Sphingolipidoses

• Lippincott Illustrated Reviews: Biochemistry, 8e, p. 594-596
• Robbins & Kumar Basic Pathology, p. 118-120

Biochemical Background

Complete Sphingolipid Degradation Pathway with Diseases

General Properties (Shared by All Sphingolipidoses)

1. Single enzyme defect - only one specific sphingolipid accumulates per disease
2. Progressive - clinical course is relentlessly progressive
3. Autosomal recessive inheritance - except Fabry disease, which is X-linked
4. Phenotypic variability - different clinical subtypes exist within a single disease (e.g., Gaucher types 1, 2, 3)
5. Population genetics - Gaucher, Tay-Sachs, and Niemann-Pick diseases are enriched in Ashkenazi Jewish populations; Tay-Sachs also has higher frequency in Irish-American, French-Canadian, and Louisiana Cajun populations
6. Diagnosis - enzyme activity measured in peripheral leukocytes or cultured fibroblasts; DNA testing; prenatal diagnosis via amniocentesis or chorionic villus sampling

Individual Diseases - Summary Table

Key Diseases in Detail

1. Gaucher Disease - Most Common Sphingolipidosis

• Enzyme replacement therapy (ERT) - recombinant glucocerebrosidase infusions (lifelong)
• Substrate reduction therapy - miglustat (oral inhibitor of glucosylceramide synthase)
• Bone marrow transplantation (macrophages are hematopoietic in origin)

2. Niemann-Pick Disease

• Type A: Infantile onset, severe neurodegeneration, hepatosplenomegaly, cherry-red macula - usually fatal by age 3
• Type B: Milder, organomegaly without CNS involvement

3. Tay-Sachs Disease

• Deficiency of β-hexosaminidase A (α subunit mutation) - GM2 accumulates exclusively in neurons
• Infants present by 6 months: poor feeding, lethargy, floppiness, then progressive neurodegeneration
• Cherry-red spot on fundoscopy - the fovea appears red because it lacks the opacified ganglion cells that surround it
• Exaggerated startle response, blindness, seizures
• Death usually by age 3-5
• NO visceral involvement (distinguishes from Sandhoff)

4. Fabry Disease - Only X-linked Sphingolipidosis

• α-Galactosidase A deficiency; X-linked recessive (females can be carriers with variable expression)
• Globotriaosylceramide (Gb3) accumulates in vascular endothelial cells throughout the body (brain, heart, kidneys, skin)
• Triad: burning/painful neuropathy in extremities, angiokeratomas (red-purple skin rash), progressive renal and cardiac failure
• ERT available (agalsidase alfa or beta)

5. Krabbe Disease (Globoid Cell Leukodystrophy)

• β-Galactocerebrosidase deficiency - galactocerebroside accumulates in white matter
• Pathology: globoid bodies - glycolipid-laden macrophages in white matter
• Near-total loss of myelin; mental and motor deterioration, blindness, deafness

6. Metachromatic Leukodystrophy (MLD)

• Arylsulfatase A deficiency (or saposin B activator protein deficiency)
• Sulfatides accumulate in CNS and peripheral nerves
• Nerves stain yellow-brown (metachromatic) with cresyl violet dye
• Progressive demyelination - peripheral neuropathy → cognitive decline → paralysis, dementia

7. Farber Disease

• Ceramidase deficiency - ceramide accumulates
• Classic triad: painful progressive joint deformity, subcutaneous lipogranulomas, hoarse cry (laryngeal involvement)
• Cherry-red macula; tissue granulomas

Diagnosis

Classification within Lysosomal Storage Diseases

Recent Evidence

Sphingolipidoses

Biochemical Basis

Complete Degradation Pathway and Disease Map

General Properties Shared by All Sphingolipidoses

1. Single substrate accumulates - only the specific substrate of the deficient enzyme accumulates per disease
2. Normal synthesis rate - the lipid continues to be made normally; only catabolism is blocked
3. Progressive - relentlessly progressive course
4. Autosomal recessive - all sphingolipidoses are AR except Fabry disease, which is X-linked recessive
5. Phenotypic and allelic variability - multiple disease subtypes from different mutations in the same gene (e.g. Gaucher types 1, 2, 3)
6. Population clustering - Gaucher, Tay-Sachs, and Niemann-Pick types A/B are enriched in Ashkenazi Jewish populations; Tay-Sachs also has increased frequency in Irish-American, French-Canadian, and Louisiana Cajun populations
7. Secondary cellular injury - beyond storage, macrophage activation and cytokine release (IL-1, IL-6, TNF) cause additional organ damage
8. Impaired autophagy - lysosomal dysfunction also impairs autophagy, leading to accumulation of dysfunctional mitochondria and polyubiquitinated proteins, which triggers free radical generation and apoptosis

