The packet lists SGSH as the known gene for MPS IIIA, with a ClinGen validity classification of definitive, located on chromosome 17. MPS IIIA is therefore caused by variants in SGSH, consistent with the packet's description of deficiency of the enzyme heparan sulfate sulfatase. The packet lists the inheritance pattern as autosomal recessive.

GeneReviews describes rapidly progressing disease in homozygotes or compound heterozygotes for certain SGSH variants, and one missense variant as associated with a slowly progressive course. It attributes the high incidence in the Cayman Islands to a high carrier frequency of an SGSH variant.

According to GeneReviews, formal diagnostic criteria for MPS III have not been established. It lists suggestive clinical findings by age: language and motor delays at ages one to three years; additional behavioral problems including hyperactivity, and sleep disturbances, at ages three to ten years; and intellectual disability, developmental regression, seizures and gait disorders from age ten years. General findings include coarse facies, hirsutism, hepatosplenomegaly, joint stiffness, hearing loss, frequent respiratory and ear infections, and hernias. GeneReviews notes that clinical findings alone are not diagnostic.

Urinary glycosaminoglycan analysis is described as suggestive of an MPS disorder when abnormal but unable to diagnose a specific enzyme deficiency; both methods have reduced sensitivity, and normal results cannot rule out an MPS disorder, particularly MPS III. Imaging findings described include mild dysostosis multiplex, brain MRI changes such as ventriculomegaly and cerebral atrophy, and valvular anomalies on echocardiogram, which may not be present in early life and are not specific to MPS III.

GeneReviews states that the diagnosis is established in a proband with suggestive findings in whom biallelic pathogenic variants in one of the MPS III genes, or deficiency of the respective lysosomal enzyme, has been identified. The packet contains no newborn screening information.

The packet lists no FDA-approved treatment records for MPS IIIA, so approved options are not certified here. GeneReviews states that no treatments are currently clinically available for the primary manifestations of MPS III, and that developmental advances made with therapy may be short-lived because of disease progression.

GeneReviews describes management of individual manifestations. Seizures are described as treated with anti-seizure medication as determined by a neurologist, and developmental delays with supportive speech, physical and occupational therapies. For behavioral issues it describes treatment as determined by a psychiatrist and a physically safe home environment, notes that behavioral therapy is unlikely to be effective, and mentions risperidone for hyperactivity. For sleep disorders it mentions melatonin. It describes physical therapy or hydrotherapy for joint stiffness, ear tubes or hearing aids for hearing loss, gastrostomy tube placement for feeding difficulties, and specialist care from orthopedic, otolaryngology, cardiology, ophthalmology and palliative care clinicians.

GeneReviews describes surveillance including annual developmental and hearing assessments, and neurologic, musculoskeletal, cardiovascular and ocular evaluations as clinically indicated. It also discusses anesthesia in ill-equipped centers and hip surgery, given osteonecrosis, as circumstances of concern.

Among investigational approaches, GeneReviews states that hematopoietic stem cell and umbilical cord blood transplantation are not currently considered effective for MPS III because of a lack of evidence of neurologic benefit, and reports that a trial of intrathecal recombinant human heparan-N-sulfatase in MPS IIIA showed good tolerability and a consistent decrease of heparan sulfate in cerebrospinal fluid. The packet's patient assistance entries relate to a different mucopolysaccharidosis and are not used here.

According to GeneReviews, clinical severity varies within and among the MPS III subtypes and even within families, and individuals may have rapidly or slowly progressing disease. It states that life span is unpredictable but shortened, with death usually secondary to neurologic disease or respiratory tract infections. GeneReviews describes some individuals with more slowly progressive, later-onset or mild disease as living into adulthood, while characterizing this as atypical, and notes that some with an extremely attenuated course present in adulthood with early-onset dementia.

The packet certifies multiple active clinical trials for MPS IIIA, including a recruiting gene transfer study and studies that are active but no longer recruiting of a biologic agent, an agent in pediatric participants, and a gene therapy using modified autologous hematopoietic stem cells. Sponsors include industry and academic organizations. The packet's literature digest describes the publication landscape as dominated by epidemiology and natural history work. Active clinical trials for this condition are listed on ClinicalTrials.gov.