MPS I

MPS I. It is a rare, inherited lysosomal storage disease caused by deficient activity of the enzyme alpha-L-iduronidase. This deficiency leads to the accumulation of complex sugar molecules called glycosaminoglycans within lysosomes of cells throughout the body, resulting in progressive cellular damage and multi-organ dysfunction. The disorder exists on a clinical spectrum historically divided into three subtypes—severe Hurler syndrome, intermediate Hurler-Scheie syndrome, and attenuated Scheie syndrome—though these represent a continuum of disease severity.

Signs and symptoms

Clinical manifestations are multisystemic and progressive. Skeletal abnormalities, collectively termed dysostosis multiplex, include kyphosis, scoliosis, and distinctive claw hand deformities. Characteristic coarse facial features develop, often with macrocephaly and prominent frontal bossing. Ocular involvement features progressive corneal clouding and potential retinal degeneration. Recurrent otitis media and conductive hearing loss are common due to middle ear effusions and structural changes. Cardiorespiratory complications involve cardiac valve disease, particularly of the mitral valve and aortic valve, alongside airway obstruction from soft tissue thickening. Abdominal findings include hepatosplenomegaly and umbilical or inguinal hernias. Neurological involvement in severe forms can lead to developmental delay, hydrocephalus, and spinal cord compression from cervical myelopathy related to odontoid hypoplasia.

Genetics

The condition follows an autosomal recessive pattern of inheritance, requiring pathogenic variants in both alleles of the IDUA gene located on chromosome 4. The IDUA gene encodes the enzyme alpha-L-iduronidase, which is essential for the stepwise degradation of the glycosaminoglycans dermatan sulfate and heparan sulfate. Over 200 different disease-causing variants have been identified in the Human Gene Mutation Database, including missense, nonsense, and frameshift types. The specific combination of variants an individual inherits largely determines the residual enzyme activity and, consequently, the position on the clinical severity spectrum, though genotype-phenotype correlation is not always perfectly predictive.

Diagnosis

Diagnosis is confirmed through biochemical and molecular testing. The primary screening test involves measuring the activity of alpha-L-iduronidase in leukocytes or cultured skin fibroblasts via an enzyme assay, which typically shows markedly reduced levels. Elevated excretion of glycosaminoglycans, specifically dermatan sulfate and heparan sulfate, in a urinary glycosaminoglycan analysis provides supportive evidence. Definitive diagnosis is achieved through genetic testing of the IDUA gene to identify biallelic pathogenic variants. Prenatal diagnosis is available for at-risk pregnancies through chorionic villus sampling or amniocentesis to assay enzyme activity or perform molecular analysis. Differential diagnosis includes other mucopolysaccharidoses such as MPS II and MPS VI, as well as multiple sulfatase deficiency.

Treatment

Management involves disease-specific therapies and comprehensive supportive care. Enzyme replacement therapy with intravenous recombinant laronidase is a standard treatment that helps reduce non-neurological somatic symptoms. For severe forms diagnosed early, hematopoietic stem cell transplantation from a compatible donor, often following myeloablative conditioning, can halt cognitive decline and improve survival by providing a source of enzyme-producing cells. Supportive care is multidisciplinary, involving specialists from orthopedic surgery for joint and spinal issues, cardiology for valve disease, otolaryngology for airway and hearing problems, and ophthalmology for corneal clouding. Surgical interventions may include adenoidectomy, tonsillectomy, ventriculoperitoneal shunt placement for hydrocephalus, and spinal fusion for instability. Ongoing clinical trials investigate advanced modalities like gene therapy.

Prognosis

The prognosis varies widely across the clinical spectrum and is heavily influenced by the timing of diagnosis and intervention. Untreated severe Hurler syndrome typically leads to progressive neurological deterioration and death in early childhood, often from cardiorespiratory failure. With successful hematopoietic stem cell transplantation performed before significant cognitive decline, individuals can achieve long-term survival into adulthood, though with significant residual somatic disease. Those with attenuated forms like Scheie syndrome often have normal intelligence and can survive into late adulthood, but face substantial morbidity from orthopedic, cardiac, and ocular complications requiring lifelong management. The advent of enzyme replacement therapy and improved supportive care has positively altered the natural history for many patients.

Category:Lysosomal storage diseases Category:Autosomal recessive disorders Category:Rare diseases