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.

Suggestive Findings

Acute infantile Tay-Sachs disease should be suspected in infants with the following clinical findings:

• Progressive weakness and loss of motor skills beginning between ages three and six months
• Decreased attentiveness
• An increased or exaggerated startle response
• A cherry-red spot of the fovea centralis of the macula of the retina
• A normal-sized liver and spleen
• Generalized muscular hypotonia with sustained ankle clonus and hyperreflexia
• Onset of seizures beginning around age 12 months
• Progressive macrocephaly with proportionate ventricular enlargement on neuroimaging beginning at age 18 months

Subacute juvenile Tay-Sachs disease should be suspected in individuals with the following clinical findings:

• A period of normal development until ages two to five years followed by a plateauing of skills and then loss of previously acquired developmental skills
• Progressive spasticity resulting in loss of independent ambulation
• Progressive dysarthria, drooling, and eventually absent speech
• Normal-sized liver and spleen
• Onset of seizures
• Progressive global brain atrophy on neuroimaging [Nestrasil et al 2018]

Late-onset Tay-Sachs disease should be suspected in individuals with the following clinical findings:

• Onset of symptoms in teens or adulthood
• Progressive neurogenic weakness of antigravity muscles in the lower extremities and frequent falls
• Dysarthria, tremor, and incoordination
• Acute psychiatric manifestations including psychosis (which can be the initial manifestation of disease)
• Isolated cerebellar atrophy on neuroimaging

Establishing the Diagnosis

The diagnosis of a HEXA disorder is established in a proband with abnormally low HEX A activity on enzyme testing and biallelic pathogenic (or likely pathogenic) variants in HEXA identified by molecular genetic testing (see Table 1).

Note: (1) Per ACMG/AMP variant interpretation guidelines, the terms "pathogenic variant" and "likely pathogenic variant" are synonymous in a clinical setting, meaning that both are considered diagnostic and can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this GeneReview is understood to include any likely pathogenic variants. (2) Identification of a heterozygous HEXA variant of uncertain significance does not establish or rule out the diagnosis.

HEX A enzymatic activity testing. Testing 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 (HEX B) enzyme [Hall et al 2014].

• Individuals with acute infantile TSD have no or extremely low HEX A enzymatic activity.
• Individuals with subacute juvenile or late-onset TSD have some minimal residual HEX A enzymatic activity.

Note: The enzyme HEX A is a heterodimer of one alpha subunit and one beta subunit (encoded by the genes HEXA and HEXB, respectively); the enzyme HEX B, on the other hand, is a homodimer composed of two beta subunits. Only HEX A is able to degrade GM2 ganglioside.

Note: 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.

Molecular genetic testing approaches can include a combination of gene-targeted testing (single-gene testing, multigene panel) and comprehensive genomic testing (exome sequencing, exome array, genome sequencing) depending on the phenotype.

Gene-targeted testing requires that the clinician determine which gene(s) are likely involved, whereas genomic testing does not. Because the phenotype of HEXA disorders is broad, infants with the distinctive findings described in Suggestive Findings are likely to be diagnosed using gene-targeted testing (see Option 1), whereas those (especially older individuals) with a phenotype indistinguishable from many other disorders presenting later in life with neurodegeneration or developmental regression are more likely to be diagnosed using comprehensive genomic testing (see Option 2).

Option 2

When the phenotype is indistinguishable from many other inherited disorders characterized by a slowly progressive neurodegeneration, comprehensive genomic testing, which does not require the clinician to determine which gene(s) are likely involved, is the best option. Exome sequencing is most commonly used; genome sequencing is also possible.

If exome sequencing is not diagnostic, exome array (when clinically available) may be considered to detect (multi)exon deletions or duplications that cannot be detected by exome sequence analysis.

For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Table 1.

Molecular Genetic Testing Used in HEXA Disorders

Clinical Description

The clinical phenotype of HEXA disorders comprises a continuum including acute infantile, subacute juvenile, and late-onset Tay-Sachs disease. Although classification into subtypes is somewhat arbitrary, it is helpful in understanding the variation observed in the timing of disease onset, presenting symptoms, rate of progression, and longevity.

While case reports of individuals abound, there is a paucity of prospective natural history studies for Tay-Sachs disease delineating the progression of disease subtypes over time.

Subtypes of HEXA disorders include the following phenotypes:

• Acute infantile Tay-Sachs disease with onset before six months, rapid progression, and death generally before age five years
• Subacute juvenile Tay-Sachs disease with later onset and survival into late childhood or adolescence
• Late-onset Tay-Sachs disease with long-term survival. Affected individuals may present with various phenotypes including lower motor neuronopathy with progressive lower-extremity weakness, atrophy and fasciculations, progressive dystonia, spinocerebellar deficits, dysarthria, and/or psychosis.

