MPS III follows an autosomal recessive inheritance pattern and results from pathogenic variants in one of four genes encoding enzymes within the heparan sulfate degradation pathway. MPS IIIA is caused by variants in SGSH (encoding heparan sulfate sulfatase); MPS IIIB by variants in NAGLU (encoding alpha-N-acetylglucosaminidase); MPS IIIC by variants in HGSNAT (encoding heparan-alpha-glucosaminide N-acetyltransferase); and MPS IIID by variants in GNS (encoding N-acetylglucosamine-6-sulfatase). Enzyme deficiency results in lysosomal accumulation of partially degraded heparan sulfate, which triggers neuroinflammatory and neurodegenerative cascades. Documented genotype-phenotype correlations exist for SGSH and NAGLU; certain variants, particularly premature termination variants, are associated with more rapidly progressing phenotypes.

Diagnosis of MPS III is established through enzymatic assay demonstrating deficient activity of the relevant lysosomal enzyme in leukocytes, fibroblasts, or plasma, combined with elevated urinary heparan sulfate detected by quantitative glycosaminoglycan analysis. Molecular genetic testing identifying biallelic pathogenic variants in SGSH, NAGLU, HGSNAT, or GNS confirms the specific subtype. Following initial diagnosis, comprehensive evaluation encompasses neurological examination, brain MRI, EEG if seizures are a concern, developmental and speech-language assessment, audiogram, echocardiogram, abdominal ultrasound for hepatosplenomegaly, skeletal survey, pulmonary evaluation for sleep apnea, and bone mineral density assessment. Clinical geneticist involvement supports subtype classification and variant interpretation.

No disease-modifying therapy is currently approved for MPS III; management is supportive and addresses specific manifestations. Seizures are managed with antiseizure medications as determined by neurology. Neurodevelopmental delays are addressed through speech therapy, physical therapy, and occupational therapy, though gains may be transient given disease progression. Behavioral and psychiatric symptoms, including hyperactivity, have been managed with risperidone and antipsychotic agents. Sleep disturbances have been treated with melatonin or other medications. Musculoskeletal issues are addressed with orthopedic care, hydrotherapy, and vitamin D therapy in the context of low bone mineral density. Hearing loss has been managed with ear tube insertion or hearing aids. Gastrostomy tube placement is documented for feeding difficulties in advanced disease stages.

MPS III is a progressive, life-limiting condition with a trajectory toward severe neurological deterioration and premature death. Developmental advances achieved through therapeutic interventions are typically short-lived due to the inexorable neurodegenerative course. The rate of progression varies by subtype and specific genotype; individuals with premature termination variants in SGSH or NAGLU are documented to have more rapidly progressing disease, while certain missense variants are associated with attenuated or slowly progressing phenotypes. Most affected individuals experience profound intellectual disability and loss of functional independence during childhood or adolescence. Survival into early to mid-adulthood has been documented, though the condition universally results in premature death. Subtype-specific genotype-phenotype data inform expectations regarding disease trajectory.

Active investigation into MPS III encompasses nine registered clinical trials as of current records. Ongoing studies include a Phase I/II/III gene transfer trial of scAAV9.U1a.hSGSH (NCT02716246, Ultragenyx Pharmaceutical Inc.) targeting MPS IIIA, with recruitment ongoing through 2029. A Phase I study of DNL126, an enzyme replacement therapeutic candidate for MPS IIIA developed by Denali Therapeutics, is active (NCT06181136). Gene therapy using modified autologous hematopoietic stem cells for MPS IIIA is under investigation at the University of Manchester (NCT04201405). A natural history study of MPS IIIC (NCT05825131, Phoenix Nest) is actively recruiting to characterize disease progression. These efforts represent multiple therapeutic modalities including gene therapy, enzyme-based approaches, and natural history characterization to support future trial endpoints.