| Category | Key points (concise) | Evidence/notes |
|---|---|---|
| Disease identifier | **L1 syndrome**; **MONDO:0017140** | Open Targets disease mapping links L1 syndrome to **MONDO_0017140** and L1CAM as the principal associated target (pqac-00000000) |
| Core synonyms / spectrum terms | Overlapping **L1CAM-related** phenotypes include **X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS)**, **MASA syndrome**, **CRASH syndrome** (*corpus callosum hypoplasia, retardation/intellectual disability, adducted thumbs, spastic paraplegia, hydrocephalus*), **SPG1** (*spastic paraplegia type 1*), and **X-linked agenesis/partial agenesis of the corpus callosum (ACC)** | Synonym set and spectrum terminology are consistently described across foundational reviews and case reports (pqac-00000004, pqac-00000006, pqac-00000002, pqac-00000003) |
| Causal gene | **L1CAM** (*L1 cell adhesion molecule*), Xq28; neural cell-adhesion glycoprotein important for CNS development | L1CAM is the established causal gene for L1 syndrome and related allelic disorders (pqac-00000000, pqac-00000004, pqac-00000002) |
| Inheritance | **X-linked recessive**; affected individuals are usually **hemizygous males**; female carriers are often asymptomatic but can occasionally manifest disease with **skewed/non-random X-inactivation** | Human clinical literature describes typical X-linked transmission and occasional manifesting females (pqac-00000004, pqac-00000001, pqac-00000005) |
| Hallmark phenotypes | Major recurrent features: **congenital hydrocephalus/ventriculomegaly** (often aqueductal stenosis), **adducted thumbs**, **spasticity/spastic paraplegia**, **agenesis/hypoplasia of corpus callosum**, **intellectual disability/developmental delay**, gait and speech problems; severity ranges from fetal-lethal hydrocephalus to milder neurodevelopmental disease | Core phenotype spectrum summarized in reviews and prenatal/fetal case series (pqac-00000004, pqac-00000006, pqac-00000005, pqac-00000002) |
| Onset / course | Often **prenatal or congenital** for severe HSAS; hydrocephalus may start **in utero**; milder presentations may survive into childhood/adulthood with chronic motor/cognitive disability | Natural-history pattern described in foundational review and fetal studies (pqac-00000004, pqac-00000002, pqac-00000006) |
| Key epidemiology: L1 syndrome / HSAS | **~1 in 30,000 live male births**; described as the **most common inherited form of hydrocephalus** | Reported in multiple L1CAM-focused papers (pqac-00000003, pqac-00000006, pqac-00000005) |
| Key epidemiology: contribution to congenital hydrocephalus | **Up to ~25% of isolated male congenital hydrocephalus** may be attributable to an **X-chromosomal gene mutation** in classic literature on L1 disease | Foundational review gives this estimate in the context of L1 disease/X-linked hydrocephalus (pqac-00000004) |
| Broader congenital hydrocephalus epidemiology | Congenital hydrocephalus (CH) affects **~1/500 births** globally; **genetic factors may contribute up to 40%** of cases, but a **precise genetic etiology has been pinpointed in <5%** of human cases | Recent CH review provides current epidemiologic framing and highlights limited solved fraction despite substantial genetic contribution (pqac-00000011) |
| Variant spectrum | Broad spectrum with **>280 reported L1CAM variants**; about **~50% missense**; many are **private/family-specific**; variant types include missense, nonsense, frameshift, splice-site, CNVs, and whole-gene deletions | Mutation spectrum summarized in case literature/reviews; CNVs are also recognized in HSP-related genes including L1CAM (pqac-00000006, pqac-00000007) |
| Genotype–phenotype correlation | General trend: **missense variants** in extracellular or cytoplasmic regions often produce **milder phenotypes**, whereas **truncating / loss-of-function variants**, especially in extracellular domains or with absent protein, are associated with **more severe disease** including severe hydrocephalus and higher infant mortality | Recurrent genotype–phenotype trend across classic and later reports (pqac-00000004, pqac-00000001, pqac-00000005, pqac-00000006) |
| Mutation classes | Proposed functional classes: **Class I** extracellular frameshift/nonsense → **loss of function, severe**; **Class II** extracellular missense → **partial function, variable severity**; **Class III** cytoplasmic variants → **signaling defects, usually milder**; **Class IV** extracellular variants associated with **aberrant splicing**, phenotype less clearly defined | Practical classification summarized in fetal hydrocephalus report/review (pqac-00000006) |
| 2023 mechanistic advance | **L1–LC3 interaction**: the **L1-70 fragment** binds **LC3** via an extracellular **LIR motif** in the 4th FNIII domain; disrupting this interaction impairs **L1-dependent neurite outgrowth** and **neuronal survival**, linking L1CAM to autophagy/mitophagy-related machinery | Direct mechanistic evidence from 2023 study; important for understanding downstream neuronal vulnerability in L1CAM dysfunction (pqac-00000009, pqac-00000010, pqac-00000014, pqac-00000015) |
| 2023-2024 broader mechanism context | L1CAM biology supports **cell migration, neurite outgrowth, neuronal survival, myelination, synaptic plasticity**; congenital hydrocephalus pathways highlighted in 2024 review include **nervous system growth/development**, **cilia synthesis/movement**, **ion channels/transport**, **Reissner’s fiber synthesis**, **cell apoptosis**, and **neurogenesis** | Recent CH review places **L1CAM** among the limited confirmed CH genes and Figure 2 summarizes pathway groupings; recent mouse work also supports consequences of extracellular-domain mutations for neuronal death/behavior (pqac-00000011, pqac-00000017, pqac-00000012, pqac-00000013) |
| Diagnostic/KB note | Evidence here is drawn from **aggregated disease-level resources and published human case series/reviews**, with mechanistic support from **mouse/in vitro** studies; useful for phenotype, mechanism, and inheritance fields in a knowledge base | Human clinical, review, and model-organism evidence are all represented in the cited contexts (pqac-00000000, pqac-00000004, pqac-00000011, pqac-00000009) |


*Table: This table summarizes high-yield knowledge base facts for L1 syndrome, including identifiers, synonyms, inheritance, hallmark phenotypes, epidemiology, genotype-phenotype patterns, and recent mechanistic advances. It is useful as a concise evidence-backed overview of L1CAM-related disease.*