakistani family, was absent from 180 Pakistani controls and then-available public databases, and affected a structurally important SAM residue. The deletion was homozygous in an affected Tunisian boy and heterozygous in his unaffected first-cousin parents. PMID: 26755636.[46][web:64dmrzKYw3F8wQLZlMe7VIVX][web:hd5SRXfhsDlQUuWJemN2EHyg]

Established risk factor: Having two disease-causing alleles. Consanguinity occurred in both reported families and increases the chance that relatives inherit the same rare allele; it is not itself a separate molecular cause. Family history is therefore clinically useful. Neither variant has an established founder prevalence, carrier frequency, or population-specific risk estimate. No sex-dependent susceptibility, environmental toxin, infection, parental-age effect, protective allele, protective diet, or demonstrated gene–environment interaction has been established for SFMMP. General knowledge about teratogens or ribotoxic stress should not be entered as an SFMMP-specific exposure association. PMID: 26755636.[46][web:64dmrzKYw3F8wQLZlMe7VIVX]

3. Phenotypes

Denominators matter. The initial report included four affected relatives in family 1 and one affected boy in family 2. Three of four affected family-1 members had a split foot—two bilateral and one unilateral—while the fourth, despite being homozygous, did not. The Tunisian boy had bilateral involvement. All four affected family-1 members had bilateral sensorineural hearing impairment; the boy passed newborn hearing screening and had normal reported language development at age two, not a documented comprehensive lifetime exclusion of hearing loss. These yield descriptive counts of 4/5 with split foot and 4/5 with reported hearing impairment, without justifying precise penetrance estimates. PMID: 26755636.[46][web:64dmrzKYw3F8wQLZlMe7VIVX][web:dbKj8JkWy7gE7ebDykFfDuZX]

The table distinguishes directly reported findings from additional Orphanet-curated HPO associations. Curated frequency bands should not be converted into numerical percentages from five cases.[web:hwHbfWnxra0B7zX87KwzsLuY][web:pwBpm7Vg3MOVPYeEasOb1gKg]

Not established as SFMMP features: behavioral or psychiatric changes, a characteristic blood/urine chemistry abnormality, and the broad skin, dental, craniosynostosis, and eye phenotype described in a different, heterozygous MAP3K20-associated disorder. A reported Tunisian child’s psychomotor and language development were normal at age two. PMIDs: 26755636, 38451290.[web:dbKj8JkWy7gE7ebDykFfDuZX][web:ZJvgk4cm9d9MxYGCAItMHt2j]

4. Genetic and molecular information

Causal gene: MAP3K20 (HGNC:17797; NCBI Gene:51776; OMIM:609479), historically ZAK, at 2q31.1. ZAKα contains an N-terminal kinase domain, leucine zipper, and C-terminal SAM domain; the shorter ZAKβ lacks that SAM domain. PMIDs: 26755636, 27816943.[95][91][46][web:Scoq7tGuP1xNmfWOudXb3Z3b]

Important 2025 mechanistic refinement: The original protein-folding findings do not prove simple loss of kinase activity. In a later cell-based, in-vitro/structural study, disease-associated p.Phe368Cys showed constitutive ZAK activation even without induced ribosome collisions or normal ribosome interaction. The authors did not show that this activity causes the human limb phenotype in vivo. Thus, annotate “disrupted SAM structure; constitutive kinase activation demonstrated in an experimental cellular system; developmental consequence unresolved,” rather than an unqualified “kinase loss of function.” The abstract describes “a known pathogenic variant of the SAM domain” among mutants that can bypass the ribosome requirement. PMID: 41261136; online November 19, 2025.[web:q7a1jL9GGAgGvNRUfGEUIrwd]

