| Domain | Evidence-based finding | Evidence level/limitations |
|---|---|---|
| Identity/identifiers | Developmental and epileptic encephalopathy 89 (DEE89) is linked to **MONDO:0030856** and **OMIM 619124**; Open Targets shows a single associated target, **GAD1**, with supporting literature including PMID **32282878**. The evidence base is disease-level aggregation plus a small primary human case series. (pqac-00000008, pqac-00000011) | Identifier linkage is strong, but disease characterization is based on very limited published human data. |
| Causal gene/inheritance | DEE89 is caused by **biallelic GAD1** variants encoding **GAD67/glutamate decarboxylase 1**; inheritance is **autosomal recessive**. In the reported cohort, **5/6 families** had homozygous variants and **1/6** had compound heterozygous variants; **4/6** affected individuals were born to consanguineous parents. (pqac-00000000, pqac-00000011, pqac-00000012) | Human genetic evidence is consistent, but numbers come from only **6 unrelated families**. |
| Foundational evidence | The foundational report is **Neuray et al., Brain, July 2020, DOI: 10.1093/brain/awaa178, PMID 32282878**, describing **6 affected individuals from 6 unrelated families** and introducing **GAD1 as a new gene associated with developmental and epileptic encephalopathy**. (pqac-00000001, pqac-00000012) | Single primary series; no larger replication cohort identified in the gathered evidence. |
| Onset/seizures | Seizure onset occurred at **2-6 months** (early-infantile onset). Seizure types were predominantly **focal motor seizures**, with some **epileptic spasms** and **bilateral motor seizures**. Seizures were **pharmacologically controlled in 3/6** and **drug-resistant in 3/6**. (pqac-00000000, pqac-00000012) | Denominators preserved from the only cohort; no standardized natural-history registry available. |
| Development/neuromuscular | All reported individuals had **severe developmental delay/intellectual disability**; most had **no speech or communication**, with only **1/6** retaining basic/simple speech. **Reduced muscle strength/weakness or hypotonia** was present in **5/6**. Variable extracerebral features included **skeletal abnormalities, dysmorphic features, and cleft palate**. (pqac-00000000, pqac-00000001, pqac-00000006, pqac-00000012) | Core phenotype is consistent, but extracerebral manifestations are variable and based on few cases. |
| MRI/EEG | **MRI** was reportedly normal in **4/6** and abnormal in **2/6**, with **ventricular enlargement** or **global atrophy**. **EEG** abnormalities included **burst-suppression**, **diffuse slowing with multifocal/generalized sharp waves**, and **hypsarrhythmia**. (pqac-00000000, pqac-00000012) | Imaging appears nonspecific; EEG spectrum is broad and not disease-exclusive. |
| Variants | Seven reported **ultrarare germline** GAD1 variants were: **c.87C>G p.Tyr29Ter**, **c.568delC p.Gln190Serfs*11**, **c.670delC p.Leu224Serfs*5**, **c.971T>G p.Phe324Cys**, **c.1040C>T p.Thr347Met**, **c.1591C>T p.Arg531***, **c.1691A>G p.Asn564Ser**. Variant classes included **nonsense, frameshift, missense**; multiple alleles are predicted **loss-of-function** via nonsense-mediated decay or catalytic disruption. (pqac-00000007, pqac-00000009, pqac-00000013) | Functional interpretation is strongest for null alleles; allele-frequency details were not extracted in this conversation. |
| Mechanism | **GAD1/GAD67** is the major CNS enzyme for converting **glutamate to GABA** and contributes about **90% of baseline GABA**. Bi-allelic loss of function is inferred to reduce GABA synthesis, disturb **inhibitory/excitatory balance**, lower seizure threshold, and impair neurodevelopment; persistent severe disability despite seizure freedom in **3/6** supports a developmental effect beyond seizures alone. (pqac-00000005, pqac-00000007, pqac-00000011, pqac-00000013) | Mechanistic chain is biologically plausible and supported by human genetics plus model data, but direct patient biochemical assays were not summarized here. |
| Treatment | No disease-specific targeted therapy was identified. Management in the cohort was standard antiseizure treatment with **3/6** achieving seizure control and **3/6** remaining drug-resistant. No DEE89-specific clinical intervention trial was found. (pqac-00000000, pqac-00000012) | Evidence is limited to case-based supportive epilepsy care; no controlled treatment studies or precision therapies are available in the gathered evidence. |
| Epidemiology/trials | No robust **prevalence**, **incidence**, or survival estimates were identified. The human literature currently consists of a **very small number of published cases**. Clinical trial search found **no relevant DEE89/GAD1-specific interventional trials**. (pqac-00000008, pqac-00000012) | Major evidence gap; epidemiology and prognosis remain undefined. |
| Model organism | **Gad1-null mice** show **neonatal lethality** and **severe cleft palate**, supporting the gene’s role in development but limiting detailed postnatal neurological modeling. Animal data also support a role for GAD/GABA pathway deficiency in seizures. (pqac-00000005, pqac-00000011) | Useful for biological plausibility, but existing model limitations reduce fidelity for full human DEE89 natural history. |


*Table: This table summarizes the highest-confidence evidence gathered for developmental and epileptic encephalopathy 89, preserving small-cohort denominators and highlighting where the literature is strong versus sparse. It is useful as a compact evidence map for knowledge-base curation.*