Working with model mice, post-mortem human brains, and people with schizophrenia, researchers at the RIKEN Center for Brain Science in Japan have discovered that a subtype of schizophrenia is related to abnormally high levels hydrogen sulfide in the brain.
Experiments showed that this abnormality likely results from a DNA-modifying reaction during development that lasts throughout life.
In addition to providing a new direction for research into drug therapies, higher than normal levels of the hydrogen sulfide-producing enzyme can act as biomarker for this type of schizophrenia.
Diagnosing disorders of thought is easier when a reliable and objective marker can be found. In the case of schizophrenia, we have known for more than 30 years that it is associated with an abnormal startle response.
Normally, we are not startled as much by a burst of noise if a smaller burst–called a prepulse–comes a little bit earlier.
This phenomenon is called prepulse inhibition (PPI) because the early pulse inhibits the startle response.
In people with schizophrenia, PPI is lowed, meaning that their startle response is not dampened as much as it should be after the prepulse.
The PPI test is a good behavioral marker, and although it cannot directly help us understand the biology behind schizophrenia, it was the starting point that led to current discoveries.
The researchers at RIKEN CBS began first looked for differences in protein expression between strains of mice that exhibit extremely low or extremely high PPI.
Ultimately, they found that the enzyme Mpst was expressed much more in the brains of the mouse strain with low PPI than in the strain with high PPI.
Knowing that this enzyme helps produce hydrogen sulfide, the team then measured hydrogen sulfide levels and found that they were higher in the low-PPI mice.
“Nobody has ever thought about a causal link between hydrogen sulfide and schizophrenia,” says team leader Takeo Toshikawa. “Once we discovered this, we had to figure out how it happens and if these findings in mice would hold true for people with schizophrenia.”
First, to be sure that Mpst was the culprit, the researchers created an Mpst knockout version of the low-PPI mice and showed that their PPI was higher than that in regular low-PPI mice.
Thus, reducing the amount of Mpst helped the mice become more normal. Next, they found that MPST gene expression was indeed higher in postmortem brains from people with schizophrenia than in those from unaffected people. MPST protein levels in these brains also correlated well with the severity of premortem symptoms.
Now the team had enough information to look at MPST expression as a biomarker for schizophrenia. They examined hair follicles from more than 150 people with schizophrenia and found that expression of MPST mRNA was much higher than people without schizophrenia.
Even though the results were not perfect–indicating that sulfide stress does not account for all cases of schizophrenia–MPST levels in hair could be a good biomarker for schizophrenia before other symptoms appear.
Whether a person develops schizophrenia is related to both their genetics and the environment. Testing in mice and postmortem brains indicated that high MPST levels were associated with changes in DNA that lead to permanently altered gene expression. So, the next step was for the team to search for environmental factors that could result in permanently increased MPST production.
MPST gene expression (which leads to hydrogen sulphide production) was higher in postmortem brains from people with schizophrenia than in those from unaffected people. MPST protein levels in these brains also correlated well with the severity of premortem symptoms.
Because hydrogen sulfide can actually protect against inflammatory stress, the group hypothesized that inflammatory stress during early development might be the root cause.
“We found that anti-oxidative markers–including the production of hydrogen sulfide–that compensate against oxidative stress and neuroinflammation during brain development were correlated with MPST levels in the brains of people with schizophrenia,” says Yoshikawa.
He proposes that once excess hydrogen sulfide production is primed, it persists throughout life due to permanent epigenetic changes to DNA, leading to “sulfide stress” induced schizophrenia.
Current treatments for schizophrenia focus on the dopamine and serotonin system in the brain. Because these drugs are not very effective and have side effects, Yoshikawa says that pharmaceutical companies have abandoned the development of new drugs. “A new paradigm is needed for the development of novel drugs,” he explains.
“Currently, about 30% of patients with schizophrenia are resistant to dopamine D2-receptor antagonist therapy. Our results provide a new principle or paradigm for designing drugs, and we are currently testing whether inhibiting the synthesis of hydrogen sulfide can alleviate symptoms in mouse models of schizophrenia.”
