Enzyme Replacement Therapy
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Direct answer
Enzyme replacement therapy works because lysosomal enzymes carrying mannose-6-phosphate are captured from the circulation through M6P receptors on the cell surface and routed to lysosomes — a partial recycling of the normal targeting pathway, exploited in Gaucher disease (imiglucerase and successors), Fabry disease, Pompe disease and several mucopolysaccharidoses. The infusions are lifelong, usually every one to four weeks, and succeed best where disease is systemic and macrophage-accessible: Gaucher type 1 responds dramatically in spleen size, blood counts and bone pain, while neuronopathic disease barely responds because intravenous enzyme does not cross the blood-brain barrier. Cost, antibody formation (worst in CRIM-negative infantile Pompe) and access are the practical constraints — the reason India's National Policy for Rare Diseases 2021 created funded centres of excellence.
What you must remember
- Targeting principle: lysosomal hydrolases are tagged with mannose-6-phosphate in the Golgi by N-acetylglucosamine phosphotransferase — the enzyme missing in I-cell disease; infused recombinant enzyme bearing M6P (or mannose, for macrophage receptors) is endocytosed and delivered to lysosomes.
- Cross-correction: cells can take up enzyme secreted by neighbours through surface M6P receptors (Neufeld's principle) — why even partial correction, or donor macrophages after transplant, nurses a whole tissue.
- Gaucher type 1: macrophage glucocerebrosidase; mannose-terminated imiglucerase, velaglucerase or taliglucerase every two weeks; platelet counts and spleen size respond first, bone disease slowest; chitotriosidase tracks burden; eliglustat (substrate reduction) offers an oral alternative.
- Pompe disease: acid α-glucosidase (alglucosidase alfa) — infantile disease with hypertrophic cardiomyopathy; CRIM-negative infants, who make no residual enzyme, mount antibodies and fare worse without immunomodulation.
- Mucopolysaccharidoses: laronidase (Hurler), idursulfase (Hunter), elosulfase alfa (Morquio A) — glycosaminoglycan excretion falls and organomegaly regresses, but skeleton and central nervous system resist.
- Fabry disease: agalsidase infusions with renal and cardiac endpoints; migalastat, a pharmacological chaperone, suits amenable GLA variants.
- Hard limits: the blood-brain barrier excludes infused enzyme, antibodies neutralise it, infusions provoke reactions, and cost is enormous — which is why haematopoietic stem cell transplantation and gene therapy are pursued for neuronopathic disease.
- India context: the National Policy for Rare Diseases 2021 notifies centres of excellence (AIIMS New Delhi among them) with public funding for high-cost therapy — the access answer to quote in a viva.
Worked example: splenomegaly in a young adult
A 24-year-old presents with a massive spleen, platelets of 62,000 per microlitre and bone pain. Marrow shows Gaucher cells — macrophages stuffed with glucocerebroside, cytoplasm resembling crumpled tissue paper. Leucocyte β-glucosidase assay confirms the deficiency; chitotriosidase, macrophage-derived, is markedly raised and will serve as the response marker. Enzyme replacement begins as a two-weekly infusion: within months platelets recover and the spleen shrinks; the skeletal response lags a year behind, so magnetic resonance monitoring continues. The teaching point is why this subtype was chosen as therapy's first success — no neuronopathic component, target cells are macrophages reachable from blood, and the mannose receptor does the delivery. Had the patient been a child with developmental regression, the same chemistry would demand a different conversation: transplant, gene therapy or supportive care, because the infused enzyme cannot cross into brain.
How the exam frames it
Expect "principle of cross-correction", "why ERT fails in neuronopathic disease", and one-liners matching disease to drug: imiglucerase-Gaucher, alglucosidase-Pompe, laronidase-Hurler, agalsidase-Fabry. The Indian convention adds the policy dimension — naming the National Policy for Rare Diseases 2021 and its centres of excellence signals awareness that biochemistry meets household economics here. The subtle trap: Hurler disease is best treated by stem cell transplantation before age two (central nervous system benefit), with ERT used as a bridge — sequencing, not either/or.
Frequently asked questions
How does infused enzyme reach lysosomes?
Mannose-6-phosphate or mannose residues on the recombinant enzyme bind receptors on the cell surface, triggering endocytosis and lysosomal delivery.
Why is enzyme replacement ineffective for brain disease?
The intravenously infused enzyme does not cross the blood-brain barrier, so neuronopathic forms need transplantation or gene therapy.
What is CRIM-negative Pompe disease?
Patients with no residual enzyme protein develop high antibody titres against the infused enzyme and respond poorly unless immunomodulated.
Which Indian policy funds these therapies?
The National Policy for Rare Diseases 2021, which notified centres of excellence with financial support for high-cost treatment.
What is cross-correction?
Uptake of secreted lysosomal enzyme by neighbouring cells through surface M6P receptors — the principle, explained by Neufeld, that makes enzyme therapy and transplantation work.