Biochemical characterization of the GM2 gangliosidosis B1 variant

Abstract

The deficiency of the A isoenzyme of ß-hexosaminidase (Hex) produced by different mutations of the gene that codes for the α subunit (Tay-Sachs disease) has two variants with enzymological differences: the B variant consists of the absence of Hex A isoenzyme and the B1 variant produces an inactive Hex A isoenzyme for the hydrolysis of the GM2 ganglioside and synthetic substrates with negative charge. In contrast to the early childhood form of the B variant, the B1 variant appears at a later clinical stage (3 to 7 years of age) with neurodegenerative symptoms leading to the death of the patient in the second decade of life. The most frequent mutation responsible for the GM2 gangliosidosis B1 variant is R178H, which has a widespread geographic and ethnic distribution. The highest incidence has been described in Portugal, which has been suggested as the point of origin of this mutation. Biochemical characterization of this lysosomal disease is carried out using negatively charged synthetic α subunit-specific sulfated substrates, since Hex A isoenzyme heat-inactivation assays are not applicable. However, the determination of the apparent activation energy of Hex using the neutral substrate 3,3'-dichlorophenolsulfonphthaleinyl N-acetyl-ß-D-glucosaminide, may offer a valid alternative. The presence of an α subunit in the αß heterodimer Hex A means that its activation energy (41.8 kJ/mol) is significantly lower than that of the ßß homodimer Hex B (75.1 kJ/mol); however, as mutation inactivates the α subunit, the Hex A of the B1 variant presents an activation energy that is similar to that of the Hex B isoenzyme. Correspondence

Myoclonic Epilepsy in Lysosomal Storage Disorders

“…There are mutations in the HEXA gene causing the B1 Variant, associated with the late onset form of TSD. This biochemical phenotype is characterized by a Hex A isoenzyme catalytically inactive against the physiological substrate, GM2 ganglioside, but active towards commonly used synthetic substrate (4-methylumbelliferyl -Nacetylglucosaminide) (Tutor, 2004). Concerning the HEXA mutations associated with the B1 Variant, the most common is the c.533G>A (p.R178H) that was first found predominantly in Portuguese patients (dos Santos et al, 1991;Gravel et al, 2001) and which has been subsequently detected in individuals with different European backgrounds (Montalvo et al, 2006).…”

“…Tay Sachs disease is confirmed by the presence of reduced levels of total Hex and very low l e v e l s o f H e x A . I t i s i m p o r t a n t t o k e e p i n m i n d t h a t t h e B 1 v a r i a n t o f T a y S a c h s i s characterized by the presence of an Hex A isoenzyme catalytically inactive against the physiological substrate, GM2 ganglioside, but active towards commonly used synthetic substrate 4-methylumbelliferyl -N-acetyl glucosaminide (Tutor, 2004). Biochemical identification of these patients requires always the use of the specific substrate MUGS.…”

Elevation of nucleocytoplasmic β-N-acetylglucosaminidase (O-GlcNAcase) activity in thyroid cancers

“…This group of hexosaminidases comprises the A, B, I, P and S isozymes (1)(2)(3)(4). Two major lysosomal hexosaminidase isozymes, A and B were extensively studied because dysfunction of either of them results in the accumulation of gangliosides and other glycoconjugates within the lysosome causing the neurodegenerative disorders known as Tay-Sachs and Sandhoff disease (2,5). Moreover, the enhancement of ß-hexosaminidase degradative activity in different human cancer tissues and in serum or urine of patients with cancers is well documented (6)(7)(8)(9)(10)(11)(12)(13)(14)(15).…”

Molecular analysis of theHEXAgene in Italian patients with infantile and late Onset Tay-Sachs disease: detection of fourteen novel alleles

“…The active site of the α subunit preferably hydrolyzes substrates with a negative charge, such as β-linked N-acetyl-glucosamine 6-sulfate containing glycosaminoglycans and GM2 ganglioside, although it may also hydrolyze neutral substrates. In contrast, the catalytic site of the ß subunit is only active with neutral substrates and as a result the Hex B isoenzyme is unable to hydrolyze the GM2 ganglioside (Mahuran, 1999;Tutor, 2004). Mutations in either gene encoding its α or β subunits result in a group of recessively inherited disorders, the G M2 Gangliosidosis.…”

“…Still there are mutations in the HEXA gene causing the B1 Variant, associated with the late onset form of Tay-Sachs. This biochemical phenotype is characterized by a Hex A isoenzyme catalytically inactive against the physiological substrate, G M2 ganglioside, but active towards commonly used synthetic substrate (4-methylumbelliferyl β-Nacetylglucosaminide) (Tutor, 2004). Biochemical identification of these patients requires a specific sulfated synthetic substrate, 4-methylumbelliferyl N-acetylglucosamine 6-sulfate (MUGS) (Bayleran, et al, 1984 ).…”