Achondrogenesis type II

Achondrogenesis type II is a skeletal genetic disease caused by autosomal dominant variants in COL2A1, acting through haploinsufficiency. Within this volume's rankable burden cohort it sits at residual rank 1 of 23, where no disease-directed therapy alters its course. That order is a provisional [H]-grade prioritisation device, not a registry-locked ranking.

Achondrogenesis type II is inherited as autosomal dominant and acts by haploinsufficiency ([L]). Its pre-treatment burden proxy is 0.9167 and, after the established therapy's efficacy offset e = 0.0000 is applied, its residual burden score is 0.9167 ([H]), placing it at residual rank 1 of 23.

Gene, inheritance, and molecular mechanism

Achondrogenesis type II is inherited as Autosomal dominant inheritance [L] and is classified mechanistically as haploinsufficiency [L]. COL2A1 encodes type II collagen, the principal structural collagen of cartilage. A defective allele degrades the cartilage template on which endochondral bone is laid down, so the skeleton fails to ossify normally.

Inheritance source: medgen:esummary:ModeOfInheritance:2026-06-17. Mechanism source: clingen:HI_score_3:2026-06-17; genes=COL2A1; PMID:17721977,27390512,27408751. These are observed, cited inputs; the inheritance and mechanism classification is the reproducible analysis layer (C-D1).

In-scope clinical involvement

The result is severe micromelic (short-limbed) dwarfism with a small chest, prominent abdomen, incomplete ossification of the vertebral bodies, and a disorganised costochondral junction. The disorder is perinatal-lethal.

Organ systems [L] from HPO phenotype.hpoa v2026-06-06 (OMIM 120140,200610); P-aspect terms rolled up to HP:0000118 organ-system categories via hp.obo.

Reproducible burden position

Of the five axes, 3 are scored (the rankability cut is ≥ 3 of 5); the pre-treatment composite is the renormalised mean over the scored axes, raw_burden = 0.9167. The registry-grade [L] axes here are onset, severity, and mortality (onset from the Orphanet AverageAgeOfOnset register / HPO).

All 3 scored axes for this disease are registry-grade [L]/[V]: its burden value is registry-locked (order_locked) and no longer rests on definition-only inference. The cohort order overall is still a provisional [H] prioritisation device, not a registry-locked ranking — this disease’s absolute rank depends on neighbours whose axes remain [H].

The natural-history registry passes have been run against Orphanet/Orphadata (CC BY 4.0, R6) and the openly published GBD 2013 disability-weights table (Salomon et al., CC BY, R7). R6 lifts onset to registry-grade [L] across most of the cohort (earliest AverageAgeOfOnset category, entity-anchored per ORPHAcode, Exact OMIM↔ORPHA only); R7 lifts disability to [L] where one dominant untreated sequela maps to a named GBD health state (published disability weight binned by declared cut-points), and independently corroborates a mortality axis from PMC survival literature where a quantitative disease-typical figure exists. Severity now lifts to registry [L] for the one disease whose dominant sequela carries a cited HPO Severity-modifier annotation (the HP:0012824 subtree, R8); for the rest the open HPO severity annotations are feature-level (using one feature as the disease tier would be a category error), so severity stays [H]/[O] with the obstacle named. Progression lifts to registry [L] where a cited PMC open-access source states a disease-level magnitude for the dominant untreated sequela and the frozen R3 tier function derives the tier from that verbatim sentence (R9, curated dominant-sequela join, non-spectrum); for the rest progression stays [H]/[O]. The OMIM clinical synopsis (the disease-level alternative) is API-key-gated and the key is unobtainable for an individual researcher — that path is removed, not guessed.

Achondrogenesis type II perturbs the cartilage-to-bone (endochondral) growth programme that the carried morphogenesis engine forward-simulates as its normal-development baseline.

Established treatment and residual burden

The established disease-directed approach is no disease-directed therapy (perinatal-lethal skeletal dysplasia); perinatal palliative care. Mechanistically: no disease-directed therapy exists [H].

Its effect on natural history is classified none, mapping to an efficacy offset e = 0.0000 and a residual factor R_treat = 1.0000. Applied to the pre-treatment proxy this gives the residual burden score 0.9167, moving the disease from raw rank 1 to residual rank 1 (shift 0).

Evidence tier: established standard of care [H] (source class: GeneReviews; OMIM clinical synopsis; no accession fabricated). Accession-dated [L] verification is deferred — evidence_status 'none' (no disease-directed therapy exists); GeneReviews chapter NBK540447 confirms supportive/absent-therapy management. No therapy to accession-date; grade retained [H] as an honest inference about absence of therapy..

An evidence_status of none (e = 0) is a positive [H] finding that no disease-directed therapy alters the course — distinct from an open [O] gap. No cure is implied (constitution C-D3).