Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disorder that progressively affects motor neurons. Gain-of-function mutations in serine palmitoyltransferase (SPT) genes, notably SPTLC1 and SPTLC2, have been linked to juvenile ALS. Here, we describe two childhood-onset ALS cases with distinct SPTLC2 mutations, providing new insights into sphingolipid dysregulation and its role in ALS pathogenesis. Two Chinese patients with early-onset ALS, both carrying SPTLC2 mutations, were recruited from Beijing Children's Hospital. We conducted whole-exome sequencing (WES) to identify genetic variants, followed by Sanger sequencing for validation. Sphingolipid profiles were analyzed using ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Clinical evaluations included neurological assessments, brain MRI and electromyography. Additionally, mutant cell lines were established to assess the functional effects of the specific mutations. Patient 1, a 6-year-old male, exhibited a novel heterozygous de-novo SPTLC2 variant (c.197T > G, p.T66R). Patient 2, a 7-year-old female, had a recurrent heterozygous de-novo SPTLC2 variant (c.778G > A, p.E260K). Both patients showed elevated levels of specific sphingolipids compared to controls, with distinct profiles between the SPTLC2-ALS and SPTLC1-hereditary sensory and autonomic neuropathy type 1 (HSAN1) cases. The novel p.T66R mutation was predicted to alter protein interactions within the SPT complex, potentially impairing sphingolipid homeostasis. Functional studies further revealed that the p.T66R variant reduces the inhibitory regulation of SPT by ORMDL proteins, leading to unrestrained SPT activity and excess sphingolipid production. Our findings identify a novel SPTLC2 variant linked to childhood-onset ALS and reveal altered sphingolipid profiles associated with different genetic mutations. These results underscore the importance of sphingolipid metabolism in ALS and suggest potential avenues for targeted therapeutic interventions. Further research is needed to explore treatment options aimed at modulating sphingolipid levels and correcting genetic defects, as well as investigating potential biomarkers for early diagnosis.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder of the motor system. Pathogenic variants in SPTLC1, encoding a subunit of serine palmitoyltransferase, cause hereditary sensory and autonomic neuropathy type 1 (HSAN1), and have recently been associated with juvenile ALS. SPTLC1 variants associated with ALS cause elevated levels of sphinganines and ceramides. Reports on ALS associated with SPTLC1 remain limited. This study aimed to investigate the frequency of SPTLC1 variants in ALS and relevant clinical characteristics. We analyzed whole-exome and whole-genome sequence data from 40 probands with familial ALS and 413 patients with sporadic ALS without previously identified causative variants. Reverse transcription polymerase chain reaction (RT-PCR) analysis and droplet digital PCR (ddPCR) were used to assess splicing and mosaicism, respectively. Plasma sphingolipid levels were quantified to analyze biochemical consequences. The heterozygous c.58G>A, p.Ala20Thr variant was identified in a 21-year-old Japanese female patient presenting with symmetric weakness which slowly progressed over 15 years. RT-PCR analysis showed no splice defects. Plasma sphingolipid levels in the patient were significantly increased compared to her asymptomatic parents. ddPCR revealed that the asymptomatic father harbored a mosaic variant with 17% relative mutant allele abundance in peripheral blood leukocytes. We identified a pathogenic c.58G>A, p.Ala20Thr SPTLC1 variant in a patient with juvenile ALS, likely inherited from an asymptomatic parent with mosaicism. Lipid analysis results are consistent with previous findings on SPTLC1-associated ALS. Further studies are necessary to determine the clinical effect of mosaic variants of SPTLC1.

