Cystathionine β‐synthase (CBS) Deficiency: Genetics

Abstract

Cystathionine β‐synthase (CBS) is an enzyme that catalyses condensation of homocysteine and serine to cystathionine. CBS deficiency, an autosomal recessive trait with estimated population frequency of around 1:10 000–1:20 000, resembles in the most severe forms Marfan syndrome with thromboembolism and neurological impairment, whereas milder forms may manifest with only thromboembolism or may remain asymptomatic. Laboratory findings include grossly elevated plasma total homocysteine and in part of patients also elevated blood methionine; the latter feature is utilised in neonatal screening. CBS binds three cofactors: pyridoxal 5′‐phosphate, an allosteric activator S‐adenosylmethionine and haem with as yet unresolved function. In the CBS gene, more than 150 different mutations have been described to date, almost 90% of them are missense variants. Although some of the mutations affect ribonucleic acid processing and its stability, the majority of mutations lead to enzyme misfolding and misassembly, which may be in part rescued by chemical or molecular chaperones.

Key Concepts:

  • Cystathionine β‐synthase catalyses the first step in homocysteine trans‐sulfuration.

  • Cystathionine β‐synthase is a modular enzyme composed of a haem‐binding N‐terminal domain, the catalytically active core and the C‐terminal autoinhibitory domain.

  • More than 150 different pathogenic mutations have been described in the CBS gene, almost 90% of all mutant CBS alleles carry missense mutations.

  • Misfolding and misassembly of mutant CBS subunits is a common pathogenic mechanism leading to CBS deficiency.

  • Mutation topology predicts in part the behaviour of mutant CBS enzymes, solvent accessible mutations have less severe effect than those buried in the enzyme globule.

  • Deficient CBS activity leads to gross elevation of plasma total homocysteine and often to elevated blood methionine levels.

  • Phenotypic consequences of mutations in the CBS gene include thromboembolism and vascular occlusion, which is accompanied in some patients by lens dislocation, marfanoid features and varying degree of neurological involvement.

  • Population frequency of clinically ascertained patients with CBS deficiency is one to two orders of magnitude lower than the frequency calculated from the prevalence of heterozygotes for pathogenic mutations; this observation indicates an ascertainment bias or lack of symptoms in many CBS‐deficient individuals.

  • Murine models of CBS deficiency recapitulate in part the pathophysiology and organ involvement observed in human patients.

Keywords: homocysteine; homocystinuria; inborn errors of metabolism; cystathionine β‐synthase; mutations; pathogenesis; misfolding; chaperones; mouse models

Figure 1.

Metabolism of sulfur amino acids. A typical Western diet contains approximately 1–3 g of methionine (Met). Met reacts with adenosine triphosphate (ATP) to yield S‐adenosylmethionine (AdoMet), a reaction catalyzed by two methionine adenosyltransferases. AdoMet is utilised in numerous transmethylation reactions yielding S‐adenosylhomocysteine (AdoHcy). The latter compound is broken down by S‐adenosylhomocysteine hydrolase into adenosine and homocysteine (Hcy). Approximately half of the Hcy molecules are remethylated to methionine by betaine:homocysteine methyltransferase or by methionine synthase, which utilises methyltetrahydrofolate (CH3‐FH4) as the source of the methyl group. The other half of the Hcy moieties are converted to cysteine in the trans‐sulfuration pathway. The first step is the condensation of Hcy with serine (Ser) catalysed by cystathionine β‐synthase. Further steps of this pathway produce cysteine (Cys), glutathione (GSH) and taurine.

Figure 2.

Modular structure of the CBS enzyme. Each CBS subunit contains the N‐terminal haem‐binding domain, the conserved catalytic core with PLP‐binding lysine 119 and the autoinhibitory C‐terminal portion, which contains two so‐called CBS domains and which binds S‐adenosylmethionine. Reproduced, with permission, from Miles and Kraus (2004).

Figure 3.

Mutant CBS alleles: variability of mutations and ethnic distribution. An updated database of mutations in the CBS gene is maintained at the Krauslab Homepage (see Websites). The proportion and ethnic origin of ∼750 independent alleles were taken from this source.

Figure 4.