Do not conflate MAP3K20 disorders. A 2024 primary report found five people with de novo heterozygous linker-region variants and a different spectrum involving craniosynostosis, ectodermal findings, hearing loss, and limb anomalies. Its abstract specifies “five individuals” with “heterozygous de novo variants in the linker region between the kinase domain and leucine zipper domain.” Separately, biallelic truncating, kinase-domain alleles cause MAP3K20-related centronuclear myopathy, with muscle weakness rather than the characteristic SFMMP foot defect. PMIDs: 38451290, 27816943.[web:ZJvgk4cm9d9MxYGCAItMHt2j][web:Scoq7tGuP1xNmfWOudXb3Z3b]

Modifiers and other genomic findings: The original linkage analysis found a 9.1-Mb chromosome-2 interval, maximum LOD 3.5. A later patient with split-hand/foot malformation had homozygous ZAK p.Ala505Ser and an approximately 2.01-Mb 7q21.3–q22.1 deletion encompassing established SHFM-related genes; its authors favored the deletion as the principal cause and proposed p.Ala505Ser only as a possible modifier. Do not register that deletion or p.Ala505Ser as established SFMMP-causing alleles or an established SFMMP modifier. No SFMMP-specific methylation signature, pathogenic aneuploidy, translocation, or inversion has been established. PMIDs: 26755636, 32266845.[web:EOAnxlclgg6vjWe7CeaEheqH][web:jQuU0ZAeiLluYP1hDlcbWWz7]

5. Environmental information

No specific toxin, radiation exposure, pollution, occupation, smoking pattern, diet, exercise pattern, alcohol exposure, or infectious agent is implicated as a cause or reproducible modifier of this Mendelian syndrome. Chemical induction of ribosome collisions in a laboratory is an experimental tool, not evidence that an exposed pregnancy will develop SFMMP. There is no infectious transmission or zoonotic risk. PMIDs: 26755636, 41261136.[46][web:q7a1jL9GGAgGvNRUfGEUIrwd]

6. Mechanism and pathophysiology

Ordered causal chain

1. Inherited biallelic SAM-region MAP3K20 lesions lead to an altered long ZAKα isoform during embryonic development; the observed missense variant and exon deletion perturb this domain in different ways. Human genetics and protein assays demonstrated; PMID: 26755636.[web:EOAnxlclgg6vjWe7CeaEheqH]
2. SAM disruption leads to impaired normal SAM structural behavior: p.Phe368Cys destabilizes the isolated domain, while exon 12–16 deletion removes its encoded region. Demonstrated or predicted according to allele; PMID: 26755636.[web:EOAnxlclgg6vjWe7CeaEheqH]
3. The missense branch: p.Phe368Cys leads to constitutive ZAK activation in transfected experimental cells, bypassing the usual requirement for ribosome-collision sensing. Demonstrated in vitro; whether this occurs in affected embryonic limb cells is inferred, not shown. PMID: 41261136.[web:q7a1jL9GGAgGvNRUfGEUIrwd]
4. The deletion-model branch: mouse SAM-domain deletion leads to abnormal hindlimb patterning and approximately 60% lower Trp63 transcript abundance in homozygous hindlimbs. Demonstrated in the model; whether reduced TP63 is the obligatory mediator in human SFMMP remains inferred. PMID: 26755636.[web:dbKj8JkWy7gE7ebDykFfDuZX]
5. Altered developmental ZAK–TP63-associated signaling is proposed to lead to disturbed distal limb-bud organization and digit specification, resulting in split foot, variable toe duplication/syndactyly, and nail anomalies. The proposed intermediate tissue and signaling links have not been directly demonstrated in affected human embryos. PMID: 26755636.[web:EOAnxlclgg6vjWe7CeaEheqH]
6. In a separate, unresolved branch, altered ZAK function is associated with sensorineural hearing impairment in one human family; the precise auditory cell injury or developmental mechanism is unproven. Mouse Zak expression in cochlear hair cells provides context, but the original SAM-deletion mice did not have a detectable abnormal Preyer reflex. PMID: 26755636.[web:dbKj8JkWy7gE7ebDykFfDuZX]