Disease-by-Disease Summary Table

Individual Diseases in Detail

1. Tay-Sachs Disease

• Onset: 3-6 months - motor weakness, poor feeding, lethargy, floppiness
• Progressive: deafness, blindness, spasticity → rigidity
• Exaggerated startle response (hyperekplexia)
• Death by age 2-3 years from respiratory infections
• No hepatosplenomegaly (distinguishes from Sandhoff)
• Carrier frequency in Ashkenazi Jews: 1 in 30; disease incidence 1:3600

2. Gaucher Disease - Most Common Sphingolipidosis

• ERT - recombinant glucocerebrosidase (imiglucerase, velaglucerase); mannose-6-phosphate modification targets enzyme to macrophage lysosomes
• Substrate reduction therapy - miglustat (oral inhibitor of glucosylceramide synthase)
• Bone marrow transplantation (curative in principle; macrophages are hematopoietic)
• Gene therapy (emerging - HSCs engineered to express GBA)

3. Niemann-Pick Disease

• Type A: Severe ASM deficiency; sphingomyelin accumulates in macrophages (liver, spleen, bone marrow, lungs) AND neurons. Macrophage cytoplasm shows fine vacuolation ("foam cells"). EM shows "zebra bodies" (concentric lamellated myelin figures). Cherry-red macula present. Fatal by age 3.
• Type B: Milder; organomegaly without CNS involvement; compatible with survival into adulthood

4. Fabry Disease

1. Burning/painful neuropathy in hands and feet (acroparesthesias) - often the earliest symptom
2. Angiokeratomas - red-purple skin rash, usually over the bathing-trunk distribution
3. Progressive renal failure + cardiomyopathy/cardiac arrhythmias

5. Krabbe Disease (Globoid Cell Leukodystrophy)

6. Metachromatic Leukodystrophy (MLD)

7. Farber Disease (Lipogranulomatosis)

• Painful progressive joint deformity
• Subcutaneous lipid-laden granulomas (nodules)
• Hoarse cry (laryngeal granuloma)

Diagnosis

Lysosomal Storage Disease Classification (Context)

Recent Evidence

• Neuroinflammation as a target (PMID 40992103): glial activation (microglia, astrocytes) is a major driver of CNS injury in sphingolipidoses beyond simple substrate accumulation - representing a new therapeutic angle for Tay-Sachs, Krabbe, and MLD
• Gene therapy (PMID 40782413): HSC gene therapy is in trials for several LSDs including Krabbe and MLD, with CNS penetration a key challenge
• Current and emerging therapies (PMID 39826077): substrate reduction therapy, pharmacological chaperones, and mRNA/gene therapies are expanding the treatment landscape beyond ERT

Fatty Liver Disease (Hepatic Steatosis)

Nomenclature (2023 Update)

Epidemiology

• 20-30% of the general population in North America and Europe has NAFLD/MASLD
• 60-75% of obese or diabetic patients have NAFLD; 20-25% of these have NASH/MASH
• Now the most common cause of chronic liver disease in the United States, affecting up to 30-46% of the population
• Expected to be (or already is) the leading indication for liver transplantation, surpassing viral hepatitis
• NASH is a major cause of cryptogenic cirrhosis (fat disappears as fibrosis advances, leaving only cirrhosis without an apparent etiology)
• Strong driver of the rising incidence of HCC in Western countries - Mulholland & Greenfield's Surgery, 7e; Tietz Textbook of Laboratory Medicine, 7e

Etiology and Risk Factors

Metabolic / Primary Causes (MASLD)

• Obesity (especially central/visceral adiposity)
• Type 2 diabetes mellitus / insulin resistance
• Dyslipidemia (hypertriglyceridemia, low HDL)
• Metabolic syndrome - nearly half of individuals meeting metabolic syndrome criteria have NAFLD
• 90% of NAFLD patients have at least one of these risk factors; 30% have three or more

