Enzyme replacement therapy prevents loss of bone and fat mass in murine homocystinuria

Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria

Cystathionine beta-synthase (CBS)-deficient homocystinuria (HCU) is the most common inborn error of sulfur amino acid metabolism. The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine, a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment. The current standard of care involves significant dietary interventions that, despite being effective, often adversely affect quality of life of HCU patients, leading to poor adherence to therapy and inadequate biochemical control with clinical complications. In recent years, the unmet need for better therapeutic options has resulted in development of novel enzyme and gene therapies and exploration of pharmacological approaches to rescue CBS folding defects caused by missense pathogenic mutations. Here, we review scientific evidence and current state of affairs in development of recent approaches to treat HCU.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Cystathionine β-synthase deficiency in the E-HOD registry-part I: pyridoxine responsiveness as a determinant of biochemical and clinical phenotype at diagnosis

Cystathionine β-synthase (CBS) deficiency has a wide clinical spectrum, ranging from neurodevelopmental problems, lens dislocation and marfanoid features in early childhood to adult onset disease with predominantly thromboembolic complications. We have analysed clinical and laboratory data at the time of diagnosis in 328 patients with CBS deficiency from the E-HOD (European network and registry for Homocystinurias and methylation Defects) registry. We developed comprehensive criteria to classify patients into four groups of pyridoxine responsivity: non-responders (NR), partial, full and extreme responders (PR, FR and ER, respectively). All groups showed overlapping concentrations of plasma total homocysteine while pyridoxine responsiveness inversely correlated with plasma/serum methionine concentrations. The FR and ER groups had a later age of onset and diagnosis and a longer diagnostic delay than NR and PR patients. Lens dislocation was common in all groups except ER but the age of dislocation increased with increasing responsiveness. Developmental delay was commonest in the NR group while no ER patient had cognitive impairment. Thromboembolism was the commonest presenting feature in ER patients, whereas it was least likely at presentation in the NR group. This probably is due to the differences in ages at presentation: all groups had a similar number of thromboembolic events per 1000 patient-years. Clinical severity of CBS deficiency depends on the degree of pyridoxine responsiveness. Therefore, a standardised pyridoxine-responsiveness test in newly diagnosed patients and a critical review of previous assessments is indispensable to ensure adequate therapy and to prevent or reduce long-term complications.

Regulation of the parental gene GRM4 by circGrm4 RNA transcript and glutamate-mediated neurovascular toxicity in eyes

Epigenetic memory plays crucial roles in gene regulation. It not only modulates the expression of specific genes but also has ripple effects on transcription as well as translation of other genes. Very often an alteration in expression occurs either via methylation or demethylation. In this context, "1-carbon metabolism" assumes a special significance since its dysregulation by higher levels of homocysteine; Hcy (known as hyperhomocysteinemia; HHcy), a byproduct of "1-Carbon Metabolism" during methionine biosynthesis leads to serious implications in cardiovascular, renal, cerebrovascular systems, and a host of other conditions. Currently, the circular RNAs (circRNAs) generated via non-canonical back-splicing events from the pre-mRNA molecules are at the center stage for their essential roles in diseases via their epigenetic manifestations. We recently identified a circular RNA transcript (circGRM4) that is significantly upregulated in the eye of cystathionine β-synthase-deficient mice. We also discovered a concurrent over-expression of the mGLUR4 receptor in the eyes of these mice. In brief, circGRM4 is selectively transcribed from its parental mGLUR4 receptor gene (GRM4) functions as a "molecular-sponge" for the miRNAs and results into excessive turnover of the mGLUR4 receptor in the eye in response to extremely high circulating glutamate concentration. We opine that this epigenetic manifestation potentially predisposes HHcy people to retinovascular malfunctioning.