Pathway detail and evidence boundaries: ZAK is a MAP kinase kinase kinase. A 2025 primary biochemical/cryo-electron-microscopy study showed that ribosome collisions position ZAK on neighboring RACK1 proteins, promote SAM-domain dimerization and activation, and engage downstream p38/JNK stress signaling; SERBP1 opposes inappropriate activation. That work explains how p.Phe368Cys can be hyperactive experimentally, not whether ordinary embryonic ribosome collisions cause SFMMP. The 2016 mouse experiments found normal Fgf8, Shh, and Trp63 spatial staining patterns in a tested context, whereas quantitative testing found reduced Trp63 amount in SAM-mutant hindlimbs. A WNT, FGF8, or SHH defect should therefore not be recorded as a directly established SFMMP molecular abnormality. PMIDs: 26755636, 41261136.[web:dbKj8JkWy7gE7ebDykFfDuZX][web:q7a1jL9GGAgGvNRUfGEUIrwd]

Ontology-ready mechanism annotations: Suggested biological-process terms are protein phosphorylation (GO:0006468), embryonic limb morphogenesis (GO:0030326), and limb development (GO:0060173); these are annotation suggestions, not claims that each process was individually measured in patients. The experimental subcellular context includes cytoplasm and ribosome; the structural target is a protein SAM domain, not a demonstrated mitochondrial, lysosomal, or metabolic defect. Suggested relevant cells are mesenchymal cell (CL:0000134), chondrocyte (CL:0000138), and keratinocyte (CL:0000312), with limb-bud ectoderm and auditory hair cells named anatomically; involvement of each particular cell class in human SFMMP has not been independently proven. No disease-specific single-cell, spatial-transcriptomic, proteomic, metabolomic, lipidomic, or validated epigenomic signature was identified in the reviewed evidence. The decisive functional-genomics application was CRISPR/Cas editing of mouse Zak. PMIDs: 26755636, 41261136.[46][web:q7a1jL9GGAgGvNRUfGEUIrwd][210][227]

7. Anatomical structures affected

Primary sites: distal lower limbs, especially the foot (UBERON:0002387), toes, phalanges/metatarsals and their developmental precursors; fingernail units are also affected. An apical ectodermal ridge (UBERON:0004356) is a useful developmental-context annotation, not a directly sampled lesion in a patient. Secondary/associated site: the auditory system and ear (UBERON:0001690); a cochlear cellular mechanism remains unconfirmed. Relevant tissue classes include developing ectoderm/epithelium, mesenchyme, and developing connective/skeletal tissues. Involvement may be unilateral or bilateral and asymmetric; one genetically affected person had no split foot. Do not assign the mouse knockout’s severe embryonic cardiac phenotype as an established organ manifestation of surviving humans with SFMMP. PMID: 26755636.[46][202][280]

Suggested cellular-component annotation: cytoplasm and ribosome for the experimentally studied ZAK stress-response machinery; not a patient-specific subcellular pathology diagnosis. PMID: 41261136.[web:q7a1jL9GGAgGvNRUfGEUIrwd]

8. Temporal development

The malformations are congenital; Orphanet specifies neonatal onset. The defensible natural-history description is persistent structural differences with variable severity, rather than an acute, episodic, staged, or relapsing disease. Neither a progression rate nor formal early/intermediate/end stages, spontaneous anatomical remission, or long-term audiometric trajectory has been established. Embryonic limb patterning is the biologically relevant vulnerability period; after birth, the clinically actionable period is early assessment of hearing and function. The Tunisian child’s normal hearing screen and development at two years must not be extrapolated to lifelong outcome. PMID: 26755636.[2][web:dbKj8JkWy7gE7ebDykFfDuZX]

