Sphingolipidoses & Mucopolysaccharidoses
A high-yield Step 1 review of the sphingolipidoses and mucopolysaccharidoses, matching each deficient enzyme and accumulated substrate to its classic vignette buzzword, and nailing the AR-vs-X-linked exceptions (Fabry, Hunter) plus next-best-step management.
The big picture
Lysosomal storage diseases arise from a deficient lysosomal enzyme, so its undegraded substrate piles up inside cells. Two families dominate Step 1:
- Sphingolipidoses — cannot degrade sphingolipids; disease is largely neurodegenerative (± hepatosplenomegaly, ± signature cells/macrophages).
- Mucopolysaccharidoses (MPS) — cannot degrade glycosaminoglycans (GAGs), so heparan sulfate + dermatan sulfate accumulate → coarse facies, skeletal dysplasia, corneal clouding, organomegaly.
Nearly every one is autosomal recessive. Memorize the two X-linked recessive exceptions the boards love: Fabry (a sphingolipidosis) and Hunter (an MPS). Three sphingolipidoses — Tay-Sachs, Niemann-Pick, and Gaucher — cluster in the Ashkenazi Jewish population. Exam strategy is pure pattern recognition: pair the missing enzyme + accumulated substrate + one buzzword (cherry-red macula, foam cell, crumpled-tissue-paper macrophage, angiokeratoma, globoid cell).
- Cherry-red macula = Tay-Sachs OR Niemann-Pick. Distinguish by hepatosplenomegaly: ABSENT in Tay-Sachs, PRESENT in Niemann-Pick.
- Tay-Sachs clues beyond the cherry-red spot: exaggerated startle to sound, hyperreflexia, progressive regression — no organomegaly.
- Signature cells: foam cells (lipid-laden macrophages) → Niemann-Pick; "crumpled/wrinkled tissue-paper" macrophages → Gaucher; globoid cells → Krabbe.
- Gaucher (most common lysosomal storage disease): hepatosplenomegaly, pancytopenia, bone crises / avascular necrosis of femoral head, Erlenmeyer-flask femur.
- Fabry (XLR): earliest sign is acroparesthesias (burning hand/foot pain) + angiokeratomas + hypohidrosis + cornea verticillata → later renal failure, cardiac disease, stroke.
- Treatment reality: enzyme replacement helps Gaucher (type 1) and Fabry; Tay-Sachs has NO disease-modifying therapy (supportive only).
Enzyme → substrate → clue (full reference)
| Disease | Deficient enzyme | Accumulates | Classic clue |
|---|---|---|---|
| Tay-Sachs | Hexosaminidase A | GM2 ganglioside | Cherry-red spot, NO HSM, startle |
| Niemann-Pick | Sphingomyelinase | Sphingomyelin | Cherry-red spot + HSM, foam cells |
| Gaucher | Glucocerebrosidase | Glucocerebroside | Crumpled-paper macrophage, bone crises |
| Fabry (XLR) | α-galactosidase A | Ceramide trihexoside (Gb3) | Angiokeratomas, acroparesthesias |
| Krabbe | Galactocerebrosidase | Galactocerebroside/psychosine | Globoid cells, neuropathy, optic atrophy |
| MLD | Arylsulfatase A | Sulfatides | Central + peripheral demyelination, ataxia |
| Hurler (MPS I) | α-L-iduronidase | Heparan + dermatan sulfate | Corneal clouding, coarse facies |
| Hunter (MPS II, XLR) | Iduronate-2-sulfatase | Heparan + dermatan sulfate | NO corneal clouding, aggression |
- MPS = defective GAG breakdown → heparan + dermatan sulfate accumulate; screen with urine GAGs.
- Hurler (MPS I) — α-L-iduronidase; autosomal recessive; severe: coarse ("gargoyle") facies, corneal clouding, hepatosplenomegaly, dysostosis multiplex, developmental delay, early death. Rx: HSCT and/or ERT (laronidase).
- Hunter (MPS II) — iduronate-2-sulfatase; X-linked recessive; milder, NO corneal clouding, aggressive behavior; ERT = idursulfase.
- Sphingolipidosis vs MPS on the exam: neuro-predominant + cherry-red spot / lipid-laden macrophage → sphingolipidosis; coarse facies + skeletal + corneal + positive urine GAGs → MPS.
- Pattern shortcuts: cherry-red macula → Tay-Sachs/Niemann-Pick; angiokeratoma → Fabry; globoid cell → Krabbe; crumpled tissue-paper macrophage → Gaucher.
Vignette: A 6-month-old Ashkenazi Jewish infant was normal at birth but now loses motor milestones, has an exaggerated startle to noise and brisk reflexes; fundoscopy shows a cherry-red macula; the abdomen is soft with no organomegaly. → Dx: Tay-Sachs (β-hexosaminidase A deficiency, GM2 accumulation). Confirm with leukocyte/serum hexosaminidase A assay. No disease-modifying therapy — supportive care.
Same cherry-red spot, but WITH hepatosplenomegaly + foam cells on marrow → Niemann-Pick (sphingomyelinase deficiency). HSM is the discriminator.
Vignette: A patient has hepatosplenomegaly, pancytopenia, and bone pain / avascular necrosis; marrow macrophages resemble crumpled tissue paper. → Dx: Gaucher (glucocerebrosidase deficiency). Next best step: β-glucosidase (glucocerebrosidase) enzyme assay; treat with enzyme replacement (imiglucerase).
Vignette: A 22-year-old man reports episodic burning pain in the hands and feet, clusters of dark-red papules over the umbilicus and groin (angiokeratomas), reduced sweating, and new proteinuria; a maternal uncle died of renal failure. → Dx: Fabry (α-galactosidase A deficiency, Gb3 accumulation; X-linked recessive). Next best step: confirm α-galactosidase A activity, then start enzyme replacement (agalsidase); monitor renal and cardiac function.
Vignette: A 2-year-old boy has coarse ("gargoyle") facies, corneal clouding, hepatosplenomegaly, umbilical hernia, and developmental delay. → Dx: Hurler (MPS I) — α-L-iduronidase deficiency (AR). Twist: if corneas are CLEAR with aggressive behavior in an X-linked pedigree → Hunter (MPS II) — iduronate-2-sulfatase deficiency.
- Tay-SaX = heXosaminidase A (the X's link them).
- "No man picks (Niemann-Pick) his nose with his sphinger (sphingomyelinase)."
- Hunters aim for the X and can see their prey: Hunter = X-linked, NO corneal clouding (+ aggressive); Hurler "hurls" — worse, AR, corneal clouding present.
- Fabry = Feet burning + X-linked (acroparesthesias, angiokeratomas).
- Gaucher cells = crumpled/wrinkled tissue paper.
- Krabbe = globoid cells (galactocerebrosidase).
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