Analysis of the Qatari R336C cystathionine ß-synthase protein in mice.

Classical homocystinuria is a recessive inborn error of metabolism caused by mutations in the cystathionine beta-synthase (CBS) gene. The highest incidence of CBS deficiency in the world is found in the country of Qatar due to the combination of high rates of consanguinity and the presence of a founder mutation, c.1006C>T (p.R336C). This mutation does not respond to pyridoxine and is considered severe. Here we describe the creation of a mouse that is null for the mouse Cbs gene and expresses human p.R336C CBS from a zinc-inducible transgene (Tg-R336C Cbs -/- ). Zinc-treated Tg-R336C Cbs -/- mice have extreme elevation in both serum total homocysteine (tHcy) and liver tHcy compared with control transgenic mice. Both the steady-state protein levels and CBS enzyme activity levels in liver lysates from Tg-R336C Cbs -/- mice are significantly reduced compared to that found in Tg-hCBS Cbs -/- mice expressing wild-type human CBS. Treatment of Tg-R336C Cbs -/- mice with the proteasome inhibitor bortezomib results in stabilization of liver CBS protein and an increase in activity to levels found in corresponding Tg-hCBS Cbs -/- wild type mice. Surprisingly, serum tHcy did not fully correct even though liver enzyme activity was as high as control animals. This discrepancy is explained by in vitro enzymatic studies of mouse liver extracts showing that p.R336C causes reduced binding affinity for the substrate serine by almost 7-fold and significantly increased dependence on pyridoxal phosphate in the reaction buffer. These studies demonstrate that the p.R336C alteration effects both protein stability and substrate/cofactor binding.

Antioxidants successfully reduce ROS production in propionic acidemia fibroblasts.

Propionic acidemia (PA), caused by a deficiency of the mitochondrial biotin dependent enzyme propionyl-CoA carboxylase (PCC) is one of the most frequent organic acidurias in humans. Most PA patients present in the neonatal period with metabolic acidosis and hyperammonemia, developing different neurological symptoms, movement disorders and cardiac complications. There is strong evidence indicating that oxidative damage could be a pathogenic factor in neurodegenerative, mitochondrial and metabolic diseases. Recently, we identified an increase in ROS levels in PA patients-derived fibroblasts. Here, we analyze the capability of seven antioxidants to scavenge ROS production in PA patients' cells. Tiron, trolox, resveratrol and MitoQ significantly reduced ROS content in patients and controls' fibroblasts. In addition, changes in the expression of two antioxidant enzymes, superoxide dismutase and glutathione peroxidase, were observed in PA patients-derived fibroblasts after tiron and resveratrol treatment. Our results in PA cellular models establish the proof of concept of the potential of antioxidants as an adjuvant therapy for PA and pave the way for future assessment of antioxidant strategies in the murine model of PA.

BH3 mimetics and azacitidine show synergistic effects on juvenile myelomonocytic leukemia.

Juvenile myelomonocytic leukemia (JMML) is an aggressive hematopoietic disorder of infancy and early childhood driven by constitutively active RAS signaling and characterized by abnormal proliferation of the granulocytic-monocytic blood cell lineage. Most JMML patients require hematopoietic stem cell transplantation for cure, but the risk of relapse is high for some JMML subtypes. Azacitidine was shown to effectively reduce leukemic burden in a subset of JMML patients. However, variable response rates to azacitidine and the risk of drug resistance highlight the need for novel therapeutic approaches. Since RAS signaling is known to interfere with the intrinsic apoptosis pathway, we combined various BH3 mimetic drugs with azacitidine in our previously established patient-derived xenograft model. We demonstrate that JMML cells require both MCL-1 and BCL-XL for survival, and that these proteins can be effectively targeted by azacitidine and BH3 mimetic combination treatment. In vivo azacitidine acts via downregulation of antiapoptotic MCL-1 and upregulation of proapoptotic BH3-only. The combination of azacitidine with BCL-XL inhibition was superior to BCL-2 inhibition in eliminating JMML cells. Our findings emphasize the need to develop clinically applicable MCL-1 or BCL-XL inhibitors in order to enable novel combination therapies in JMML refractory to standard therapy.

