Sphingolipidoses
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Direct answer
Tay-Sachs disease results from deficiency of hexosaminidase A, causing GM2 ganglioside to accumulate in neurons and producing the cherry-red spot, startle response and death in early childhood without any hepatosplenomegaly. Each sphingolipidosis blocks one lysosomal hydrolase acting on the ceramide backbone of sphingomyelin, cerebrosides or gangliosides, so the stored material and the organ picture identify the enzyme at fault. All are autosomal recessive except Fabry disease, which is X-linked recessive.
What you must remember
- Sphingolipids are built on ceramide (sphingosine plus a long-chain fatty acid); adding phosphocholine gives sphingomyelin, glucose gives glucocerebroside, and sialic acid-containing oligosaccharides give gangliosides, enriched in grey matter.
- Tay-Sachs: hexosaminidase A deficiency, GM2 gangliosidosis, Ashkenazi Jewish carrier frequency, cherry-red spot on macula, macrocephaly, startle to sound, death by 3–4 years, no visceral enlargement.
- Sandhoff disease: hexosaminidase A and B deficiency — resembles Tay-Sachs but with hepatosplenomegaly.
- Gaucher disease: beta-glucocerebrosidase deficiency — the commonest sphingolipidosis; glucocerebroside-filled macrophages with crumpled-tissue-paper cytoplasm, hepatosplenomegaly, pancytopenia, Erlenmeyer-flask femur; type 1 spares the brain and responds to enzyme replacement with imiglucerase.
- Niemann-Pick disease: sphingomyelinase deficiency; foam cells, cherry-red spot, hepatosplenomegaly with neurological decline in the infantile type A.
- Fabry disease: alpha-galactosidase A deficiency, X-linked recessive; angiokeratomas in a bathing-trunk distribution, acroparaesthesia, corneal verticillata, renal and cardiac failure in adults.
- Krabbe disease: galactocerebrosidase deficiency with globoid cells and peripheral neuropathy; metachromatic leukodystrophy: arylsulfatase A deficiency with sulphatide storage, demyelination and metachromatically staining granules.
- Enzyme assay in leucocytes or fibroblasts confirms the diagnosis; carrier detection and prenatal diagnosis are possible for all.
Sorting one infant from another at the bedside
An infant of Ashkenazi parentage, normal at birth, loses milestones by six months, startles violently to any sound, and fixation drifts; fundoscopy shows a cherry-red spot against a pale retina (the macula keeps its translucency while ganglion cells laden with GM2 whiten the surrounding retina). The abdomen is soft — no spleen — which is the discriminator from Niemann-Pick, where sphingomyelin-stuffed foam cells swell both liver and spleen alongside a similar cherry-red spot. Now consider the child of two or three with a massively enlarged spleen, thrombocytopenia and painful bone crises whose marrow aspirate shows Gaucher cells, their cytoplasm like crumpled tissue paper: glucocerebroside storage from beta-glucocerebrosidase deficiency, type 1, the form common in Ashkenazi Jews that does not touch the brain and is the poster child of enzyme replacement therapy. Finally, an adult man with burning hands since boyhood, scattered red-black angiokeratomas over hips and scrotum, proteinuria and concentric left ventricular hypertrophy has Fabry disease — the lone X-linked member of the family — and his enzyme is alpha-galactosidase A; his renal prognosis depends on starting enzyme replacement or migalastat (for amenable variants) before irreversible fibrosis.
Where students slip
Examiners deliberately give a cherry-red spot plus hepatosplenomegaly, and students who memorised "cherry-red spot equals Tay-Sachs" miss Niemann-Pick. The second trap is Gaucher cells versus Niemann-Pick foam cells: crumpled tissue paper (fibrillary, PAS-positive) is Gaucher; uniformly foamy or mulberry cytoplasm is Niemann-Pick. Third, remember that enzyme names encode the substrate: glucocerebrosidase for glucocerebroside, galactocerebrosidase for galactocerebroside, sphingomyelinase for sphingomyelin — when in doubt in the hall, decode the substrate suffix. A final one-liner: hexosaminidase A can be assayed in serum for carrier detection in Tay-Sachs, a classic population-screening application in genetic counselling.
Frequently asked questions
Which sphingolipidoses show a cherry-red spot at the macula?
Tay-Sachs and Niemann-Pick classically (also GM1 gangliosidosis). The red macula stands out against storage-pale ganglion cells; in Tay-Sachs the absence of organomegaly distinguishes it from Niemann-Pick.
What are Gaucher cells and which enzyme is deficient?
Tissue macrophages engorged with glucocerebroside, showing characteristic crumpled-tissue-paper fibrillary cytoplasm on smear; beta-glucocerebrosidase is deficient, and type 1 disease is treated with enzyme replacement (imiglucerase) or substrate reduction therapy.
Why is Fabry disease the odd one out among sphingolipidoses?
It is the only X-linked recessive member (alpha-galactosidase A deficiency, globotriaosylceramide storage); all others are autosomal recessive. Female carriers can be symptomatic due to random X-inactivation.
Which enzyme defect causes metachromatic leukodystrophy and what accumulates?
Arylsulfatase A (cerebroside sulfatase) deficiency, causing sulphatide storage with central and peripheral demyelination; stored granules stain metachromatically (brownish-gold with cresyl violet) and sulphatide appears in urine.
How is Sandhoff disease distinguished from Tay-Sachs biochemically?
Total hexosaminidase deficiency (A plus B) in Sandhoff, so enzyme assay shows near-total loss including in serum, and visceral organs store globoside as well — Tay-Sachs spares hexosaminidase B and the viscera.