9. Inheritance and population

Autosomal recessive. When both biological parents carry the same established pathogenic allele, the standard Mendelian risk per pregnancy is 25% affected, 50% carrier, and 25% inheriting neither familial allele; counseling should confirm the actual familial genotypes. Variable expressivity is directly observed: one homozygous affected relative lacked a split foot, while another had unilateral involvement. That observation alone cannot yield a population penetrance percentage. No anticipation, germline mosaicism rate, founder effect, or carrier frequency has been established. PMID: 26755636.[46][web:64dmrzKYw3F8wQLZlMe7VIVX]

Orphanet’s estimate is prevalence <1 per 1,000,000, not a measured national incidence. Annual incidence, population age distribution, geographic prevalence, and population sex ratio are unknown. The original cases were from families in Pakistan and Tunisia; their locations do not establish ethnic restriction or an endemic region. A later comparison table records three males and one female in the original Pakistani family and one male in the Tunisian family, but 4:1 is a five-person report count, not a disease sex ratio. PMIDs: 26755636, 38451290.[2][web:ZJvgk4cm9d9MxYGCAItMHt2j]

10. Diagnostics and screening

Clinical suspicion: congenital split/atypical feet, central toe duplication or syndactyly, characteristic fingernails, and potentially bilateral sensorineural hearing loss, especially with similarly affected relatives or consanguinity. Examine all four limbs and nails, obtain a pedigree, and arrange foot radiographs to define skeletal anatomy when management requires it. Obtain age-appropriate formal audiology even if an initial newborn screen was passed, because the molecular syndrome can include hearing loss and a normal screen is not a complete lifelong audiological evaluation. Foot imaging and audiology are phenotype-directed clinical recommendations, not validated SFMMP-specific diagnostic criteria. PMID: 26755636.[46][web:dbKj8JkWy7gE7ebDykFfDuZX][76]

Molecular confirmation: seek biallelic pathogenic/likely pathogenic MAP3K20 variants, considering SNVs and intragenic exon deletions. An appropriate limb-malformation/SHFM panel with deletion–duplication analysis, exome sequencing plus reliable CNV calling, or genome sequencing can be selected according to the phenotype and laboratory capabilities. Single-gene sequencing is reasonable when a familial allele is known, but sequencing alone may miss the documented exon 12–16 deletion; use validated copy-number or breakpoint assessment. The original study used linkage, exome sequencing, array-CGH, qPCR, breakpoint PCR, and segregation analysis; GTR lists tests associated with MAP3K20 and this condition. PMID: 26755636.[web:EOAnxlclgg6vjWe7CeaEheqH][web:RjRxYfl6gV7WymF9gF9gk2y6]

Differential diagnosis: other molecular forms of split-hand/foot malformation, including 7q21/DLX5–DLX6 abnormalities and other SHFM loci; heterozygous MAP3K20 linker-associated disease, particularly if craniosynostosis or broad ectodermal findings predominate; and biallelic kinase-domain MAP3K20 myopathy, if generalized weakness, hypotonia, or abnormal muscle biopsy dominates. These are distinct molecular diagnoses despite overlap in gene or limb appearance. PMIDs: 32266845, 38451290, 27816943.[web:jQuU0ZAeiLluYP1hDlcbWWz7][web:ZJvgk4cm9d9MxYGCAItMHt2j][web:Scoq7tGuP1xNmfWOudXb3Z3b]

Tests without an established routine SFMMP role: characteristic blood/urine biomarkers, enzyme assays, biopsy, ECG/EEG/EMG, mitochondrial sequencing, repeat-expansion testing, liquid biopsy, and clinical metabolomic/proteomic/epigenomic assays. Chromosomal microarray or cytogenetic testing can be useful for a broader unresolved differential diagnosis, not because the two established SFMMP alleles are aneuploidies. There are no demonstrated SFMMP-specific formal diagnostic-score criteria or population newborn molecular-screening program. PMIDs: 26755636, 32266845.[web:EOAnxlclgg6vjWe7CeaEheqH][web:jQuU0ZAeiLluYP1hDlcbWWz7]

