Cystathionine ␤-synthase (CBS) deficiency causes severe hyperhomocysteinemia and other signs of homocystinuria syndrome, in particular a premature atherosclerosis with multiple thrombosis. However, the molecular mechanisms by which homocysteine could interfere with normal cell function are poorly understood in a whole organ like the liver, which is central to the catabolism of homocysteine. We used a combination of differential display and cDNA arrays to analyze differential gene expression in association with elevated hepatic homocysteine levels in CBS-deficient mice, a murine model of hyperhomocysteinemia. Expression of several genes was found to be reproducibly abnormal in the livers of heterozygous and homozygous CBS-deficient mice. We report altered expression of genes encoding ribosomal protein S3a and methylthioadenosine phosphorylase, suggesting such cellular growth and proliferation perturbations may occur in homozygous CBS-deficient mice liver. Many up-or down-regulated genes encoded cytochromes P450, evidence of perturbations of the redox potential in heterozygous and homozygous CBS-deficient mice liver. The expression of various genes involved in severe oxidative processes was also abnormal in homozygous CBS-deficient mice liver. Among them, the expression of heme oxygenase 1 gene was increased, concomitant with overexpression of heme oxygenase 1 at the protein level. Commensurate with the difference in hepatic mRNA paraoxonase 1 abundance, the mean hepatic activity of paraoxonase 1, an enzyme that protects low density lipoprotein from oxidation, was 3-fold lower in homozygous CBS-deficient mice. Heterozygous CBS-deficient mice, when fed a hyperhomocysteinemic diet, have also reduced PON1 activity, which demonstrates the effect of hyperhomocysteinemia in the paraoxonase 1 activity.

The human cystathionine beta-synthase (CBS) gene spans in excess of 30 kb and consists of 19 exons, with three different 5' untranslated regions including three different exons 1 (exons 1 a, b, and c). Exon 1a and 1b are 390 bp apart from each other and are linked to exon 2 in cDNA << a >> and cDNA << b >>. Exon 1c, which linked to exon 5 in cDNA << c >>, is 7 kb apart from exon 1b. All splice sites conform to the GT/AG rule, including those from exon 1a or 1b to exon 2 and from exon 1c to exon 5. Upstream of exons 1a and 1b, we found two putative promoter sequences with high C + G nucleotide content, one CAAT box at -70 nucleotides (for exon 1b), no TATA box, several Spl binding regulatory consensus sequences, and some other regulatory sequences. Human adult and fetal Northern blots hybridized with total cDNA containing exon 1b, or specific probes from exons 1 (b and c) showed mRNAs of 2.5 kb, 2.7 kb, and 3.7 kb. These results suggest that the mRNAs containing the different exons 1 are under the control of different promoters.

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