Tay-Sachs Disease via the French Canadian Deletion in the HEXA Gene

Tay-Sachs disease (TSD), also known as GM2 Gangliosidosis Type 1, is a neurodegenerative lysosomal storage disorder due to deficiency in the enzyme beta-hexosaminidase A. The enzymatic deficiency results in the accumulation of the lipid GM2 ganglioside, the natural substrate for beta-hexosaminidase A, mainly in nerve cells of the brain and retina. Two main variants of TSD are recognized.

Infantile acute TSD is the most severe and most prevalent variant. It is characterized by onset before the age of 6 months, rapid progression and death by 4 years of age, usually from bronchopneumonia. Symptoms at onset include muscle weakness and atrophy, hypotonia, exaggerated startle response, cherry-red spot in the macula, and decline in psychomotor functions. Abnormal eye movements, visual difficulties and seizures of variable types occur later. Symptoms progress rapidly to blindness, deafness, and generalized paralysis, leading eventually to death (Gravel et al. In: Scriver et al. 2001).

Late-onset TSD is clinically heterogeneous in regards to the age of onset of symptoms, clinical features, and progression rate. It is further divided into subacute TSD and chronic TSD. In the subacute TSD, symptoms begin between 2-5 years of age and death occurs in the second decade of life, usually from infections. Symptoms include ataxia, loss of movement coordination, spasticity, dystonia, slow deterioration of speech, gait and posture, cerebellar atrophy, dementia, and loss of vision (Maegawa et al. 2006). In chronic TSD, age of onset varies from childhood to late adulthood. Symptoms include seizures, unsteady gait and slow neurological deterioration with cognitive loss and psychosis (Johnson 1981; Navon and Proia 1989).

All TSD pathogenic variants are inherited in an autosomal recessive manner and result from mutations in the HEXA gene (Myerowitz and Hogikyan 1986). TSD occurs worldwide, with a carrier frequency of 1/250 in the general population. It is, however, more prevalent in the Ashkenazi Jewish and French Canadian populations with carrier frequencies of 1/30 and 1/14, respectively (Kaback 2000). 

A genomic deletion of 7.6 kb in the HEXA gene was found in about 80% of the disease alleles in Tay Sachs disease patients from the French Canadian population (Hetchman et al. 1990). This deletion includes the promoter, the entire exon 1 and part of intron 1 of the gene (Myerowitz and Hogikyan, 1986). 

The HEXA gene encodes the alpha-subunit of the beta-hexosaminidase A enzyme, which is involved in the biodegradation of GM2 ganglioside.

A genomic deletion of 7.6 kb in the HEXA gene was found in about 80% of the disease alleles in Tay Sachs disease patients from the French Canadian population (Hetchman et al. 1990).  

This test involves amplification of patient DNA with specific PCR primers that flank the deletion. Positive control samples include samples from two individuals from the French Canadian population, who are known to be heterozygous for the deletion. A healthy individual known to be a non-carrier of the deletion is included as a negative control.