Abstract
Propionic acid and methylmalonic acid, which are intermediaries (BCAA-I) of branched-chain amino acid (leucine, isoleucine, valine) metabolism, can up-regulate HbF during definitive erythropoiesis [italics]in vitro[/italics] and [italics]in vivo [/italics](Little 1995). We tested ‘proximal’ (prior to thiolester formation with Coenzyme A) BCAA-Is, which would be elevated in maple syrup urine disease (MSUD), and ‘distal’, which would be elevated in Isovaleric Acidemia (IVA) Propionic Acidemia (PA) or Methylmalonic Acidemia (MMA), for their ability to simultaneously stimulate adult erythroid differentiation and embryonic globin gene expression in an adult erythroid cell line, murine erythroleukemia (MEL) cells after 96 hours in culture. Compounds and maximal concentrations (X) tested were: α-keto isocaproic acid, α−keto β−methylvaleric acid, and α−keto isovaleric acid, all at 50 mM, which are elevated in ‘proximal’ MSUD; isobutyric acid (IB), isovaleric acid (IV), methylmalonic acid, all at 50 mM, and propionate (5mM), all of which are elevated in ‘distal’ perturbations of BCAA metabolism, plus butyrate (2mM), which has been used pharmacologically to increase HbF (Atweh et al, 1999), and DMSO (adult erythroid differentiating agent in MELs). Concentration ranges were, relative to maximum, X, [X], [0.5X], [0.2X], [0.1X] and [0.01X]. RNA was analyzed by real-time PCR for murine adult alpha (α) and adult beta-major (βmaj) globin gene expression and embryonic α-type zeta (ζ) and β-type beta-H1 (βH1) and epsilon-y (εY). Only four BCAA-I compounds, plus DMSO, induced adult α−globin gene expression in MEL cells (by >100-fold, relative to uninduced). These compounds, but not DMSO, also augmented β−type embryonic globin gene expression ((βH1+εY)/(β−total))*100, as shown: propionate (5mM) 24%, butyrate (1 mM) 8% (2mM) 10%, IBA (25 mM) 5%, IVA (5 mM) 9% and (10mM) 13%. Samples from 10 individuals with ‘proximal’ abnormalities in BCAA-I (n=4, median age 5.5 years) due to MSUD and ‘distal’ abnormalities in BCAA-I (n=6, median age 6 years) due to IVA, PA, and MMA were examined by HPLC for % HbF. All patients were under good metabolic control at evaluation; mean Hgb (g/dL) was 13.1±0.9 in ‘proximal’ (MSUD) patients, and 12.2±1.6 in patients with distal perturbations in BCAA metabolism (p=n.s.). HbF was 0.9±0.3% (one beta-thalassemia excluded) and 0.2±0.4%(p<.01) in proximal and distal BCAA-I defects, respectively. Gamma (γ)-globin gene composition was examined by HPLC-mass spectromety in 3 patients with proximal BCAA-I and 5 with distal BCAA-I. No γ chains were detectable in patients with MSUD, and only Gγ chains, at >5% of total β−type chains, were present in 3 of 5 patients with distal BCAA-I (p<.05). Our data show that, as predicted by MEL cells [italics]in vitro[/italics], a subset of metabolic intermediaries can up-regulate fetal globin gene expression in non-anemic patients with endogenous elevations in BCAA-I due to inborn metabolic errors. Gγ, which predominates during normal fetal life, may be most affected by these compounds.
Branched-chain amino acid aminotransferase 2 regulates ferroptotic cell death in cancer cells