Acute intermittent porphyria

Acute intermittent porphyria is a metabolic genetic disease caused by autosomal dominant variants in HMBS. It is not placed in the rankable burden order: only 2 of five axes are scored (cut: three of five), so the remaining 3 stay open [O], not guessed. Mechanism, scored axes, and treatment follow below.

Acute intermittent porphyria is inherited as autosomal dominant and acts by haploinsufficiency ([L]). Of the five burden axes, 2 are scored (O, S) and 3 are open [O] (P, M, D); because fewer than three are scored the disease is reported as “not placed — insufficient axis coverage” (rank null), not ranked on partial data.

Gene, inheritance, and molecular mechanism

Acute intermittent porphyria is inherited as Autosomal dominant inheritance [L] and is classified mechanistically as haploinsufficiency [L]. HMBS encodes hydroxymethylbilane synthase, the third enzyme of haem biosynthesis. Haploinsufficiency lets the neurotoxic precursors delta-aminolevulinic acid and porphobilinogen accumulate when haem demand rises.

Inheritance source: medgen:esummary:ModeOfInheritance:2026-06-17. Mechanism source: clingen:HI_score_3:2026-06-17; genes=HMBS; PMID:10790212,12372055,19460837. These are observed, cited inputs; the classification is the reproducible analysis layer (C-D1).

In-scope clinical involvement

Acute neurovisceral attacks bring severe abdominal pain, autonomic instability, hyponatraemia, peripheral neuropathy, and neuropsychiatric features, typically provoked by certain drugs, fasting, or hormonal cycles.

Organ systems [L] from HPO phenotype.hpoa v2026-06-06 (OMIM 176000,609806); P-aspect terms rolled up to HP:0000118 organ-system categories via hp.obo.

Burden axes and why this disease is not placed

For transparency the partial composite over the 2 scored axis/axes is raw_burden = 0.6500, but it is not used to rank the disease (below the ≥ 3-axis cut). The open axes (progression, mortality, disability) are graded [O] with a named obstacle, never imputed.

The natural-history registry passes lift onset (Orphanet, R6) and disability (GBD 2013 disability weights, R7) to registry [L] where an entity-anchored mapping exists, lift severity (HPO Severity-modifier subtree, R8) where the dominant sequela is annotated, and lift progression (curated PMC open-access literature with a frozen-R3-derived tier, R9) where a cited disease-level magnitude for the dominant untreated sequela exists. For some diseases the added disability axis was enough to cross the ≥ 3-axis cut and enter the placed residual order; for this one it was not — and its dominant sequela carries no open HPO severity annotation, so no registry severity tier is available (the open HPO severity annotations elsewhere are feature-level; the OMIM clinical synopsis, the disease-level alternative, is API-key-gated and the key is unobtainable for an individual researcher). Scoring three or more axes at registry grade (via published functional & survival literature) would let this disease enter the rankable residual order; until then it is reported here, never imputed.

Established treatment

The established disease-directed approach is hemin for acute attacks and givosiran (siRNA) for prophylaxis, with trigger avoidance and carbohydrate loading. Mechanistically: hemin and givosiran downregulate hepatic ALAS1, lowering neurotoxic ALA and porphobilinogen [L]. Its effect on natural history is classified disease-modifying (substantial) (efficacy offset e = 0.5500, residual factor R_treat = 0.4500); this offset would apply to the burden score once the disease is placed.

Evidence tier: accession-dated to the GeneReviews NBK1193 Management section [L] (initial posting September 27, 2005; last revision February 8, 2024; retrieved 2026-06-18; corroborating term(s): “hemin, acute, attacks, trigger, carbohydrate”).