Hyperhomocysteinemia (HHcy), a medical condition of elevated homocysteine (Hcy) concentration in the plasma, usually defined as >15 µM [1,2], is prevalent in the general population and may have severe implications for the human health . Cardiovascular disease and stroke , dementia , schizophrenia , infertility, pregnancy complications, birth defects , cancer [8,9], liver injury , and osteoporosis  are all associated with HHcy. Elevated Hcy is often present in patients with kidney disease and uraemia  and nonalcoholic fatty liver disease [13,14].
HHcy is a risk factor for atherosclerosis, a chronic inflammatory disorder of large and intermediate arteries. Hcy levels tend to increase with age and disease, including atherosclerosis and thrombosis . HHcy causes vascular injury, manifested by endothelial desquamation, which in turn provokes smooth muscle cell proliferation leading to atherosclerosis as shown in a baboon model . Two recent reviews provide an overview of pathologies associated with HHcy [17,18].
HHcy is a consequence of insufficient intake of B-vitamins (B12, B9, B6) and betaine (cofactors of Hcy-metabolizing enzymes), excessive intake of Met, the only known source of Hcy in the human body, as well as of mutations in genes encoding cystathionine β-synthase (CBS), 5,10-methylenetetrahydrofolate reductase (MTHFR), or methionine synthase (MS) .
The studies of molecular mechanisms of HHcy toxicity are complicated by the fact that there are multiple molecular forms of Hcy [20,21], each of which may exert different and sex-specific effects on biological molecules , tissues and organs [23,24].
HHcy has a wide range of biological effects and the pathophysiological changes associated with HHcy are most likely accumulated outcomes of Hcy and its metabolites, such as Hcy-thiolactone (HTL), N-Hcy-proteins, S-Hcy-proteins, S-adenosylhomocysteine (SAH), and low molecular weight disulfides of Hcy .
In the majority of Hcy-lowering clinical trials, so called ‘total Hcy’ (tHcy) was examined as a marker for outcomes. However, because tHcy is a complex marker, which nevertheless does not include other important Hcy metabolites (such as HTL, N-Hcy-protein, SAH), the conclusions from those trials may not be informative.
A case in point is a recent study with a WENBIT cohort that found that HTL is a predictor of acute myocardial infarction but is not affected by tHcy-lowering therapy .
In the human body Hcy arises from dietary Met as a result of multiple reactions known as Met cycle (Figure 1). First, Met is activated to S-adenosylmethionine (SAM) by Met S-adenosyl transferase. SAM is a donor of methyl group to multiple acceptor molecules, e.g. proteins, DNA, RNA, lipids, and neurotransmitters, and after donating the methyl group, gives rise to SAH.
SAH is subsequently hydrolyzed by SAH hydrolase (AHCY) to yield Hcy. Hcy is then metabolized via four reactions (Figure 1): remethylation to Met by Met synthase (MS) or betaine:Hcy methyltransferase (BHMT), transsulfuration to Cys by CBS and cystathionine γ-lyase (CSE), conversion to HTL by Met-tRNA synthetase (MARS), or oxidation with thiol groups to form S-Hcy-protein or low molecular weight disulfides .
Epigenetic regulation, as a response to endogenous and exogenous factors, leads to alterations in gene expression via DNA methylation, histone modification, and the action of noncoding RNA.
Epigenetic remodeling plays an important role in both normal and pathological conditions of an organism. Modifications of DNA and histones regulate gene expression by changing chromatin structure, making it transcriptionally permissive (euchromatin) or inactive (heterochromatin).
Hcy occupies an important junction between one carbon metabolism and epigenetic processes (Figure 1). Methyl group used for methylation reactions originates form SAM, an intermediate in Hcy metabolism. Upon methylation SAM is converted to SAH, which inhibits transmethylation reactions.
HHcy, through SAH accumulation and diminished methylation capacity (decreased SAM/SAH ratio) could lead to global hypomethylation . However, this simplistic view is not supported by experimental evidence from cell culture (Table 1) and animal (Table 2) studies showing that decreased SAM/SAH ratio in general does not always lead to DNA hypomethylation [28,29].
A novel mechanism of epigenetic dysregulation in HHcy has been revealed by studies of Hcy-editing by MARS during protein biosynthesis , which generates HTL (Figure 1). HTL has a propensity to modify protein lysine residues [31,32], affording N-Hcy-protein (Figure 1), a process that also involves histones and interferes with their normal acetylation/methylation .