Functional characterization of novel genotypes and cellular oxidative stress studies in propionic acidemia.

Propionic acidemia (PA), caused by a deficiency of the mitochondrial biotin dependent enzyme propionyl-CoA carboxylase (PCC) is one of the most frequent organic acidurias in humans. PA is caused by mutations in either the PCCA or PCCB genes encoding the α- and ß-subunits of the PCC enzyme which are assembled as an α6ß6 dodecamer. In this study we have investigated the molecular basis of the defect in ten fibroblast samples from PA patients. Using homology modeling with the recently solved crystal structure of the PCC holoenzyme and a eukaryotic expression system we have analyzed the structural and functional effect of novel point mutations, also revealing a novel splice defect by minigene analysis. In addition, we have investigated the contribution of oxidative stress to cellular damage measuring reactive oxygen species (ROS) levels and apoptosis parameters in patient fibroblasts, as recent studies point to a secondary mitochondrial dysfunction as pathophysiological mechanism in this disorder. The results show an increase in intracellular ROS content compared to controls, correlating with the activation of the JNK and p38 signaling pathways. Highest ROS levels were present in cells harboring functionally null mutations, including one severe missense mutation. This work provides molecular insight into the pathogenicity of PA variants and indicates that oxidative stress may be a major contributing factor to the cellular damage, supporting the proposal of antioxidant strategies as novel supplementary therapy in this rare disease.

Treatment of Cystathionine ß-Synthase Deficiency in Mice Using a Minicircle-Based Naked DNA Vector.

Cystathionine ß-synthase (CBS) deficiency is a recessive inborn error of metabolism characterized by extremely elevated total homocysteine (tHcy) in the blood. Patients diagnosed with CBS deficiency have a variety of clinical problems, including dislocated lenses, osteoporosis, cognitive and behavioral issues, and a significantly increased risk of thrombosis. Current treatment strategies involve a combination of vitamin supplementation and restriction of foods containing the homocysteine precursor methionine. Here, a mouse model for CBS deficiency (Tg-I278T Cbs-/-) was used to evaluate the potential of minicircle-based naked DNA gene therapy to treat CBS deficiency. A 2.3 kb DNA-minicircle containing the liver-specific P3 promoter driving the human CBS cDNA (MC.P3-hCBS) was delivered into Tg-I278T Cbs-/- mice via a single hydrodynamic tail vein injection. Mean serum tHcy decreased from 351 µM before injection to 176 µM 7 days after injection (p = 0.0005), and remained decreased for at least 42 days. Western blot analysis reveals significant minicircle-directed CBS expression in the liver tissue. Liver CBS activity increased 34-fold (12.8 vs. 432 units; p = 0.0004) in MC.P3-hCBS-injected animals. Injection of MC.P3-hCBS in young mice, subsequently followed for 202 days, showed that the vector can ameliorate the mouse homocystinuria alopecia phenotype. The present findings show that minicircle-based gene therapy can lower tHcy in a mouse model of CBS deficiency.

Altered Redox Homeostasis in Branched-Chain Amino Acid Disorders, Organic Acidurias, and Homocystinuria.

Inborn errors of metabolism (IEMs) are a group of monogenic disorders characterized by dysregulation of the metabolic networks that underlie development and homeostasis. Emerging evidence points to oxidative stress and mitochondrial dysfunction as major contributors to the multiorgan alterations observed in several IEMs. The accumulation of toxic metabolites in organic acidurias, respiratory chain, and fatty acid oxidation disorders inhibits mitochondrial enzymes and processes resulting in elevated levels of reactive oxygen species (ROS). In other IEMs, as in homocystinuria, different sources of ROS have been proposed. In patients' samples, as well as in cellular and animal models, several studies have identified significant increases in ROS levels along with decreases in antioxidant defences, correlating with oxidative damage to proteins, lipids, and DNA. Elevated ROS disturb redox-signaling pathways regulating biological processes such as cell growth, differentiation, or cell death; however, there are few studies investigating these processes in IEMs. In this review, we describe the published data on mitochondrial dysfunction, oxidative stress, and impaired redox signaling in branched-chain amino acid disorders, other organic acidurias, and homocystinuria, along with recent studies exploring the efficiency of antioxidants and mitochondria-targeted therapies as therapeutic compounds in these diseases.