Expression of mutant CBS enzymes in Escherichia coli. (a) The series of 27 mutants was expressed in E. coli and further analysed for the presence of CBS antigen in nonparticulate water‐soluble fractions of bacterial extracts (Sup, supernatant) as well as in the sodium dodecyl sulfate (SDS)‐soluble particulate fraction (Pel, pellet) obtained by centrifugation using SDS‐polyacrylamide gel electrophoresis (PAGE) and Western blotting. The CBS antigen is present in all fractions of all mutants. (b) The quaternary structure of CBS mutants was assessed in the water‐soluble nonparticulate fraction of bacterial extracts by electrophoresis under native conditions followed by Western blotting; sharply demarcated fractions are tetramers and higher oligomers. These fractions are missing in some of the mutants indicating that they misfold and aggregate. (a) Mutant proteins expressed at 37°C. (b) Results of expression of mutant proteins at 18°C demonstrating increased formation of tetramers for several mutants. Solvent‐exposed mutations are mutations with accessible surface area larger than 40 Å2 and with relative accessible surface larger than 9%; buried mutations are the remaining mutations. pKK 388.1, extracts of bacteria transformed with the empty pKK 388.1 plasmid lacking any CBS; WT, bacterial extracts containing wild‐type CBS. Reproduced, with permission, from Kožich et al..

Figure 5.

Solvent exposure and misassembly of mutants. The individual data points show the correspondence between the amount of tetramer and activity of the mutants, which are both normalised to the amount and activity of wild‐type CBS that has been expressed in each series. Data are here combined separately for all solvent‐exposed (see panel a) and all buried mutations (see panel b); ○ and ▵ indicate data for expression at 37°C and 18°C, respectively. Linear regression analysis shows much stronger correlation between activity and the amount of tetramers for solvent‐exposed mutations than for the buried ones. Reproduced, with permission, from Kožich et al..

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Further Reading

Banerjee R and Zou CG (2005) Redox regulation and reaction mechanism of human cystathionine‐beta‐synthase: a PLP‐dependent hemesensor protein. Archives of Biochemistry and Biophysics 433(1): 144–156.

Carmel R and Jacobsen DW (2001) Homocysteine in Health and Disease. Cambridge, UK; New York: Cambridge University Press.

Kraus JP, Janosik M, Kožich V et al. (1999) Cystathionine beta‐synthase mutations in homocystinuria. Human Mutation 13(5): 362–375.

Kraus JP, Oliveriusova J, Sokalova J et al. (1998) The human cystathionine beta‐synthase (CBS) gene: complete sequence, alternative splicing, and polymorphisms. Genomics 52(3): 312–324.

Miles EW and Kraus JP (2004) Cystathionine beta‐synthase: structure, function, regulation, and location of homocystinuria‐causing mutations. Journal of Biological Chemistry 279(29): 29871–29874.

Mudd SH, Skovby F, Levy HL et al. (1985) The natural‐history of homocystinuria due to cystathionine beta‐synthase deficiency. American Journal of Human Genetics 37(1): 1–31.

Watson MS, Mann MY, Lloyd‐Puryear MA, Rinaldo P and Howell RR (2006) Newborn screening: toward a uniform screening panel and system. Genetics in Medicine 8(suppl. 1): 1S–252S.

Websites

CBS gene map http://cbs.lf1.cuni.cz/cbsdata/genome.htm

Coenzymes and catalysis http://academic.brooklyn.cuny.edu/biology/bio4fv/page/coenz_ca.htm

Cystathionine‐beta‐synthase (CBS); Locus ID: 875 LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink/LocRpt.cgi?l=875

Homocystinuria due to cystathionine beta‐synthase deficiency; MIM number: 236200 OMIM: http://www.ncbi.nlm.nih.gov/omim/236200

Krauslab Homepage–University of Colorado Health Sciences Center http://medschool.ucdenver.edu/krauslab

Map of all CBS mutations http://cbs.lf1.cuni.cz/index.php

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How to Cite close
Kožich, Viktor, Kruger, Warren D, and Kraus, Jan Peter(Dec 2010) Cystathionine β‐synthase (CBS) Deficiency: Genetics. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005935.pub2]