| domain | disease-specific finding | quantitative/detail | evidence type/limitations |
|---|---|---|---|
| Disease identity | Amyotrophic lateral sclerosis 27, juvenile; Mendelian juvenile ALS subtype linked to **SPTLC1** | **MONDO:0859529**; target association **SPTLC1 / ENSG00000090054** (pqac-00000000) | Disease-level ontology and target-association resource; does not provide full clinical detail (pqac-00000000) |
| Causal gene / mechanism class | **SPTLC1** pathogenic variants cause a **dominant gain-of-function** disorder of sphingolipid biosynthesis | Pathogenic effect is increased/unrestrained SPT activity via impaired ORMDL regulation rather than classic loss of function (pqac-00000012, pqac-00000015) | Primary mechanistic human/cell studies and review synthesis; selective motor-neuron vulnerability remains incompletely explained (pqac-00000012, pqac-00000015) |
| Inheritance | Usually **autosomal dominant**, commonly **de novo** | 2023 compilation table lists many cases as **de novo**; AD familial cases also reported, including multiple **p.Leu39del** relatives (pqac-00000011, pqac-00000015) | Derived largely from case reports/small pedigrees; penetrance not well quantified (pqac-00000011, pqac-00000015) |
| Established disease variants | Recurrently implicated ALS27 variants cluster in **SPTLC1 exon 2 / N-terminal transmembrane domain** | Five established variants emphasized in reviews/comparative data: **p.A20S, p.Y23F, p.L38R, p.Leu39del, p.Phe40_Ser41del**; **p.A20T** reported as a novel case in 2023 (pqac-00000011, pqac-00000015) | Variant set based on early case series/reviews through 2024; later expansion possible (pqac-00000011, pqac-00000015) |
| Age at onset / natural history | Very early childhood onset with unusually slow course relative to FUS-JALS | In 17-patient SPTLC1 JALS comparison: **AAO 7.9 ± 4.6 years**, **100% spinal onset**, disease duration **512.0 months** (95% CI **416.7–607.3**) vs FUS-JALS **33.4 months** (95% CI **21.6–45.1**) (pqac-00000011) | Cohort assembled from literature plus new cases; small numbers and publication bias likely (pqac-00000011) |
| Core phenotype | Combined upper and lower motor neuron disease, often beginning in legs | Early **lower-limb spasticity**, toe walking/gait abnormality, weakness/atrophy; symptoms may start as early as **3–4 years**; bulbar/respiratory involvement can occur later (pqac-00000011, pqac-00000015) | Mostly retrospective case data; severity varies across variants/families (pqac-00000011, pqac-00000015) |
| Sensory / cognitive profile | Sensory system usually spared; cognition usually preserved | **No sensory neuropathy** is typical even late; normal sensory studies and at least one normal sural biopsy reported; cognitive dysfunction generally **not reported** in SPTLC1-ALS, unlike some SPTLC2 cases (pqac-00000014, pqac-00000015) | Important disease discriminator, but systematic neuropsychology is limited (pqac-00000015) |
| Electrophysiology / pathology | Motor neuron disorder pattern supports ALS over HSP | EMG/NCS: **normal sensory studies** with **diffuse acute and chronic denervation in multiple myotomes** and no demyelinating features; extensive neurogenic damage helps distinguish from HSP (pqac-00000011, pqac-00000015) | Disease-specific but based on relatively few deeply phenotyped patients (pqac-00000011, pqac-00000015) |
| Molecular mechanism | Variants disrupt **ORMDL-SPTLC1** interaction at the ER SPT complex | SPT resides in **ER**/ER-mitochondrial contact sites; ALS variants in the **N-terminal TMD** impair ORMDL binding and feedback inhibition, causing excess canonical sphingolipid synthesis (pqac-00000012, pqac-00000006, pqac-00000015) | Strong cell-biochemical evidence; downstream pathway from lipid excess to motor-neuron death remains partly unresolved (pqac-00000006, pqac-00000015) |
| Biomarker signature | Distinct **canonical sphingolipid / ceramide** signature rather than HSAN1-like deoxySL excess | Elevated canonical sphingolipids, including unusual **C18:0, C20:0, C22:0** acyl-chain species; generally **not 1-deoxysphingolipid-driven** unless substrate conditions shift (pqac-00000006, pqac-00000012, pqac-00000015) | Promising biochemical biomarker from serum/fibroblast/cell studies; no standardized clinical cutoff/assay yet (pqac-00000006, pqac-00000015) |
| Diagnostic approach | Suspect in children with slowly progressive spastic-paraparetic/motor neuron syndrome and no sensory loss | Recommended work-up from disease-specific evidence: **broad genetic testing (WES/WGS or ALS/HSP panels including SPTLC1)** + EMG/NCS + clinical exclusion of HSP/HSAN1; lipidomics may support mechanism where available (pqac-00000011, pqac-00000015) | No dedicated formal diagnostic criteria for ALS27; practice extrapolated from juvenile ALS genetics literature (pqac-00000011, pqac-00000015) |
| Differential diagnosis | Often confused early with **hereditary spastic paraplegia**; biochemically distinct from **HSAN1** | HSP mimic: early spastic gait; ALS27 favored by LMN involvement/EMG denervation. HSAN1 differs by sensory neuropathy and deoxysphingolipid excess (pqac-00000011, pqac-00000012, pqac-00000015) | Differential framework is strong, but based on expert synthesis more than prospective studies (pqac-00000011, pqac-00000015) |
| Treatment caveat | **Avoid L-serine supplementation** in SPT-related motor-neuron disease | Reviews and mechanistic studies predict serine may **exacerbate canonical sphingolipid overproduction**; iPSC-derived motor neurons with **p.F40_S41del** reportedly worsened with serine supplementation (pqac-00000013, pqac-00000015) | Key actionable caution; not based on controlled clinical trials in ALS27 (pqac-00000013, pqac-00000015) |
| Current treatment status | No disease-specific approved therapy or registered disease-specific trial identified | Standard care remains supportive/ALS multidisciplinary management; **partial SPT inhibition** is proposed conceptually but not established clinically for ALS27 (pqac-00000013) | No disease-specific interventional trial retrieved; evidence for therapy is preclinical/expert-opinion level (pqac-00000013) |
| Models | Human cell systems provide main mechanistic evidence | Evidence includes **HEK293/COS-7** systems, **patient fibroblasts**, and **iPSC-derived lower motor neuron-like cells** with elevated canonical sphingolipids; **no murine SPT-related ALS model reported so far** in 2024 review (pqac-00000012, pqac-00000015) | Major translational gap: limited in vivo disease modeling for ALS27 specifically (pqac-00000015) |
| Evidence gaps | Major unknowns remain despite strong gene-mechanism link | Missing/limited: prevalence and incidence, penetrance, validated prognosis markers, standardized lipid biomarker thresholds, controlled treatment data, and explanation for selective motor-neuron vulnerability (pqac-00000011, pqac-00000015) | Knowledge base should mark many epidemiology/outcome fields as **not established** rather than infer from general ALS (pqac-00000011, pqac-00000015) |


*Table: This compact table summarizes the most actionable disease-specific facts for SPTLC1-associated juvenile ALS (ALS27), including identity, variants, phenotype, mechanism, diagnostics, and treatment caveats. It is designed for direct reuse in a knowledge-base entry while clearly flagging evidence limitations and unknowns.*