What disease is caused by lysosome malfunction?

Lysosomal Storage Diseases

Lysosomal malfunction causes a heterogeneous group of almost 50 inherited metabolic disorders collectively known as lysosomal storage diseases (LSDs), which result from genetic defects in lysosomal enzymes, receptors, activator proteins, membrane proteins, or transporters. 1

Disease Mechanism and Pathophysiology

LSDs arise from mutations in proteins critical for lysosomal function, leading to progressive intra-lysosomal accumulation of specific undegraded substrates. 1 This accumulation causes:

• Progressive deterioration of cellular and tissue function across multiple organ systems 1
• Disruption of downstream pathways affecting autophagy, phagocytosis, and calcium homeostasis 1
• Decreased lifespan and significant morbidity in most affected patients 1
• Central nervous system involvement in over two-thirds of cases, often representing the most debilitating manifestations 2

Major Categories and Specific Diseases

LSDs are categorized by the type of substrate that accumulates: 1

Mucopolysaccharidoses (MPS)

• MPS types I, II, and VI present with coarse facial features, joint stiffness, organomegaly, and dysostosis multiplex 1
• Sanfilippo disease (MPS III) can result from deficiency in any one of four different hydrolases 1

Sphingolipidoses

• Gaucher disease (beta-glucocerebrosidase deficiency) causes hepatosplenomegaly and bone disease 1, 3
• Fabry disease (alpha-galactosidase A deficiency) is X-linked and affects vascular, renal, and autonomic nervous systems 1
• Tay-Sachs disease (hexosaminidase A deficiency) causes progressive CNS deterioration 1
• Niemann-Pick types A and B (acid sphingomyelinase deficiency) present with hepatosplenomegaly 1
• Krabbe disease (globoid cell leukodystrophy) causes progressive hypotonia and seizures 1
• Metachromatic leukodystrophy presents with progressive neurological decline 1

Glycogen Storage Disorders

• Pompe disease (glycogen storage type II) results from acid alpha-glucosidase deficiency 1, 4

Other LSDs

• Niemann-Pick type C involves defective cholesterol export from lysosomes 5
• GM1- and GM2-gangliosidoses present with cherry-red spot on fundoscopic examination 1

Inheritance Patterns and Epidemiology

• Most LSDs follow autosomal recessive inheritance, except Fabry disease, Hunter syndrome (MPS II), and Danon disease, which are X-linked 1
• Combined prevalence is approximately 1 in 7,000-8,000 live births, though this may underestimate true frequency as more mild and adult-onset forms are identified 1
• Certain populations have higher frequencies of specific disorders: Gaucher, Tay-Sachs, Niemann-Pick type A, and mucolipidosis IV are more common in Ashkenazi Jews 1

Clinical Presentation

Individuals with LSDs typically have no distinctive features at birth but experience progressive worsening as substrates accumulate. 1

Key clinical features include:

• Developmental delay and/or regression in most LSDs 1
• Variable age of onset and severity, with early onset correlating with more severe disease 1
• Multi-systemic involvement affecting visceral organs, neurological function, skeletal system, and hematologic parameters 5

Diagnostic Approach

Diagnosis must be established through specific enzyme assays and mutational analysis in all cases. 1, 6

• Urinary mucopolysaccharides and oligosaccharides are useful for screening but can be normal or nonspecifically elevated in healthy neonates 1, 6
• Biomarker analysis provides additional diagnostic and monitoring tools for specific disorders 1
• Presymptomatic identification occurs through newborn screening, family-based testing after proband identification, carrier screening, or prenatal testing 1, 6

Treatment Considerations

Earlier therapeutic intervention has been shown to reduce disease symptoms and slow progression, emphasizing the critical importance of prompt diagnosis. 1, 7

Available treatments include:

• Enzyme replacement therapy (ERT) for Fabry, Gaucher, Pompe, and certain mucopolysaccharidoses 1, 6
• Hematopoietic stem cell transplantation for select disorders 6
• Substrate reduction therapy as alternative or combination therapy 6, 8

Important Clinical Pitfalls

• Genotype-phenotype correlations are typically not strong: patients with identical mutations may have vastly different clinical presentations, even within families 1
• For X-linked disorders in females, disease severity may be determined primarily by X-chromosomal inactivation patterns 1, 6
• Storage may begin during early embryonic development, meaning irreversible damage can occur before clinical symptoms appear 9