Interplay of Enzyme Therapy and Dietary Management of Murine Homocystinuria

Albeit effective, methionine/protein restriction in the management of classical homocystinuria (HCU) is suboptimal and hard to follow. To address unmet need, we developed an enzyme therapy (OT-58), which effectively corrected disease symptoms in various mouse models of HCU in the absence of methionine restriction. Here we evaluated short- and long-term efficacy of OT-58 on the background of current dietary management of HCU. Methionine restriction resulted in the lowering of total homocysteine (tHcy) by 38–63% directly proportional to a decreased methionine intake (50–12.5% of normal). Supplemental betaine resulted in additional lowering of tHcy. OT-58 successfully competed with betaine and normalized tHcy on the background of reduced methionine intake, while substantially lowering tHcy in mice on normal methionine intake. Betaine was less effective in lowering tHcy on the background of normal or increased methionine intake, while exacerbating hypermethioninemia. OT-58 markedly reduced both hyperhomocysteinemia and hypermethioninemia caused by the diets and betaine in HCU mice. Withdrawal of betaine did not affect improved metabolic balance, which was established and solely maintained by OT-58 during periods of fluctuating dietary methionine intake. Taken together, OT-58 may represent novel, highly effective enzyme therapy for HCU performing optimally in the presence or absence of dietary management of HCU.

Hypermethioninemia Leads to Fatal Bleeding and Increased Mortality in a Transgenic I278T Mouse Model of Homocystinuria

Severely elevated plasma homocysteine and methionine lead to thromboembolic events and strokes in homocystinuric (HCU) patients. Mouse models of HCU failed to exhibit prothrombotic phenotype, presumably due to lack of hypermethioninemia. We evaluated the impact of hypermethioninemia together with hyperhomocysteinemia on murine HCU phenotype and compared the efficacy of the current and novel therapies for HCU. High methionine intake decreased survival of I278T mice, which died from intestinal bleeding with hepatic and pancreatic failure. I278T mice on normal or increased methionine intake developed endothelial dysfunction, but paradoxically demonstrated delayed occlusion in an induced arterial thrombosis model. RNA-seq analysis suggested that expression of coagulation factor XI (FXI) is downregulated in livers of I278T mice. Indeed, plasma concentrations of FXI were decreased in I278T mice on normal diet and further reduced by increased methionine intake. Dietary methionine restriction normalized the observed phenotype. Similarly, treatment with OT-58, a novel enzyme therapy for HCU, corrected the phenotype in I278T mice regardless of their dietary methionine intake. Hypermethioninemia does not contribute to prothrombotic phenotype in murine HCU. Downregulation of FXI may contribute to the lack of prothrombotic tendency in I278T mice. Methionine restriction or treatment with OT-58 corrects vascular disease in the I278T mouse model of HCU.

Behavior, body composition, and vascular phenotype of homocystinuric mice on methionine-restricted diet or enzyme replacement therapy

Classic homocystinuria (HCU) is an inherited disorder characterized by elevated homocysteine (Hcy) in plasma and tissues resulting from cystathionine β-synthase (CBS) deficiency. There is no cure, and patients are predominantly managed by methionine-restricted diet (MRD) to limit the production of Hcy. In this study, we used the I278T mouse model of HCU to evaluate the long-term impact of a novel enzyme replacement therapy [truncated human CBS C15S mutant modified with linear 20-kDa N-hydroxysuccinimide ester polyethylene glycol (OT-58)] on clinical end points relevant to human patients with HCU. In addition, we compared its efficacy on a background of either MRD or normal methionine intake [regular diet (REG)] to that of MRD alone. We found that, compared with untreated I278T mice, OT-58 treatment of I278T mice fed with the REG diet resulted in a 90% decrease in plasma Hcy concentrations and correction of learning/cognition, endothelial dysfunction, hemostasis, bone mineralization, and body composition. On background of the MRD, OT-58 performed equally well with plasma Hcy entirely normalized. The MRD alone decreased plasma Hcy by 67% and corrected the HCU phenotype in I278T mice. However, the MRD increased anxiety and reduced bone mineral content in both I278T mice and wild-type controls. This study shows that OT-58 is a highly efficacious novel treatment for HCU on the background of either normal or restricted methionine intake.-Majtan, T., Park, I., Cox, A., Branchford, B. R., di Paola, J., Bublil, E. M., Kraus, J. P. Behavior, body composition, and vascular phenotype of homocystinuric mice on methionine-restricted diet or enzyme replacement therapy.

