Mucolipidosis IV

Mucolipidosis IV is an autosomal recessive lysosomal disorder due to biallelic variants in MCOLN1, with psychomotor delay, corneal clouding, retinal degeneration, and lifelong achlorhydria with iron deficiency anaemia. It is relatively common in Ashkenazi Jewish populations through two founder variants.

Pathophysiology. MCOLN1 encodes TRPML1, a lysosomal cation channel needed for lysosomal exocytosis, trafficking and autophagy. Loss of the channel leads to lipid and mucopolysaccharide storage in neurons and epithelium.

Gastric acid secretion is absent, which raises serum gastrin.

Neurological findings are static or slowly progressive for years.

• Hypotonia and developmental delay from infancy; many never walk or speak, while mild forms speak
• Spastic quadriparesis later
• Corneal clouding from infancy, with photophobia
• Retinal degeneration, optic atrophy, and strabismus
• Iron deficiency anaemia with achlorhydria and raised gastrin
• Feeding difficulty and constipation
• Recurrent respiratory infections
• Hand clasping and stereotypies

• MCOLN1 sequencing with deletion and duplication analysis. Two variants account for most cases in Ashkenazi Jewish individuals.
• Serum gastrin: markedly raised, with achlorhydria on gastric pH testing.
• Full blood count, ferritin and iron studies.
• Ophthalmology: corneal clouding, retinal changes, and electroretinography.
• Brain MRI: thin corpus callosum and white matter changes.
• Electron microscopy of skin or conjunctiva shows storage bodies.
• Carrier testing in at risk populations.

• Iron deficiency: ferrous sulphate 200 mg (65 mg elemental iron) orally once or twice daily in adults, and elemental iron 3 to 6 mg/kg daily in children. Because acid is absent, absorption is poor, so use intravenous iron (iron sucrose 200 mg intravenously for adults, repeated to a calculated deficit) when oral iron fails.
• Vitamin B12 status: check yearly, and give hydroxocobalamin 1 mg intramuscularly every 3 months if low.
• Eye care: lubricants, treatment of strabismus, corneal surface management, and low vision support. Corneal transplant has had limited success because of recurrent clouding.
• Spasticity: baclofen 0.3 mg/kg daily in divided doses increasing weekly to 2 mg/kg daily (maximum 60 mg), physiotherapy and orthotics.
• Feeding and constipation: feeding therapy, gastrostomy for unsafe swallow, and macrogol 0.4 to 0.8 g/kg daily (adults 1 to 3 sachets).
• Development and education: early intervention, communication aids and special education.
• Disease modifying therapy: none is established; research on drugs affecting lysosomal function is in progress.
• Genetic counselling: a 25% recurrence risk in each pregnancy, with carrier screening and prenatal diagnosis in at risk populations.

Special groups.

• Patients with persistent anaemia despite oral iron: arrange intravenous iron and check for gastrointestinal blood loss.

Monitoring. Full blood count and ferritin every 3 to 6 months. Eye review yearly, vitamin B12 yearly, and development every 6 to 12 months.

Referral and escalation. Refer to clinical genetics, metabolic medicine, ophthalmology and haematology. Urgent for symptomatic anaemia or sudden visual loss.