| Domain | Best-supported finding | Evidence type | Certainty / gap |
|---|---|---|---|
| Identity | Maleylacetoacetate isomerase deficiency (MAAID; GSTZ1/MAAI deficiency), MONDO:0060527 and OMIM phenotype #617596, is caused by deficient glutathione S-transferase zeta 1/maleylacetoacetate isomerase encoded by **GSTZ1**. (pqac-00000000, pqac-00000002) | Curated disease–gene association; human molecular cases | High-confidence identity and gene association; no disease-specific ICD or MeSH identifier was established in the gathered evidence. |
| Inheritance | Biallelic **GSTZ1** variants cause autosomal-recessive MAAID. Homozygous and compound-heterozygous individuals have been reported; segregation was demonstrated in multiple families. (pqac-00000001, pqac-00000018) | Human pedigrees and molecular testing | High confidence for autosomal-recessive inheritance; penetrance cannot be quantified and appears low for overt clinical disease. |
| Biochemical phenotype | Mild, persistent or intermittent elevation of succinylacetone is the principal finding, often with normal tyrosine, liver tests and coagulation. Quantitative urinary maleic acid is elevated in reported genetically confirmed cases and may distinguish MAAID from tyrosinemia type I. (pqac-00000013, pqac-00000021) | Human newborn-screening and biomarker studies | Strong evidence for mild hypersuccinylacetonemia; maleic acid is promising but its assay, stability, reference intervals and dried-blood-spot implementation remain insufficiently standardized. |
| Clinical phenotype and natural history | Most molecularly confirmed individuals have been asymptomatic. Reported follow-up includes untreated children without liver or neurologic complications and an untreated homozygous adult clinically well at age 32; isolated microcephaly, short stature, obesity and mild hyperbilirubinemia have been reported without proof that they are disease-caused. (pqac-00000006, pqac-00000017, pqac-00000022) | Small human case series and family follow-up | Evidence favors a benign or predominantly biochemical phenotype, but fewer than 20 individuals have been reported and long-term surveillance is sparse. |
| 2024 screening statistics | In Heidelberg, 516,803 newborns were screened during August 2016–December 2020. Among 42 elevated-succinylacetone screens, two had tyrosinemia type I, two were suspected of MAAID and one MAAID case was genetically confirmed; the index value was 2.61 µmol/L. (pqac-00000004, pqac-00000018) | Large regional newborn-screening cohort | Reliable center-level data, not a population prevalence estimate; ascertainment depends on assay and cutoff. |
| Diagnostics and differential | After elevated succinylacetone, confirm with repeat dried-blood-spot/plasma testing, urine organic acids or quantitative maleic acid, liver/coagulation studies, and molecular testing. Exclude **FAH**-related tyrosinemia type I first, then analyze **GSTZ1** by single-gene testing, a tyrosinemia panel, exome or genome sequencing. (pqac-00000007, pqac-00000018, pqac-00000021) | Human diagnostic workflows | Molecular confirmation is decisive. Low succinylacetone cannot safely exclude mild tyrosinemia type I, so simply raising screening cutoffs risks missed cases. |
| Treatment | Most reported individuals received neither protein restriction nor medication and remained well. Nitisinone and tyrosine/phenylalanine restriction initiated while tyrosinemia type I was unresolved were discontinued after MAAID diagnosis without deterioration. (pqac-00000001, pqac-00000022) | Case reports and observational follow-up | Current evidence argues against routine disease-specific treatment, but there are no controlled trials or formal guidelines; surveillance and individualized intervention if liver dysfunction appears are prudent. |
| Mechanism | GSTZ1/MAAI normally catalyzes glutathione-dependent isomerization of maleylacetoacetate to fumarylacetoacetate in phenylalanine/tyrosine catabolism. Loss of activity permits upstream metabolites and succinylacetone to accumulate; a glutathione-dependent nonenzymatic bypass can still generate fumarylacetoacetate and probably explains the mild baseline phenotype. (pqac-00000009, pqac-00000012) | Enzyme biochemistry, in vitro experiments and knockout mice | Core enzymatic defect and bypass are well supported experimentally; their quantitative contribution in affected humans has not been directly measured. |
| Mouse model | **Gstz1**-null mice excrete fumarylacetoacetate and succinylacetone and can appear relatively healthy on standard chow, but some backgrounds show enlarged liver/kidneys, hepatitis, renal abnormalities, splenic atrophy and antioxidant-response induction. Phenylalanine, tyrosine/homogentisate challenge or glutathione depletion causes severe hepatic/renal injury and age-dependent lethality. (pqac-00000005, pqac-00000010, pqac-00000015) | Germline knockout mouse studies | Strong evidence for conditional metabolic toxicity; severe challenged-mouse phenotypes have not been observed in the small human cohort and must not be directly extrapolated. |
| Major evidence gaps | No robust incidence or prevalence, carrier frequency, penetrance estimate, genotype–phenotype model, validated clinical criteria, disease-specific quality-of-life data, controlled treatment study, clinical trial, gene therapy, human tissue omics, single-cell/spatial study or proven congenital-MAAID cancer risk is available. Cancer studies of acquired GSTZ1 downregulation are mechanistically informative but not direct evidence for the inherited disorder. (pqac-00000017, pqac-00000020) | Evidence-gap assessment across retrieved literature | Very substantial uncertainty due to ultra-rarity, screening ascertainment, predominantly asymptomatic cases and limited longitudinal follow-up. |


*Table: Compact evidence map for GSTZ1-related maleylacetoacetate isomerase deficiency, integrating human screening and natural-history data with mechanistic and mouse-model findings. The final column highlights where evidence is strong and where ultra-rarity limits inference.*