Cystathionine beta-synthase p.S466L mutation causes hyperhomocysteinemia in mice.
Missense mutations in the cystathionine beta-synthase (CBS) gene are the most common cause of clinical homocystinuria in humans. The p.S466L mutation was identified in a homocystinuric patient, but enzymatic studies with recombinant protein show this mutant to be highly active. To understand how this mutation causes disease in vivo, we have created mice lacking endogenous mouse CBS and expressing either wild-type (Tg-hCBS) or p.S466L (Tg-S466L) human CBS under control of zinc inducible metallothionein promoter. In the presence of zinc, we found that the mean serum total homocysteine (tHcy) of Tg-S466L mice was 142+/-55 microM compared to 16+/-13 microM for hCBS mice. Tg-S466L mice also had significantly higher levels of total free homocysteine and S-adenosylhomocysteine in liver and kidney. Only 48% of Tg-S466L mice had detectable CBS protein in the liver, whereas all the Tg-hCBS animals had detectable protein. Surprisingly, CBS mRNA was significantly elevated in Tg-S466L animals compared to Tg-hCBS, implying that the reduction in p.S466L protein was occurring due to posttranscriptional mechanisms. In Tg-S466L animals with detectable liver CBS, the enzyme formed tetramers and was active, but lacked inducibility by S-adenosylmethionine (AdoMet). However, even in Tg-S466L animals that had in vitro liver CBS activity equivalent to Tg-hCBS animals there was significant elevation of serum tHcy. Our results show that p.S466L causes homocystinuria by affecting both the steady state level of CBS protein and by reducing the efficiency of the enzyme in vivo.
Gupta
Sapna
S
Division of Population Science, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.
Wang
Liqun
L
Hua
Xiang
X
Krijt
Jakub
J
Kozich
Viktor
V
Kruger
Warren D
WD
D000818:Animals; D003541:Cystathionine beta-Synthase / Q000235:genetics* / Q000378:metabolism; D006710:Homocysteine / Q000378:metabolism; D006801:Humans; D020138:Hyperhomocysteinemia / Q000235:genetics; D051379:Mice; D008822:Mice, Transgenic; D009154:Mutation; D012441:Saccharomyces cerevisiae / Q000235:genetics
D006710:Homocysteine; D003541:Cystathionine beta-Synthase