CBS mutations are good predictors for B6-responsiveness: A study based on the analysis of 35 Brazilian Classical Homocystinuria patients.

BACKGROUND: Classical homocystinuria (HCU) is a monogenic disease caused by the deficient activity of cystathionine ß-synthase (CßS). The objective of this study was to identify the CBS mutations in Brazilian patients with HCU. METHODS: gDNA samples were obtained for 35 patients (30 families) with biochemically confirmed diagnosis of HCU. All exons and exon-intron boundaries of CBS gene were sequenced. Gene expression analysis by qRT-PCR was performed in six patients. Novel missense point mutations were expressed in E. coli by site-directed mutagenesis. RESULTS: Parental consanguinity was reported in 16 families, and pyridoxine responsiveness in five (15%) patients. Among individuals from the same family, all presented the same phenotype. Both pathogenic mutations were identified in 29/30 patients. Twenty-one different mutations were detected in nine exons and three introns; being six common mutations. Most prevalent were p.Ile278Thr (18.2%), p.Trp323Ter (11.3%), p.Thr191Met (11.3%), and c.828+1G>A (11.3%). Eight novel mutations were found [c.2T>C, c.209+1delG, c.284T>C, c.329A>T, c.444delG, c.864_868delGAG c.989_991delAGG, and c.1223+5G>T]. Enzyme activity in E. coli-expressed mutations was 1.5% for c.329A>T and 17.5% for c.284T>C. qRT-PCR analysis revealed reduced gene expression in all evaluated genotypes: [c.209+1delG; c.572C>T]; [c.2T>C; c.828+1G>A]; [c.828+1G>A; c.1126G>A]; [c.833T>C; c.989_991delAGG]; [c.1058C>T; c.146C>T]; and [c.444delG; c.444delG]. The expected phenotype according to the genotype (pyridoxine responsiveness) matched in all cases. CONCLUSIONS: Most patients studied were pyridoxine nonresponsive and presented early manifestations, suggesting severe phenotypes. Many private mutations were observed, but the four most prevalent mutations together accounted for over 50% of mutated alleles. A good genotype-phenotype relationship was observed within families and for the four most common mutations.

A sensitive assay system to test antisense oligonucleotides for splice suppression therapy in the mouse liver.

We have previously demonstrated the efficacy of antisense therapy for splicing defects in cellular models of metabolic diseases, suppressing the use of cryptic splice sites or pseudoexon insertions. To date, no animal models with these defects are available. Here, we propose exon skipping of the phenylalanine hydroxylase (Pah) gene expressed in liver and kidney to generate systemic hyperphenylalaninemia in mice as a sensitive in vivo assay to test splice suppression. Systemic elevation of blood L-Phe can be quantified using tandem MS/MS. Exon 11 and/or 12 skipping for the normal PAH gene was validated in hepatoma cells for comparing two oligonucleotide chemistries, morpholinos and locked nucleic acids. Subsequently, Vivo-morpholinos (VMO) were tested in wild-type and in phenotypically normal Pah(enu2/+) heterozygous mice to target exon 11 and/or 12 of the murine Pah gene using different VMO dosing, mode of injection and treatment regimes. Consecutive intravenous injections of VMO resulted in transient hyperphenylalaninemia correlating with complete exon skipping and absence of PAH protein and enzyme activity. Sustained effect required repeated injection of VMOs. Our results provide not only a sensitive in vivo assay to test for splice-modulating antisense oligonucleotides, but also a simple method to generate murine models for genetic liver diseases.