11. Outcome and prognosis

Life expectancy, 5-/10-year survival, mortality rates, disease-specific deaths, EQ-5D/SF-36 scores, and validated prognostic biomarkers are unavailable for SFMMP. No progressive multiorgan failure has been established. The main plausible long-term burdens are those of persistent foot anatomy and potentially hearing impairment; their individual severity, footwear needs, and hearing trajectory require assessment, not prediction from genotype alone. The single reported two-year-old Tunisian child had normal psychomotor and language development at that examination. There are no reliable genotype-specific response rates or natural-history cohorts. PMID: 26755636.[46][web:dbKj8JkWy7gE7ebDykFfDuZX]

12. Treatment and implementation

There is no established medication or molecular treatment that reverses the congenital SFMMP lesion. Management is individualized and function-oriented; the table separates general clinical practice or evidence from other cleft-foot cases from outcomes demonstrated in genetically confirmed SFMMP.[46][76][256]

Not established for this disorder: disease-modifying pharmacotherapy or pharmacogenomic dosing rules; gene, cell, RNA, immune, or targeted therapy; a validated genotype-guided treatment algorithm; an SFMMP-specific interventional trial or NCT identifier; treatment response rates or disease-specific adverse-event rates. In particular, experimental ZAK inhibition used to interrogate a cellular p.Phe368Cys mechanism is not an indicated treatment for a congenital limb malformation. PMID: 41261136.[web:q7a1jL9GGAgGvNRUfGEUIrwd]

13. Prevention

Primary prevention of the underlying allele is not possible through vaccination, diet, or avoidance of a known SFMMP-specific exposure. For a family with established variants, genetic counseling, targeted carrier testing, and discussion of prenatal or preimplantation genetic testing can support reproductive decisions; these are options, not evidence of environmental prevention or a population screening recommendation. Secondary prevention means identifying an affected child’s hearing or functional needs early; tertiary prevention means managing footwear, mobility, and communication-related consequences. Routine immunizations remain appropriate but do not prevent SFMMP. PMID: 26755636.[46][76]

14. Other species and naturally occurring disease

Human: Homo sapiens, NCBI Taxon:9606; mouse: Mus musculus, NCBI Taxon:10090, ortholog Map3k20, NCBI Gene:65964; MGI:2443258. Other species have MAP3K20 orthologs, but an authenticated, naturally occurring veterinary equivalent caused by the human SFMMP SAM-domain alleles was not established in the reviewed evidence. Naturally occurring polydactyly in an animal should not be equated with this molecular syndrome. Cross-species conservation supports experimental comparison; there is no zoonotic transmission. PMID: 26755636.[121][126][128][130][134]

15. Model organisms and experimental systems

Knowledge-base interpretation: Record the human biallelic genotype–phenotype association as established; record the mouse SAM-deletion/Trp63 result and cellular p.Phe368Cys hyperactivity as different, complementary mechanistic evidence types. The exact link between those results—and the cause of hearing impairment—remains unresolved. PMIDs: 26755636, 41261136.[46][web:q7a1jL9GGAgGvNRUfGEUIrwd]

Reference Validation

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References that may not be about this subject

These identifiers resolve, so they are not fabrications, but the records they resolve to share almost none of this report's vocabulary. That is a clue and not a verdict - a paper can be relevant in ways its title and abstract do not spell out - so read them before deciding:

• PMC:PMC11805609 (1 mention) - Fun in the sun: ribosomes defend against UV irradiation.
• shared terms: none

Weighed against this report's own most characteristic terms: web, established, sfmmp, hearing, foot, zak, affected, family, molecular, normal, variant, allele, phenotype, dbkj8jkwy7ge7ebdykffduzx, loss, map3k20, limb, sfmmp-specific, disease, variable.

All extracted references resolved successfully. Resolving is not the same as being relevant, though - see the references listed above as possibly off topic.

Term Validation

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Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA, Gene, OMIM, Taxon, MGI.

25 of 33 terms resolved to a current term; the rest could not be looked up either way.