Limb and Digit Patterning Serial Homology Module

Module limb_digit_patterning_serial_homology 2 disorders Pathograph 3

A conserved developmental-patterning module that explains why a single patterning lesion produces a phenotype "bundle" repeated across serially homologous skeletal elements — most strikingly the autopod, where fingers and toes are patterned by the same molecular machinery and therefore tend to be malformed together. The anteroposterior axis of the limb is set by a gradient of GLI3 transcriptional repressor, generated by PKA-dependent processing of GLI3 and antagonized by long-range Sonic hedgehog (SHH) signaling from the posterior zone of polarizing activity; Indian hedgehog (IHH), the HOXD gene cluster, FGF8 from the apical ectodermal ridge, and WNT signaling provide the other patterning inputs that specify digit number, identity, and segment length. A dosage or activity change in any of these conserved signals perturbs digit-number and digit-identity specification, and because the homologous program operates in fore- and hindlimb autopods, the resulting malformation — polydactyly, syndactyly, brachydactyly, ectrodactyly, or triphalangism — characteristically appears in both hands and feet. Greig cephalopolysyndactyly and Pallister-Hall syndrome (GLI3 dosage), brachydactyly type A1 (IHH), and split-hand/foot malformation (HOXD/AER) differ in the upstream signal but converge on this serially homologous autopod phenotype. Conforming disorder entries declare conformance via conforms_to, substituting the disorder-specific patterning gene (GLI3, IHH, SHH/ZRS, HOXD13, TP63, WNT10B) while preserving the perturbed-patterning-to-serially-homologous-malformation chain.

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Pathophysiology Nodes

3 shared nodes are defined in this module.

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Cell Types

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Biological Processes

Notes

This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "limb_digit_patterning_serial_homology#Serially Homologous Autopod Malformation"). The biological motivation is serial homology: the autopod patterning program is reused across fore- and hindlimb, so a single lesion yields a recurrent multi-element phenotype bundle (hand + foot digit anomalies) rather than an isolated defect. Conforming nodes should substitute the disorder-specific patterning input at the trigger node (GLI3 repressor dosage in the Greig/Pallister-Hall spectrum, IHH in brachydactyly A1, SHH/ZRS in preaxial polydactyly, HOXD13 in synpolydactyly, TP63/WNT10B in split-hand/foot malformation) and specialize the autopod phenotype (preaxial vs. postaxial polydactyly, syndactyly, brachydactyly, ectrodactyly) at the consequence node. Modules bind GO and CL terms only.

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Used By Disorder Entries

Brachydactyly Type A1 via Impaired IHH-Patched1 Receptor Signaling → Limb Patterning Signal Perturbation

Greig Cephalopolysyndactyly Syndrome via GLI3 Haploinsufficiency → Limb Patterning Signal Perturbation , Disrupted Anterior-Posterior Limb Patterning → Disrupted Digit Number and Identity Specification , Serially Homologous Autopod Malformation

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Pathograph

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Pathophysiology

Limb Patterning Signal Perturbation

trigger

The conserved initiating lesion is a dosage or activity change in a core limb-patterning signal. The anteroposterior axis is governed by a GLI3 repressor gradient that is generated by PKA-dependent GLI3 processing and antagonized by posterior SHH; loss of GLI3 dosage, deregulated production of truncated GLI3, or altered Hedgehog ligand (SHH, IHH) signaling perturbs this gradient. HOXD-cluster, FGF8/apical-ectodermal-ridge, and WNT inputs provide the additional patterning information that the lesion disrupts.

Smoothened Signaling Pathway GO:0007224 ABNORMAL

Used by disorders

Brachydactyly Type A1 as Impaired IHH-Patched1 Receptor Signaling

Greig Cephalopolysyndactyly Syndrome as GLI3 Haploinsufficiency

Downstream

Disrupted Digit Number and Identity Specification

Disrupted Digit Number and Identity Specification

central effector

Perturbation of the patterning gradients changes how digit number, digit identity, and phalangeal segment length are specified in the autopod. Increased Hedgehog activity (or loss of GLI3 repressor) tends to add digits (polydactyly), whereas reduced Hedgehog/IHH signaling shortens or deletes skeletal segments (brachydactyly); the same deregulated patterning output underlies the polydactyly and Pallister-Hall phenotypes from truncated GLI3.

Chondrocyte CL:0000138

Anterior/Posterior Pattern Specification GO:0009952 ABNORMAL Embryonic Limb Morphogenesis GO:0030326 ABNORMAL

Used by disorders

Greig Cephalopolysyndactyly Syndrome as Disrupted Anterior-Posterior Limb Patterning

Downstream

Serially Homologous Autopod Malformation

Serially Homologous Autopod Malformation

consequence

Because the autopod patterning program is serially reused in fore- and hindlimb, the disrupted specification produces a recurrent phenotype bundle across homologous elements: digit anomalies — polydactyly, syndactyly, brachydactyly, ectrodactyly, or triphalangism — characteristically affecting both hands and feet. This serial homology is why a single patterning lesion yields a multi-element malformation bundle rather than an isolated defect.

Chondrocyte CL:0000138

Limb Morphogenesis GO:0035108 ABNORMAL

Used by disorders

Greig Cephalopolysyndactyly Syndrome