DNA and histone modifications as well as microRNAs and circular RNAs have been identified to have a crucial role in the progression of atherosclerosis [26,33], play an important role in stroke pathogenesis , Alzheimer’s disease (AD) development , neural tube defects , and cancer .
In this review we summarize the recent progress in studies of epigenetic dysregulation of gene expression by HHcy and its role in the etiology of human disease.
Elucidation of mechanisms of HHcy toxicity is crucial for prevention and treatment of major human diseases, including atherosclerosis, cancer, dementia and brain disease, liver injury, osteoporosis, and pregnancy complications.
Recent findings have revealed that the pathogenesis of many diseases, including those induced by HHcy, involves dysregulation of epigenetic mechanisms that control cell growth and differentiation. Details of the molecular and cellular mechanisms involving interactions of HHcy with DNA methylation, histone modifications, miRNA, and lncRNA are beginning to emerge.
The failure of Hcy-lowering trials to reduce heart attacks, suggests that some pathological changes caused by HHcy are irreversible and highlights the need for additional research into the mechanisms by which HHcy causes disease. It is likely that epigenetic dysregulation induced by HHcy cannot be reversed by dietary interventions targeting Hcy.
In fact, folic acid supplementation, which lowers plasma Hcy levels and elevates SAM/SAH ratio, does not affect global DNA methylation in HHcy subjects with normal renal function [198,199]. Although, in patients with uremia, folate treatment corrects global DNA hypomethylation , this is not always associated with better outcomes .
Folic acid supplementation also does not lower HTL  and anti-N-Hcy-protein autoantibodies  in CAD patients. Accumulating evidence suggests that targeting HTL, which is involved in the pathology of HHcy [19,25], might be an effective strategy for disease prevention. One possible therapeutic approach is to pharmacologically inhibit MARS using Hcy analogues, which reduce HTL synthesis at the MARS active site, which in turn attenuates protein N-homocysteinylation .
Another approach would be to up-regulate HTL-hydrolyzing enzymes, such as PON1, BLMH, or BPHL, which also would reduce HTL levels and N-Hcy-protein accumulation [202,203]. Although epigenetic dysregulation in HHcy has been proposed to be secondary to the accumulation of SAH, recent evidence indicates that in general SAH may not be associated with DNA hypomethylation.
Post-translational modification of histone lysine residues by HTL has been established as a new epigenetic mechanism responsible for neural tube defects. Analogous modifications of other proteins by HTL are involved in other diseases such as atherosclerosis, colorectal cancer, and Alzheimer’s disease. Additional studies are required to determine how general these mechanisms are and whether they can explain the pathology of other HHcy-related diseases.
|MDPI||Multidisciplinary Digital Publishing Institute|
|DOAJ||Directory of open access journals|
|TLA||Three letter acronym|
|APP||Amyloid-beta precursor protein|
|BPHL||Biphenyl hydrolase like|
|CFTR||Cystic fibrosis transmembrane conductance regulator|
|EPCs||Endothelial progenitor cells|
|FABP4||Fatty acid-binding protein 4|
|FATP1||Long-chain fatty acid transport protein 1|
|HUVECs||Human umbilical vein endothelial cells|
|MAPK||Mitogen-activated protein kinase|
|NASH||Nonalcoholic fatty liver disease|
|NTD||Neural tube defect|
|PDGF||Platelet-derived growth factor|
|PPAR||Peroxisome proliferator-activated receptor|
|ROS||Reactive oxygen species|
|She||Soluble epoxide hydrolase|
|TIMP||Tissue inhibitors of metaloproteinases|
|TERT||Telomerase reverse transcriptase|
|TLR4||Toll-like receptor 4|
|VSMCs||Vascular smooth muscle cells|
|TLA||Three letter acronym|
Adam Phillips – RIKEN
The image is credited to RIKEN.
Original Research: Open access
“Excess hydrogen sulfide and polysulfides production underlies a schizophrenia pathophysiology”. Reference: Ide et al.
EMBO Molecular Medicine doi:10.15252/emmm.201910695.