Neuropathology

Children with the acute infantile form of TSD have excessive and ubiquitous neuronal glycolipid storage (≤12% of the brain dry weight), of which the enormous predominance is the specific glycolipid GM2 ganglioside. Individuals with the adult-onset forms have less accumulation of glycolipid; it may even be restricted to specific brain regions. For example, in LOTS the neocortex may be spared, while the hippocampus, brain stem nuclei, and the spinal cord are markedly affected [Gravel et al 2001].

Genotype-Phenotype Correlations

In general, individuals with two null (nonexpressing) alleles have the infantile form, individuals with one null allele and one missense allele have the subacute juvenile-onset phenotype, and individuals with two missense alleles have the milder late-onset phenotype. This reflects the inverse correlation of the level of the residual activity of the HEX A enzyme with the severity of the disease: the lower the level of the enzymatic activity, the more severe the phenotype is likely to be.

Nomenclature

Tay-Sachs disease was originally described as "infantile amaurotic idiocy" and "amaurotic familial infantile idiocy" by Tay and Sachs, respectively.

When GM2 ganglioside was identified as the major accumulating substrate, the nomenclature included the terms "infantile ganglioside lipidosis," "type 1 GM2 gangliosidosis," and "acute infantile GM2 gangliosidosis."

When deficient HEX A enzymatic activity was identified, the disease was then referred to as "hexosaminidase A deficiency," "HEX A deficiency," or "type 1 hexosaminidase A deficiency."

When the subacute juvenile and late-onset phenotypes were identified, they were referred to as the "B1 variant of GM2 gangliosidoses" or "juvenile (subacute) hexosaminidase deficiency" and "chronic or adult-onset hexosaminidase A deficiency," respectively.

Prevalence

Before the advent of population-based carrier screening, education, and counseling programs for the prevention of TSD in Jewish communities, the incidence of TSD was estimated at 1:3,600 Ashkenazi Jewish births. At that birth rate, the carrier rate for TSD is approximately 1:30 among Jewish Americans of Ashkenazi extraction (i.e., from Central and Eastern Europe).

Carrier screening studies have indicated that the frequency of the Ashkenazi Jewish founder variants in individuals whose parents and respective grandparents were of Ashkenazi Jewish descent is 1:27.4 [Scott et al 2010].

As the result of extensive genetic counseling of carriers identified through carrier screening programs and monitoring of at-risk pregnancies, the incidence of TSD in the Ashkenazi Jewish population of North America has been reduced by greater than 90% [Kaback et al 1993, Kaback 2000].

Among Sephardic Jews and all non-Jews, the disease incidence has been observed to be about 100 times lower, corresponding to a tenfold lower carrier frequency (between 1:250 and 1:300).

TSD has been reported in children in virtually all population groups studied.

Other genetically isolated populations have been found to carry founder HEXA pathogenic variants at frequencies comparable to or even greater than those observed in Ashkenazi Jews. These include:

• French Canadians of the eastern St Lawrence Valley, Quebec;
• Cajuns from Louisiana.

Hereditary Disorders

Infantile Onset

Table 2.

Genetic Disorders of Interest in the Differential Diagnosis of Acute Infantile Tay-Sachs Disease

Subacute Juvenile Onset

Table 3.

Genetic Disorders of Interest in the Differential Diagnosis of Subacute Juvenile Tay-Sachs disease

Spinocerebellar ataxia (SCA). Some spinocerebellar ataxia syndromes (e.g., ataxia caused by mutation of FGF14, MTCL1, or TXN2 or SCA7 with extreme anticipation) may be associated with early onset and can be considered in the differential diagnosis of subacute juvenile TSD (see Hereditary Ataxia Overview).

Late Onset

Table 4.

Genetic Disorders in the Differential Diagnosis of Late-Onset Tay-Sachs Disease

Spinocerebellar ataxia (SCA). Similar to late-onset TSD, SCA is associated with tremor, cerebellar atrophy, and dysarthria and can be considered in the differential diagnosis (see Hereditary Ataxia Overview).

Acquired Disorders

Lead and other heavy metal poisoning, infectious and postinfectious meningoencephalitis, subacute sclerosing panencephalitis, hydrocephalus, and neurologic manifestations of other systemic diseases may mimic the neurologic findings associated with HEXA disorders.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with a HEXA disorder, the evaluations summarized in Tables 5, 6, and 7 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Table 5.

