Preferential beta-hexosaminidase (Hex) A (alpha beta) formation in the absence of beta-Hex B (beta beta) due to heterozygous point mutations present in beta-Hex beta-chain alleles of a motor neuron disease patient.
Deficiency of the lysosomal enzyme beta-hexosaminidase B (beta-Hex B) (a homodimer, beta beta), caused by a defect in the HEX B gene encoding the beta-chain, is usually accompanied by an absence of beta-Hex A (a heterodimer, alpha beta), thereby causing Sandhoff disease. However, we have earlier demonstrated the presence of partial beta-Hex A (30-50% of normal) even in the absence of beta-Hex B in an adult with motor neuron disease. The patient is a compound heterozygote with normal beta-chain message and one HEX B point mutation originating from each asymptomatic parent. Since the non-expression of beta-Hex B was post-transcriptional, we transfected COS-7 cells to understand the effect of each mutation on beta-Hex B activity. Transfection of the A1367-->C mutant (maternal allele) construct produced no overexpressed beta-Hex B, indicating that the encoded Tyr456-->Ser beta-chain was non-functional. Chou-Fasman analysis predicted that the Tyr456-->Ser mutation would cause a dramatic change in beta-chain folding (which often inhibits formation of functional dimers). This explains the complete lack of beta-Hex B in the transfectants and a partial deficiency of beta-Hex A and B (50% of normal) in the patient's mother. Since immunoprecipitated beta-Hex A (alpha beta) protein from patient fibroblasts showed the presence of mature beta-chains even though there was no beta-Hex B (beta beta) protein, the mutant beta-chain inherited from the father (who has normal beta-Hex A and B) must undergo preferential association with the normal alpha-chains in the patient, thus producing only beta-Hex A. Transient expression of the A619-->G mutant (paternal allele) construct produced beta-Hex B activity comparable to the wild type (approximately 10-20-fold over mock-transfected) whereas stable expression produced normal message but no beta-Hex B activity (wild type beta-Hex B expression: only 2-fold over mock-transfected). The lack of increased beta-Hex B after stable expression of the Ile207-->Val beta-chains at a lower copy number indicates the absence of self-association at low concentrations of Ile207-->Val beta-chain. In the patient who also has a non-functional Tyr456-->Ser allele, the effective concentration of beta-chains is reduced to 50% of normal and the remaining Ile207-->Val beta-chains fail to self-associate but can still dimerize with the abundant normal alpha-chains thus producing partial beta-Hex A and no beta-Hex B.
Banerjee
P
P
Department of Pediatrics, University of Chicago, Illinois 60637.
Boyers
M J
MJ
Berry-Kravis
E
E
Dawson
G
G
D000328:Adult; D000483:Alleles; D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D005091:Exons; D005260:Female; D006580:Genetic Carrier Screening; D054818:Hexosaminidase A; D054819:Hexosaminidase B; D006801:Humans; D046911:Macromolecular Substances; D008297:Male; D016472:Motor Neuron Disease / Q000201:enzymology* / Q000235:genetics; D017354:Point Mutation; D017433:Protein Structure, Secondary; D012497:Sandhoff Disease / Q000201:enzymology* / Q000235:genetics; D012694:Serine; D014162:Transfection; D014443:Tyrosine; D001619:beta-N-Acetylhexosaminidases / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics*
D046911:Macromolecular Substances; D014443:Tyrosine; D012694:Serine; D054818:Hexosaminidase A; D054819:Hexosaminidase B; D001619:beta-N-Acetylhexosaminidases