Cystathionine beta-synthase p.S466L mutation causes hyperhomocysteinemia in mice.

Sapna Gupta,Xiang Hua,Jakub Krijt,Viktor KožIch,Warren D Kruger,Liqun Wang

doi:10.1002/humu.20773

Sapna Gupta, Xiang Hua + Show 4 more

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https://doi.org/10.1002/humu.20773

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Journal: Human Mutation	Publication Date: May 2, 2008
Citations: 38	License type: cc-by

Affiliation: Fox Chase Cancer Center, Charles University

Abstract

Missense mutations in the cystathionine β-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 µM compared to 16 ± 13 µM 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. Hum Mutat 29(8), 1048–1054, 2008. © 2008 Wiley-Liss, Inc.

Highlights

Cystathionine b-synthase (CBS; EC 4.2.1.22; MIM] 236200) is a key enzyme in the transsulfuration pathway that regulates homocysteine homeostasis
We found that all of the p.S466L animals had significantly more CBS mRNA compared to the Tg-hCBS animals (Supplementary Fig. S2)
We found that there was a linear relationship for both Tg-hCBS and TgS466L mice, but that the slope of the regression line differed dramatically between the two groups (Po0.0003; Fig. 5B)

Summary

Introduction

Cystathionine b-synthase (CBS; EC 4.2.1.22; MIM] 236200) is a key enzyme in the transsulfuration pathway that regulates homocysteine homeostasis. Mutations in the CBS gene cause clinical CBS deficiency (homocystinuria), a metabolic disorder characterized by extreme elevations in plasma total homocysteine (tHcy). The natural history of untreated CBS deficiency includes the development of venous thrombosis, arteriosclerosis, ectopia lentis, developmental delay, and osteoporosis [Mudd et al, 2001]. CBS has three functional domains: a heme binding domain, a catalytic core domain, and a C-terminal regulatory domain [Jhee and Kruger, 2005]. The C-terminal regulatory domain binds S-adenosylmethionine (AdoMet), an allosteric effector that results in a three-fold increase in Vmax for the enzyme [Taoka et al, 1998]. Most disease-causing mutations in CBS are of the missense variety [Kraus et al, 1999]

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