Recommended Evaluations Following Initial Diagnosis in Individuals with Acute Infantile Tay-Sachs Disease

Table 6.

Recommended Evaluations Following Initial Diagnosis in Individuals with Subacute Juvenile Tay-Sachs Disease

Table 7.

Recommended Evaluations Following Initial Diagnosis in Individuals with Late-Onset Tay-Sachs Disease

Treatment of Manifestations

For the most part, treatment for acute infantile Tay-Sachs disease (TSD) is 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 TSD 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.

Table 8.

Treatment of Manifestations in Individuals with Acute Infantile Tay-Sachs Disease

Table 9.

Treatment of Manifestations in Individuals with Subacute Juvenile Tay-Sachs Disease

Table 10.

Treatment of Manifestations in Individuals with Late-Onset Tay-Sachs Disease

Surveillance

There are no formal guidelines for surveillance for those affected with HEXA disorders.

Neurology evaluations should commence at the time of diagnosis for all subtypes of TSD if not previously established, and follow up should be dictated by emergent clinical concerns.

Agents/Circumstances to Avoid

For individuals with acute infantile TSD, avoid:

• Positioning that increases aspiration risk during feedings;
• Seizure medication dosages that result in excessive sedation.

For individuals with subacute juvenile TSD, avoid:

• Situations that increase the likelihood of contractures or pressure sores, such as extended periods of immobility;
• Circumstances that exacerbate the risk of falls.

For individuals with late-onset TSD, avoid:

• Situations that exacerbate fall risk (i.e., walking on uneven or unstable surfaces);
• Psychiatric medications that have been associated with disease worsening (e.g., haloperidol, risperidone, and chlorpromazine) [Shapiro et al 2006].

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Current studies include:

• A Phase II study to assess the safety and efficacy of N-acetyl-L-leucine for the treatment of GM2 gangliosidosis (Tay-Sachs disease and Sandhoff disease);
• A multicenter study to assess the efficacy and pharmacodynamics of daily oral dosing of venglustat when administered over a 104-week period in late-onset and subacute juvenile GM2 gangliosidosis (Tay-Sachs disease and Sandhoff disease);
• A combination therapy using miglustat and the ketogenic diet for infantile and juvenile individuals with gangliosidoses;
• A survey of miglustat therapeutic effects on neurologic and systemic symptoms of infantile types of Tay-Sachs and Sandhoff disease.

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on other clinical studies.

Mode of Inheritance

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.

Risk to Family Members

Parents of a proband

• The parents of an affected individual are obligate heterozygotes (i.e., presumed to be carriers of one HEXA pathogenic variant based on family history).
• Molecular genetic testing is recommended for the parents of a proband to confirm that both parents are heterozygous for a HEXA pathogenic variant and to allow reliable recurrence risk assessment. (De novo variants are known to occur at a low but appreciable rate in autosomal recessive disorders [Jónsson et al 2017].)
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

• If both parents are known to be heterozygous for a HEXA pathogenic variant, each sib of an affected individual has at conception 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.
• Sibs who inherit biallelic HEXA pathogenic variants will have the same HEXA disorder phenotype (i.e., acute infantile, subacute juvenile, or late-onset TSD) as the proband (see Genotype-Phenotype Correlations). However, the subacute juvenile and late-onset phenotypes are associated with significant intrafamilial clinical variability (see Clinical Description).
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband. Unless an individual with late-onset TSD has children with an affected individual or a carrier, offspring will be obligate heterozygotes (carriers) for a pathogenic variant in HEXA; it is appropriate to offer carrier testing to the reproductive partners of individuals with late-onset TSD (see Related Genetic Counseling Issues, Population screening).

Other family members. Each sib of a proband's parents is at a 50% risk of being a carrier of a HEXA pathogenic variant.

Carrier Detection

Molecular genetic testing. 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.

Biochemical testing. Assay of beta-hexosaminidase A activity is a highly sensitive method for the identification of carriers; however, follow-up molecular testing is required if a carrier couple wishes to pursue prenatal/preimplantation genetic testing. Note: Leukocyte testing (rather than serum testing) should be ordered for TSD carrier detection in women who are pregnant or using oral contraceptive medication. Additional limitations of enzyme testing are addressed in Related Genetic Counseling Issues, Population screening.

Carrier testing recommendations for the reproductive partners of known carriers (or the reproductive partners of individuals with late-onset TSD) who do not have a family history of TSD are addressed in Population screening.

