A novel transgenic mouse model of CBS-deficient homocystinuria does not incur hepatic steatosis or fibrosis and exhibits a hypercoagulative phenotype that is ameliorated by betaine treatment

Abstract

Cystathionine beta-synthase (CBS) catalyzes the condensation of homocysteine (Hcy) and serine to cystathionine, which is then hydrolyzed to cysteine by cystathionine gamma-lyase. Inactivation of CBS results in CBS-deficient homocystinuria more commonly referred to as classical homocystinuria, which, if untreated, results in mental retardation, thromboembolic complications, and a range of connective tissue disorders. The molecular mechanisms that underlie the pathology of this disease are poorly understood. We report here the generation of a new mouse model of classical homocystinuria in which the mouse cbs gene is inactivated and that exhibits low-level expression of the human CBS transgene under the control of the human CBS promoter. This mouse model, designated “human only” (HO), exhibits severe elevations in both plasma and tissue levels of Hcy, methionine, S-adenosylmethionine, and S-adenosylhomocysteine and a concomitant decrease in plasma and hepatic levels of cysteine. HO mice exhibit mild hepatopathy but, in contrast to previous models of classical homocystinuria, do not incur hepatic steatosis, fibrosis, or neonatal death with approximately 90% of HO mice living for at least 6 months. Tail bleeding determinations indicate that HO mice are in a hypercoagulative state that is significantly ameliorated by betaine treatment in a manner that recapitulates the disease as it occurs in humans. Our findings indicate that this mouse model will be a valuable tool in the study of pathogenesis in classical homocystinuria and the rational design of novel treatments.

A functional variant in the cystathionine β-synthase gene promoter significantly reduces congenital heart disease susceptibility in a Han Chinese population

“…And among the CBSdeficient HCU patients or Cbs knockout mice, there were no reports that they were affected by CHD, although they had dramatically elevated plasma homocysteine levels [9,[25][26][27][28][29][30]. We presume that it reflects the different roles of transsulfuration pathway catalyzed by CBS and remethylation pathway by MTR/MTRR in homocyestine removal in different developmental stages.…”

“…In both Down syndrome children and the CBS transgenic mice, a significant decrease in plasma homocysteine levels was observed with the overexpression of CBS [20][21][22][23][24]. In contrast, both the patients with inherited homocystinuria caused by CBS deficiency and the Cbs knockout mice had significantly elevated plasma homocysteine levels [9,[25][26][27][28][29][30]. Therefore, the CBS expression level demonstrated a correlation with the plasma homocysteine levels, although in this study we did not observe a statistically significant difference in plasma homocysteine levels in 522 fasting undergraduates with different CBS −4673 genotypes.…”

“…However, the overloaded homocysteine due to CBS deficiency after birth could not be compensated anymore, and CBS activity undergoes dramatic change after birth to maintain normal plasma total homocysteine (tHcy) level. Thus, HCU patients and Cbs knockout mice had extremely high plasma tHcy levels without any CHD phenotype [9,[25][26][27][28][29][30]. On the other hand, the decreasing MTR/MTRR activity, which plays a vital role in homocysteine removal, could not be compensated either by CBS activity in the same cells or by hepatic BHMT before fetal liver formation on day 32.…”

Betaine treatment of cystathionine β-synthase-deficient homocystinuria; does it work and can it be improved?

“…55 Betaine treatment in the absence of dietary methionine restriction significantly decreased plasma tHcy levels (P , 0.0001) and concomitantly increased the clotting time in the HO mice (P = 0.0005). 56 Subsequent work has found that HO HCU mice exhibit a highly significant induction of the pro-inflammatory cytokines interleukin 1a (IL-1a), IL-1b, and tumor necrosis factor-α (TNF-α). 57 Similar, constitutive inductions of multiple proinflammatory cytokines (IL-1α, IL-6, TNF-α, IL-17, and IL-12 [p70]) and chemotactic chemokines (fractalkine, macrophage inflammatory protein [MIP]-1α and MIP-1β) were observed in untreated/poorly compliant human subjects with HCU.…”

“…This mouse model, designated "human only" (HO), exhibits severe elevations in both plasma and tissue levels of Hcy, methionine, AdoMet, and AdoHcy and a concomitant decrease in plasma and hepatic levels of cysteine. 56 Betaine treatment of HO mice resulted in a highly significant lowering of average tHcy levels from 257 (standard deviation [SD] = 65) to 50 µM (SD = 16.1; P , 0.001) (Figure 2A receiving conventional therapy for HCU might benefit from adjuvant anti-inflammatory therapy.…”