Short tandem repeats and methylation in the promoter region affect expression of cystathionine beta-synthase gene in the laying hen

Homocysteine can be converted to cysteine via the transsulfuration pathway where cystathionine beta-synthase (CBS) is a rate-limiting enzyme. Homocysteine is thought to play a role in bone remodelling and strength. Previous results indicated that some of the difference in bone strength of end-of-lay hens may be associated with CBS expression level. To investigate if differences in the promoter region of the CBS gene might be responsible for observed differences in gene expression between CBS alleles post mortem- and in-vitro expression studies have been undertaken. Transfection of the DF-1 avian cell line with a series of deletion fragments of the 5' promoter, or constructs containing three CBS allele sequences, with a luciferase reporter revealed that a core region of -155 to +131 bp in the CBS promoter was essential for mRNA expression. We found that a variable number of short tandem repeats (7 nucleotide motif and 6 nucleotide repeats) in the core region of the promoter affecting the transcriptional activity and a strong effect for gene expression. However, methylation of the 6 nucleotide repeats varied between allelic variants and these maybe responsible for differences in promoter activity. Our findings indicated variable short tandem repeats and the differentially methylated sites in the promoter region may be responsible for CBS expression differences in the bone of laying hens.

Pharmacokinetics and pharmacodynamics of PEGylated truncated human cystathionine beta-synthase for treatment of homocystinuria

AIMS

PEGylated human truncated cystathionine beta-synthase, lacking the C-terminal regulatory domain (PEG-CBS), is a promising preclinical candidate for enzyme replacement therapy in homocystinuria (HCU). It was designed to function as a metabolic sink to decrease the severely elevated plasma and tissue homocysteine concentrations. In this communication, we evaluated pharmacokinetics (PK), pharmacodynamics (PD) and sub-chronic toxicity of PEG-CBS in homocystinuric mice, wild type rats and monkeys to estimate the minimum human efficacious dose for clinical trials.

MAIN METHODS

Animal models received single or multiple doses of PEG-CBS. Activity of PEG-CBS and sulfur amino acid metabolites were determined in plasma and used to determine PK and PD.

KEY FINDINGS

The plasma half-lives of PEG-CBS after a single subcutaneous (SC) injection were approximately 20, 44 and 73 h in mouse, rat and monkey, respectively. The SC administration of PEG-CBS resulted in a significant improvement or full correction of metabolic imbalance in both blood and tissues of homocystinuric mice. The PD of PEG-CBS in mouse was dose-dependent, but less than dose-proportional, with the maximal efficacy achieved at 8 mg/kg. PEG-CBS was well-tolerated in mice and monkeys, but resulted in dose-dependent minimal-to-moderate inflammation at the injection sites and vacuolated macrophages in rats. Allometric scaling of animal data was linear and the estimated human efficacious dose was determined as 0.66 mg/kg administered once a week.

SIGNIFICANCE

These results provide critical preclinical data for the design of first-in-human PEG-CBS clinical trial.

Enzyme Replacement Therapy Ameliorates Multiple Symptoms of Murine Homocystinuria

Classical homocystinuria (HCU) is the most common inherited disorder of sulfur amino acid metabolism caused by deficiency in cystathionine beta-synthase (CBS) activity and characterized by severe elevation of homocysteine in blood and tissues. Treatment with dietary methionine restriction is not optimal, and poor compliance leads to serious complications. We developed an enzyme replacement therapy (ERT) and studied its efficacy in a severe form of HCU in mouse (the I278T model). Treatment was initiated before or after the onset of clinical symptoms in an effort to prevent or reverse the phenotype. ERT substantially reduced and sustained plasma homocysteine concentration at around 100 μM and normalized plasma cysteine for up to 9 months of treatment. Biochemical balance was also restored in the liver, kidney, and brain. Furthermore, ERT corrected liver glucose and lipid metabolism. The treatment prevented or reversed facial alopecia, fragile and lean phenotype, and low bone mass. In addition, structurally defective ciliary zonules in the eyes of I278T mice contained low density and/or broken fibers, while administration of ERT from birth partially rescued the ocular phenotype. In conclusion, ERT maintained an improved metabolic pattern and ameliorated many of the clinical complications in the I278T mouse model of HCU.