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SMPD1
sphingomyelin phosphodiesterase 1
SMPD1 is located on the short (p) arm of chromosome 11, at band 11p15.4. Arm ratio per GRCh38 - banding schematic.
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Clinical tests that include this
Overview
SMPD1 encodes acid sphingomyelinase, an enzyme housed within lysosomes that catalyses the breakdown of sphingomyelin into ceramide. This conversion is essential for maintaining normal lipid composition in cell membranes and enabling various cellular signalling processes. The gene is located on chromosome 11 and follows an autosomal recessive inheritance pattern, meaning that pathogenic variants in both gene copies are required to cause disease.
Pathogenic variants in SMPD1 result in deficient or absent enzyme activity, leading to progressive accumulation of sphingomyelin in cells throughout the body. This storage disorder particularly affects the liver, spleen, lungs, and in severe cases, the nervous system. The SMPD1 gene is included on multiple NHS clinical panels related to lysosomal storage disorders and metabolic conditions.
What the gene does
Acid sphingomyelinase functions as a phosphodiesterase enzyme that cleaves sphingomyelin, a phospholipid abundant in cell membranes, into ceramide and phosphocholine. This enzymatic reaction occurs within the acidic environment of lysosomes, where the enzyme achieves optimal activity. Ceramide serves as a key signalling molecule involved in cell growth, differentiation, and programmed cell death pathways.
Beyond its role in lipid catabolism, the enzyme helps regulate cellular cholesterol distribution, as sphingomyelin molecules interact with cholesterol to form specialised membrane microdomains. These lipid rafts are important for receptor signalling and membrane trafficking processes. The enzyme's activity is particularly crucial in tissues with high rates of membrane turnover, including cells of the reticuloendothelial system. Disruption of this enzymatic function impairs the cell's ability to recycle lipid components, leading to their pathological accumulation within lysosomes and subsequent cellular dysfunction.
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Chromosome location
SMPD1 is positioned on the short arm of chromosome 11 at band p15.4, a region rich in genes involved in metabolic processes. The gene spans multiple exons that encode a 631 amino acid protein. This chromosomal location places SMPD1 within a region that has been associated with several inherited metabolic disorders, reflecting the clustering of functionally related genes involved in cellular metabolism and lysosomal function.
Protein structure
The acid sphingomyelinase protein consists of 631 amino acids and contains distinct structural features that enable its lysosomal function. The protein includes a Saposin B-type domain spanning amino acids 87 to 171, which is characteristic of proteins involved in lipid binding and membrane interaction. This domain facilitates the enzyme's ability to access its sphingomyelin substrate embedded within lipid membranes. The N-terminal region from amino acids 1 to 23 is disordered, lacking fixed secondary structure, which may provide flexibility for protein trafficking or substrate recognition. The protein undergoes post-translational modifications and processing as it traffics from the endoplasmic reticulum through the Golgi apparatus to reach its final destination in lysosomes.
Key variants
Over 175 pathogenic variants have been identified in SMPD1, including missense mutations, nonsense mutations, and small deletions that disrupt enzyme production or activity. The specific variant determines disease severity, with complete loss of enzyme function typically associated with the more severe type A presentation, whilst variants retaining residual activity correlate with the milder type B phenotype. Certain populations carry founder mutations at higher frequencies, influencing carrier screening strategies in those communities.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Pathogenic variants in SMPD1 cause Niemann-Pick disease type A and Niemann-Pick disease type B, both inherited in an autosomal recessive manner. Type A represents the severe infantile form characterised by rapid neurological deterioration alongside enlargement of the liver and spleen, with most affected children not surviving beyond early childhood. Type B presents as a milder condition with prominent lung and liver involvement but typically preserved neurological function, allowing survival into adulthood. Both conditions result from accumulation of sphingomyelin in tissues, though the distribution and rate of accumulation differ between the two forms.
Inheritance pattern
Conditions caused by pathogenic SMPD1 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
SMPD1 holds green (high evidence) status across multiple NHS Genomic Medicine Service panels, reflecting its well-established clinical validity. The gene appears on the Niemann-Pick disease type A or B panel (R282), the Lysosomal storage disorder panel (R276), and the Likely inborn error of metabolism panel (R98). Additional panel memberships include Cholestasis (R171), Intellectual disability (R29), Neonatal cholestasis, Familial pulmonary fibrosis, and Fetal hydrops panels. This broad panel representation reflects the diverse clinical presentations of SMPD1-related disease and supports diagnostic testing across various clinical scenarios within the NHS.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the difference between Niemann-Pick disease type A and type B?
Type A is the severe infantile form with rapid neurological deterioration and limited survival beyond early childhood, whilst type B is milder with prominent lung and liver disease but generally preserved neurological function, allowing survival into adulthood. The distinction relates to the amount of residual enzyme activity determined by the specific SMPD1 variants inherited.
How is SMPD1 inherited?
SMPD1 follows an autosomal recessive inheritance pattern, meaning an individual must inherit a pathogenic variant from both parents to develop Niemann-Pick disease. Carriers with one pathogenic variant typically remain healthy but can pass the variant to their children.
Why is SMPD1 included on carrier screening panels?
SMPD1 is included on carrier screening panels because it causes a severe autosomal recessive condition, and certain populations have higher carrier frequencies for specific variants. Identifying carrier couples before or during pregnancy enables informed reproductive choices and early diagnosis if a child is affected.
References
- Irun P, Mallén M, Dominguez C. Identification of seven novel SMPD1 mutations causing Niemann-Pick disease types A and B. Clinical genetics. 2013. PMID: 23252888
- Hollak CE, de Sonnaville ES, Cassiman D. Acid sphingomyelinase (Asm) deficiency patients in The Netherlands and Belgium: disease spectrum and natural course in attenuated patients. Molecular genetics and metabolism. 2012. PMID: 22818240
- Schuchman EH. The pathogenesis and treatment of acid sphingomyelinase-deficient Niemann-Pick disease. Journal of inherited metabolic disease. 2007. PMID: 17632693
- Wasserstein MP, Aron A, Brodie SE. Acid sphingomyelinase deficiency: prevalence and characterization of an intermediate phenotype of Niemann-Pick disease. The Journal of pediatrics. 2006. PMID: 17011332
- Wasserstein MP, Desnick RJ, Schuchman EH. The natural history of type B Niemann-Pick disease: results from a 10-year longitudinal study. Pediatrics. 2004. PMID: 15545621
- Simonaro CM, Desnick RJ, McGovern MM. The demographics and distribution of type B Niemann-Pick disease: novel mutations lead to new genotype/phenotype correlations. American journal of human genetics. 2002. PMID: 12369017