Mucopolysaccharidosis I

Mucopolysaccharidosis I. Mucopolysaccharidosis I is a rare, inherited lysosomal storage disease caused by a deficiency of the enzyme alpha-L-iduronidase. This deficiency leads to the accumulation of complex sugar molecules called glycosaminoglycans within lysosomes of various tissues and organs, resulting in progressive cellular damage. The disorder encompasses a clinical spectrum historically divided into three syndromes: severe Hurler syndrome, intermediate Hurler-Scheie syndrome, and attenuated Scheie syndrome.

Signs and symptoms

Clinical manifestations are multisystemic and vary in severity. Common features include coarse facial features, corneal clouding, hepatosplenomegaly, and joint stiffness. Skeletal abnormalities, collectively termed dysostosis multiplex, are prominent and may include kyphosis, scoliosis, and malformed vertebrae. Progressive cardiovascular disease often involves valvular heart disease and coronary artery disease. Upper airway obstruction, sleep apnea, and recurrent otitis media are frequent. Neurological involvement in severe forms includes progressive cognitive impairment, hydrocephalus, and spinal cord compression from atlantoaxial instability or cervical myelopathy.

Genetics

Mucopolysaccharidosis I is inherited in an autosomal recessive manner. The causative gene, IDUA, is located on chromosome 4 at locus 4p16.3. Disease-causing mutations in the IDUA gene lead to deficient activity of the enzyme alpha-L-iduronidase. More than 200 distinct pathogenic variants have been identified, including missense mutations, nonsense mutations, and frameshift mutations. The specific genotype often correlates with residual enzyme activity and influences the phenotype, though prediction can be complex due to compound heterozygosity and other genetic modifiers.

Diagnosis

Diagnosis is suspected based on clinical presentation and confirmed through laboratory testing. Initial screening often involves detecting elevated levels of glycosaminoglycans, specifically dermatan sulfate and heparan sulfate, in the urine. Definitive diagnosis is established by demonstrating deficient alpha-L-iduronidase enzyme activity in leukocytes or fibroblasts cultured from a skin biopsy. Molecular genetic testing of the IDUA gene identifies pathogenic variants and can aid in prognosis and genetic counseling. Prenatal diagnosis is available via amniocentesis or chorionic villus sampling for at-risk pregnancies.

Treatment

Management requires a multidisciplinary team at specialized centers like the Lysosomal Disease Network. The standard of care is enzyme replacement therapy with laronidase, a recombinant form of alpha-L-iduronidase approved by the U.S. Food and Drug Administration and the European Medicines Agency. For severe patients diagnosed early, hematopoietic stem cell transplantation can stabilize cognitive function and improve somatic symptoms. Supportive care is critical and may include surgeries for carpal tunnel syndrome, hernia repair, cardiac valve replacement, and ventriculoperitoneal shunting for hydrocephalus. Ongoing care involves specialists in orthopedics, cardiology, pulmonology, and ophthalmology.

Prognosis

Prognosis is highly variable and depends on the clinical phenotype and timing of intervention. Untreated severe Hurler syndrome typically leads to death in early childhood from cardiorespiratory failure. With successful hematopoietic stem cell transplantation performed before significant cognitive decline, life expectancy and quality of life can be significantly improved, though skeletal and ocular complications often persist. Individuals with attenuated forms like Scheie syndrome may have normal intelligence and survive into adulthood, but they face significant morbidity from valvular heart disease, airway disease, and orthopedic issues. Long-term outcomes continue to be studied through registries like the MPS I Registry.

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