Secondary Causes

Pathogenesis

The "Two-Hit" Model (Classic)

1. First hit - insulin resistance → hepatic steatosis (sensitizes liver to further injury)
2. Second hit - oxidative stress, inflammation, lipotoxicity → steatohepatitis → fibrosis

Biochemical Mechanisms of Fat Accumulation

• Oxidized by mitochondria (β-oxidation)
• Esterified into TG and exported as VLDL
• Incorporated into phospholipids and cholesteryl esters

Progression to Steatohepatitis (MASH/NASH)

• Lipotoxicity → mitochondrial dysfunction, ER stress
• Unfolded protein response activation
• Inflammasome activation → IL-1β, IL-18 secretion
• Gut-derived bacterial endotoxin (LPS) → hepatic stellate cell activation → fibrosis

Histopathology

Simple Steatosis (MASLD without MASH)

Steatohepatitis (MASH/NASH) - requires ALL of:

1. Steatosis (macrovesicular, zone 3 predominant)
2. Hepatocyte ballooning degeneration - swollen, rarefied cytoplasm
3. Lobular inflammation - mixed inflammatory infiltrate
4. Mallory-Denk bodies - eosinophilic cytoplasmic inclusions (aggregates of cytokeratins 8/18)
5. Pericellular/perisinusoidal fibrosis (zone 3, "chicken-wire" pattern) on trichrome stain

Grading and Staging (NAS - NAFLD Activity Score)

Disease Spectrum and Natural History

Simple Steatosis (MASLD)
     ↓ (~20% progress over 10+ years)
Steatohepatitis (MASH/NASH)
     ↓ (~20% progress)
Fibrosis (F1→F4)
     ↓
Cirrhosis
     ↓
HCC / Liver failure / Transplant

• Up to 20% of patients with NASH progress to cirrhosis
• >30% macrovesicular steatosis in donor livers correlates with increased graft failure risk in transplantation

Diagnosis

Clinical Presentation

• Usually asymptomatic - discovered incidentally on imaging or elevated LFTs
• May have vague RUQ discomfort or fatigue
• Signs of metabolic syndrome (obesity, hypertension, dyslipidemia, T2DM)
• Advanced disease: signs of portal hypertension, hepatomegaly

Laboratory Tests

Non-Invasive Fibrosis Scores

Imaging

• Ultrasound - increased echogenicity ("bright liver"); sensitive for >30% steatosis
• CT - fatty liver appears hypodense relative to spleen (normal liver HU > spleen); liver:spleen ratio <0.8 indicates steatosis (see CT image above)
• MRI-PDFF (proton density fat fraction) - most accurate non-invasive quantification of hepatic fat
• FibroScan/Transient elastography - liver stiffness measurement for fibrosis staging; CAP (controlled attenuation parameter) for steatosis

Liver Biopsy

• Distinguishing simple steatosis from NASH
• Staging fibrosis
• Excluding other diagnoses

Management

Lifestyle Modification (First-line for all)

• Weight loss 5-10% - associated with significant histologic improvement in steatosis and inflammation
• >10% weight loss - may improve fibrosis
• Aerobic exercise - reduces hepatic fat independent of weight loss
• Mediterranean diet, avoidance of fructose/high-sugar beverages
• Alcohol avoidance - even modest alcohol accelerates fibrosis in NASH

Pharmacological Therapy

Bariatric Surgery

• For patients with obesity (BMI >40 or >35 with comorbidities) who fail lifestyle modification
• Produces dramatic improvement or resolution of NAFLD/NASH
• Improves fibrosis in most patients

Liver Transplantation

• Indicated for NASH cirrhosis with decompensation or HCC
• NAFLD/NASH now a leading or the leading indication for liver transplantation in the US
• Post-transplant: NAFLD can recur in the graft; metabolic syndrome management is essential
• Donor livers with >30% macrovesicular steatosis have increased graft failure risk

Alcoholic Fatty Liver Disease (ALD)

• Acetaldehyde toxicity
• Generation of reactive oxygen species
• Altered NAD⁺/NADH ratio → impaired fatty acid oxidation → fat accumulation

Special Situations

Cardiovascular Relevance