Hereditary sensory and autonomic neuropathy type 1 is an autosomal dominant neuropathy caused by the SPTLC1 or SPTLC2 variants. These variants modify the preferred substrate of serine palmitoyl transferase, responsible for the first step of de novo sphingolipids synthesis, leading to accumulation of cytotoxic deoxysphingolipids. Diagnosis of HSAN1 is based on clinical symptoms, mainly progressive loss of distal sensory keep, and genetic analysis. Aim: Identifying new SPTLC1 or SPTLC2 "gain-of-function" variants raises the question as to their pathogenicity. This work focused on characterizing six new SPTLC1 variants using in silico prediction tools, new meta-scores, 3D modeling, and functional testing to establish their pathogenicity. Methods: Variants from six patients with HSAN1 were studied. In silico, CADD and REVEL scores and the 3D modeling software MITZLI were used to characterize the pathogenic effect of the variants. Functional tests based on plasma sphingolipids quantification (total deoxysphinganine, ceramides, and dihydroceramides) were performed by tandem mass spectrometry. Results: In silico predictors did not provide very contrasting results when functional tests discriminated the different variants according to their impact on deoxysphinganine level or canonical sphingolipids synthesis. Two SPTLC1 variants were newly described as pathogenic: SPTLC1 NM_006415.4:c.998A>G and NM_006415.4:c.1015G>A. Discussion: The combination of the different tools provides arguments to establish the pathogenicity of these new variants. When available, functional testing remains the best option to establish the in vivo impact of a variant. Moreover, the comprehension of metabolic dysregulation offers opportunities to develop new therapeutic strategies for these genetic disorders.

Motor neuron diseases and peripheral neuropathies are heterogeneous groups of neurodegenerative disorders that manifest with distinct symptoms due to progressive dysfunction or loss of specific neuronal subpopulations during different stages of development. A few monogenic, neurodegenerative diseases associated with primary metabolic disruptions of sphingolipid biosynthesis have been recently discovered. Sphingolipids are a subclass of lipids that form critical building blocks of all cellular and subcellular organelle membranes including the membrane components of the nervous system cells. They are especially abundant within the lipid portion of myelin. In this review, we will focus on our current understanding of disease phenotypes in three monogenic, neuromuscular diseases associated with pathogenic variants in components of serine palmitoyltransferase, the first step in sphingolipid biosynthesis. These include hereditary sensory and autonomic neuropathy type 1 (HSAN1), a sensory predominant peripheral neuropathy, and two neurodegenerative disorders: juvenile amyotrophic lateral sclerosis affecting the upper and lower motor neurons with sparing of sensory neurons, and a complicated form of hereditary spastic paraplegia with selective involvement of the upper motor neurons and more broad CNS neurodegeneration. We will also review our current understanding of disease pathomechanisms, therapeutic approaches, and the unanswered questions to explore in future studies.

Charcot-Marie-Tooth Disease (CMT) is a commonly inherited peripheral polyneuropathy. Clinical manifestations for this disease include symmetrical distal polyneuropathy, altered deep tendon reflexes, distal sensory loss, foot deformities, and gait abnormalities. Genetic mutations in heat shock proteins have been linked to CMT2. Specifically, mutations in the heat shock protein B1 (HSPB1) gene encoding for heat shock protein 27 (Hsp27) have been linked to CMT2F and distal hereditary motor and sensory neuropathy type 2B (dHMSN2B) subtype. The goal of the study was to examine the role of an endogenous mutation in HSPB1 in vivo and to define the effects of this mutation on motor function and pathology in a novel animal model. As sphingolipids have been implicated in hereditary and sensory neuropathies, we examined sphingolipid metabolism in central and peripheral nervous tissues in 3-month-old HspS139F mice. Though sphingolipid levels were not altered in sciatic nerves from HspS139F mice, ceramides and deoxyceramides, as well as sphingomyelins (SMs) were elevated in brain tissues from HspS139F mice. Histology was utilized to further characterize HspS139F mice. HspS139F mice exhibited no alterations to the expression and phosphorylation of neurofilaments, or in the expression of acetylated α-tubulin in the brain or sciatic nerve. Interestingly, HspS139F mice demonstrated cerebellar demyelination. Locomotor function, grip strength and gait were examined to define the role of HspS139F in the clinical phenotypes associated with CMT2F. Gait analysis revealed no differences between HspWT and HspS139F mice. However, both coordination and grip strength were decreased in 3-month-old HspS139F mice. Together these data suggest that the endogenous S139F mutation in HSPB1 may serve as a mouse model for hereditary and sensory neuropathies such as CMT2F.