Pyrimethamine increases β-hexosaminidase A activity in patients with Late Onset Tay Sachs
Introduction
Late Onset Tay Sachs disease (LOTS) is a rare variant of the better known infantile Tay–Sachs disease, which results from mutations in the gene encoding the α subunit of β-hexosaminidase A (HexA), a lysosomal enzyme in charge of normal degradation of the ganglioside GM2. Human cells express two major β-hexosaminidase (Hex) isozymes, HexA and HexB. HexA is a heterodimer made up of α and β subunits, whereas HexB is a homodimer of two identical β-subunits [1]. Based on correlations between HexA activity and phenotype, it has been estimated that preservation of only 10% of the normal HexA activity may provide just enough GM2 degrading capacity to avoid clinically evident disease [2] In the classical infantile form, severe mutations in the α subunit gene lead to severe HexA deficiency leaving < 0.5% of the normal enzyme activity, which allows rapid intracellular accumulation of GM2 in brain neurons, inducing accelerated neural cell distortion and apoptosis and early onset of a rapidly progressive neurodegenerative disease, culminating in death during infancy [1] In contrast, LOTS occurs in individuals harboring compound heterozygote or mild homozygote α subunit gene mutations and is linked to higher residual HexA catalytic efficacy amounting to 2–5% of normal activity. This results in a more gradual course, with delayed onset, slower decline in motor, cerebral and spinocerebellar function and the evolution of psychiatric disorders such as depression, bipolar disorder and psychosis [3].
At the present time, no effective treatment for LOTS exists. Enzyme replacement therapy is impractical, predominantly due to the inability to deliver large molecules across the blood brain barrier (BBB) [4].Attempts to decrease the accumulation of GM2 via pharmacological inhibition of its synthesis, by blocking ceramide glucosyltransferase, have yielded promising results in a mouse model of Sandhoff's disease, but failed to provide appreciable relief in human subjects afflicted with LOTS [5], [6].
Recently, Mahuran's group [7] has explored the possibility that pharmacological chaperones can augment HexA. Chaperones are small molecules that can assist the folding of mutant misfolded proteins retaining partial catalyic function and thus allow better access of such partially functional enzymes to the lysosome, resulting in an increase in total enzyme activity. Following extensive screening of candidate molecules, pyrimethamine (PMT), an FDA approved anti-marial/anti-toxoplasmosis agent already used in humans [8], [9], [10], [11] which is capable of entering the central nervous system (CNS), was shown to enhance HexA activity in vitro in human fibroblasts carrying some (e.g., αG269S), but not all LOTS related mutations [12].
Here we examined the effect of PMT in LOTS in a cohort of Israeli patients, all of whom were of Jewish Ashkenazi extraction. All patients were compound heterozygotes carrying the αG269S/c.1278insTACT mutations. We present the initial biochemical effect on HexA activity and preliminary results of the clinical effects of PMT in 9 LOTS patients, in an open label trial which included active PMT treatment for the duration of up to 10 months.
Section snippets
Patient recruitment and study design
Inclusion criteria for participation in this study were a genetically and clinically confirmed diagnosis of LOTS, and interest in close surveillance at the study center. Exclusion criteria for participation in this study were any serious medical illness, cardiac disease or hematologic abnormalities including bleeding diathesis and active uncontrolled bleeding. The presence of contraindications to the use of PMT was also excluded in each case prior to inclusion in this trial, including a history
Baseline clinical assessment
All patients were Ashkenazi Jews with the αG269S/c.1278insTACT mutation in the HexA gene and their baseline demographic and clinical characteristics are presented in Table 1. Mean patients' age was 37.9 ± 16.1 (± SD) years and the mean duration of the pre-trial symptomatic disease was 20.2 ± 11.9 years.
All patients had the same mutation in the HexA gene, but there was a considerable variability in the clinical phenotype as subjects had neurological manifestations at a different degree of severity.
Discussion
The key finding in this study is that PMT can enhance HexA activity in patients with LOTS, in association with some limited clinical benefits. The observed short- to mid-term effects of PMT on HexA in LOTS are small to moderate in magnitude and fall far from the theoretically desirable increase in HexA to "10% of normal HexA", above which a disease-free phenotype has been documented despite the presence of LOTS mutations [20]. Very recently, after the submission of our paper for publication,
Acknowledgment
This study was supported, in part, by a grant from the Chief Scientist of the Ministry of Health, Israel.
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