Related Genetic Counseling Issues

Population screening. Recent studies suggest that full-exon HEXA next-generation sequencing (NGS) is equally or more sensitive for the detection of carriers than targeted testing for specific variants and HEX A enzyme testing [Hoffman et al 2013, Cecchi et al 2019]. These findings are reflected in the 2019 position statement of the National Tay-Sachs and Allied Disorders Association (NTSAD Position Statement); see Table 11 for a summary of the NTSAD recommendations.

Table 11.

Population Screening for HEXA Disorders Based on Recommendations from the 2019 Update of the NTSAD Position Statement on Standards for Tay-Sachs Carrier Screening

Assisted reproductive technologies. Individuals who are pursuing reproductive technologies that involve gamete (egg or sperm) donation and who are at increased risk of being heterozygous for a HEXA pathogenic variant because of family history (see Carrier Detection) or ethnic background (see Related Genetic Counseling Issues, Population screening; Prevalence) should be offered testing. If the gamete recipient is a known carrier, any potential gamete donor must undergo molecular testing to determine if the donor is also a carrier.

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.

DNA banking. Because it is likely that testing methodology and our understanding of genes, pathogenic mechanisms, and diseases will improve in the future, consideration should be given to banking DNA from probands in whom a molecular diagnosis has not been confirmed (i.e., the causative pathogenic mechanism is unknown). For more information, see Huang et al [2022].

Prenatal Testing and Preimplantation Genetic Testing

Positive family history. Once the HEXA pathogenic variants have been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Population screening. If both members of a reproductive couple are known to be heterozygous for a HEXA pathogenic variant, prenatal and preimplantation genetic testing for the HEXA pathogenic variants identified in the parents are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing. While most centers would consider use of prenatal testing to be a personal decision, discussion of these issues may be helpful.

Molecular Pathogenesis

Biallelic pathogenic variants in HEXA lead to absent or reduced activity in β-hexosaminidase A, a lysosomal hydrolytic enzyme required for the breakdown of ganglioside GM2 in neurons, where synthesis of complex gangliosides is the highest. The buildup of GM2 ganglioside, normally present in neurons in very small quantities, leads to impairment and subsequent progressive loss of neurons and resultant neurodegeneration.

Mechanism of disease causation. Loss-of-function variants cause decreased to absent β-hexosaminidase activity.

HEXA-specific laboratory technical considerations. Pseudodeficiency refers to an in vitro phenomenon caused by specific HEXA alleles (see Table 12) that renders the β-hexosaminidase A enzyme unable to process the synthetic (but not the natural) GM2 substrates, and leads to false positive enzyme testing results.

In contrast, the so-called B1 variant allele results in a β-hexosaminidase A enzyme that is able to degrade the artificial substrate, but not the natural GM2 ganglioside, which leads to false negative enzyme testing results.

Table 12.

Notable HEXA Variants

Author Notes

Authors' websites:

Cynthia Tifft, MD, PhD

Camilo Toro, MD

Acknowledgments

This work was supported by funds from the Intramural Research Program of the National Human Genome Research Institute, National Institutes of Health, Bethesda, MD, USA.

The authors wish to acknowledge all participants in the Neurodegeneration in Glycosphingolipid Storage Disorders' Natural History Study at the NIH (ClinicalTrials.gov Identifier: NCT00029965) and the longstanding contribution of the National Tay-Sachs and Allied Diseases Association (www.ntsad.org) to the support and education of patients and families with GM1 and GM2 gangliosidosis.

Author History

Robert J Desnick, PhD, MD, FACMG; Icahn School of Medicine at Mount Sinai (1999-2020)
Michael M Kaback, MD, FACMG; University of California, San Diego (1999-2020)
Leila Shirvan, BA (2020-present)
Cynthia Tifft, MD, PhD (2020-present)
Camilo Toro, MD (2020-present)

Revision History

• 1 October 2020 (ha) Comprehensive update posted live
• 11 August 2011 (me) Comprehensive update posted live
• 19 May 2006 (me) Comprehensive update posted live
• 9 January 2004 (me) Comprehensive update posted live
• 30 October 2001 (me) Comprehensive update posted live
• 11 March 1999 (me) Review posted live
• April 1998 (mk) Original submission

Note: Pursuant to 17 USC Section 105 of the United States Copyright Act, the GeneReview "HEXA Disorders" is in the public domain in the United States of America.

Published Guidelines / Consensus Statements

• NTSAD. Position Statement 2019 Update – "Standards for Tay-Sachs Carrier Screening." Available online. 2019. Accessed 12-21-22.

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