HEXA Disorders

Clinical characteristics: HEXA disorders are best considered as a disease continuum based on the amount of residual beta-hexosaminidase A (HEX A) enzyme activity. This, in turn, depends on the molecular characteristics and biological impact of the HEXA pathogenic variants. HEX A is necessary for degradation of GM2 ganglioside; without well-functioning enzymes, GM2 ganglioside builds up in the lysosomes of brain and nerve cells.

The classic clinical phenotype is known as Tay-Sachs disease (TSD), characterized by progressive weakness, loss of motor skills beginning between ages three and six months, decreased visual attentiveness, and increased or exaggerated startle response with a cherry-red spot observable on the retina followed by developmental plateau and loss of skills after eight to ten months. Seizures are common by 12 months with further deterioration in the second year of life and death occurring between ages two and three years with some survival to five to seven years.

Subacute juvenile TSD is associated with normal developmental milestones until age two years, when the emergence of abnormal gait or dysarthria is noted followed by loss of previously acquired skills and cognitive decline. Spasticity, dysphagia, and seizures are present by the end of the first decade of life, with death within the second decade of life, usually by aspiration.

Late-onset TSD presents in older teens or young adults with a slowly progressive spectrum of neurologic symptoms including lower-extremity weakness with muscle atrophy, dysarthria, incoordination, tremor, mild spasticity and/or dystonia, and psychiatric manifestations including acute psychosis. Clinical variability even among affected members of the same family is observed in both the subacute juvenile and the late-onset TSD phenotypes.

Diagnosis/testing: The diagnosis of a HEXA disorder is established in a proband with abnormally low HEX A activity on enzyme testing and biallelic pathogenic variants in HEXA identified by molecular genetic testing. Targeted analysis for certain pathogenic variants can be performed first in individuals of specific ethnicity (e.g., French Canadian, Ashkenazi Jewish). Enzyme testing of affected individuals identifies absent to near-absent HEX A enzymatic activity in the serum, white blood cells, or other tissues in the presence of normal or elevated activity of the beta-hexosaminidase B enzyme. Pseudodeficiency refers to an in vitro phenomenon caused by specific HEXA variants that renders the enzyme unable to process the synthetic (but not the natural) GM2 substrates, and leads to false positive enzyme testing results.

Management: Treatment of manifestations: Treatment is mostly supportive and directed to providing adequate nutrition and hydration, managing infectious disease, protecting the airway, and controlling seizures. The treatment for the subacute juvenile and late-onset Tay-Sachs phenotypes is directed to providing the services of a physiatrist and team of physical, occupational, and speech therapists for maximizing function and providing aids for activities of daily living.

Agents/circumstances to avoid: Positioning that increases aspiration risk during feedings and seizure medication dosages that result in excessive sedation for those with acute infantile TSD; situations that increase the likelihood of contractures or pressure sores, such as extended periods of immobility; circumstances that exacerbate the risk of falls (i.e., walking on uneven or unstable surfaces) in those with subacute juvenile TSD; psychiatric medications that have been associated with disease worsening, including haloperidol, risperidone, and chlorpromazine.

Genetic counseling: Acute infantile Tay-Sachs disease (TSD), subacute juvenile TSD, and late-onset TSD (comprising the clinical spectrum of HEXA disorders) are inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Heterozygotes (carriers) are asymptomatic. Once both HEXA pathogenic variants have been identified in an affected family member, targeted analysis for the specific familial variants can be used for carrier testing in at-risk relatives. Molecular genetic testing and/or HEX A enzyme testing can be used for carrier detection in individuals who do not have a family history of TSD. If both members of a reproductive couple are known to be heterozygous for a HEXA pathogenic variant, molecular genetic prenatal testing and preimplantation genetic testing for the HEXA pathogenic variants identified in the parents are possible.