Yin-Ming Kuo (Presenter)
Fox Chase Cancer Center
Authorship: Yin-Ming Kuo (1), Sapna Gupta (1), Hyung-Ok Lee (1), Eoin P. Quinlivan (2), Warren D. Kruger (1), and Andrew J. Andrews (1)
(1) Fox Chase Cancer Center, Philadelphia, PA USA, (2) Southeast Center for Integrated Metabolomics, University of Florida, Gainesville, FL USA
| Short Abstract Cystathionine β-synthase (CBS) deficiency is an inborn error of metabolism characterized by high levels of serum total homocysteine. To better understand how hyperhomocysteine (alteration of one-carbon metabolism) affects pathogenesis, we utilized LC-MS/MS methods to examine the levels of S-adenosylhomocysteine (AdoHcy), S-adenosylmethionine (AdoMet), and the percentage of 5-methyldeoxycytidine in tissues from two mouse models of CBS deficiency: Tg-hCBS Cbs-/-, and Eµ-myc mice. Our results reveal that CBS deficiency results in elevated AdoHcy levels and reduced AdoMet/AdoHcy ratio in liver, kidney, and tumor tissues. However, there is no clear evidence linking this change to further epigenetic modulation and biological effect. |
Long Abstract
Cystathionine β-synthase (CBS) deficiency is an inborn error of sulfur amino acid metabolism characterized by high levels of serum total homocysteine (tHcy). Patients with this disorder suffer from various pathologies including thrombosis, osteoporosis, mental retardation, and dislocated lenses. A possible mechanisms by which elevated tHcy causes disease, is that elevated tHcy may cause an increase in the steady state level of its immediate upstream precursor, S-adenosylhomocysteine (AdoHcy). AdoHcy is a competitive inhibitor of S-adenosylmethionine (AdoMet), the key substrate in almost all biological methylation reactions, including the methylation of the DNA and histones. Thus elevated tHcy may lead to increases in cellular AdoHcy resulting in inhibition of DNA methylation. To test this idea, we have examined AdoMet, AdoHcy, and 5-methyldeoxycytidine (%MdCyt) levels using an LC-MS/MS assay in tissues from two different mouse models of CBS deficiency.
Tg-hCBS Cbs-/- have a null allele for mouse Cbs, but have a transgene in which the human CBS cDNA expressed under the control of metallothionine promoter. When given regular water, these mice have tHcy of 157.9 ± 18.5 µM, while when given zinc the tHcy is only 12.8 ± 13.3 µM. We found significantly elevated AdoHcy levels in liver (171 ± 71.3 vs. 45.2 ± 19.7 nmol/g) and in kidney (24.2 ± 8.2 vs. 8.1 ± 2.8 nmol/g), as well as significantly decreased AdoMet/AdoHcy ratios (0.6 vs. 1.7 in liver; 2.8 vs. 5.0 in kidney). However, despite these differences, we did not observe any difference in the levels of %MdCyt in the liver (3.79 ± 0.13 vs. 3.70 ± 0.17).
In second mouse model, we asked the question whether there is any effect of altered AdoMet/AdoHcy in pathogenesis of cancer. Here, we have generated an Eµ-myc mouse model which is either wild type, heterozygous or homozygous for a deletion allele of Cbs (Eµ myc+ Tg-I278T Cbs+/+; Eµ myc+ Tg-I278T Cbs+/-; Eµ myc+ Tg-I278T Cbs-/-). These mice were aged until tumors were detected (median 4.7 months, range 2.6-12.6 months), at which times tumors were harvested and analyzed for AdoMet, AdoHcy, AdoMet/AdoHcy, and %MdCyt. The results for each of the three genotypes are: AdoHcy (9.3 ± 3.1; 13 ± 5.2; 92 ± 23 nmol/g), AdoMet (74 ± 2.1; 73 ± 6.7; 123 ± 7.7 nmol/g), AdoMet/AdoHcy (8.6 ± 2.8; 6.2 ± 2.0; 1.4 ± 0.4) and %MdCyt (3.86 ± 0.17; 3.88 ± 0.16; 3.60 ± 0.12). Interestingly, despite the differences in AdoMet/AdoHcy and %MdCyt values, we observed no significant difference in survival of the three different genotypes.
In summary, our results show that CBS deficiency does cause increased AdoHcy and reduced AdoMet/AdoHcy in several tissues. However, the evidence is less clear as to whether this alteration has meaningful effects on epigenetic or biological function.
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