Epigenetically mediated inhibition of S-adenosylhomocysteine hydrolase and the associated dysregulation of 1-carbon metabolism in nonalcoholic steatohepatitis and hepatocellular carcinoma

Integrated single-cell transcriptomics reveals the hypoxia-induced inflammation-cancer transformation in NASH-derived hepatocellular carcinoma

Non-alcoholic fatty liver disease (NAFLD) has emerged as the primary risk factor for hepatocellular carcinoma (HCC), owing to improved vaccination rates of Hepatitis B and the increasing prevalence of metabolic syndrome related to obesity. Although the importance of innate and adaptive immune cells has been emphasized, the malignant transformation of hepatocytes and their intricate cellular network with the immune system remain unclear. The study incorporated four single-cell transcriptomic datasets of liver tissues covering healthy and NAFLD-related disease status. To identify the subsets and functions of hepatocytes and macrophages, we employed differential composition analysis, functional enrichment analysis, pseudotime analysis, and scenic analysis. Furthermore, an experimental mouse model for the transformation of nonalcoholic steatohepatitis into hepatocellular carcinoma was established for validation purposes. We defined CYP7A1+ hepatocytes enriched in precancerous lesions as 'Transitional Cells' in the progression from NAFLD to HCC. CYP7A1+ hepatocytes upregulated genes associated with stress response, inflammation and cancer-associated pathways and downregulated the normal hepatocyte signature. We observed that hypoxia activation accompanied the entire process of inflammation-cancer transformation. Hepatocyte-derived HIF1A was gradually activated during the progression of NAFLD disease to adapt to the hypoxic microenvironment and hepatocytes under hypoxic environment led to changes in the metabolism, proliferation and angiogenesis, promoting the occurrence of tumours. Meanwhile, hypoxia induced the polarization of RACK1+ macrophages that enriched in the liver tissues of NASH towards immunosuppressed TREM2+ macrophages. Moreover, immunosuppressive TREM2+ macrophages were recruited by tumour cells through the CCL15-CCR1 axis to enhance immunosuppressive microenvironment and promote NAFLD-related HCC progression. The study provides a deep understanding of the development mechanism of NAFLD-related HCC and offers theoretical support and experimental basis for biological targets, drug research, and clinical application.

Comprehensive analysis of epigenetic and epitranscriptomic genes' expression in human NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a multifactorial condition with a complex etiology. Its incidence is increasing globally in parallel with the obesity epidemic, and it is now considered the most common liver disease in Western countries. The precise mechanisms underlying the development and progression of NAFLD are complex and still poorly understood. The dysregulation of epigenetic and epitranscriptomic mechanisms is increasingly recognized to play pathogenic roles in multiple conditions, including chronic liver diseases. Here, we have performed a comprehensive analysis of the expression of epigenetic and epitranscriptomic genes in a total of 903 liver tissue samples corresponding to patients with normal liver, obese patients, and patients with non-alcoholic fatty liver (NAFL) and non-alcoholic steatohepatitis (NASH), advancing stages in NAFLD progression. We integrated ten transcriptomic datasets in an unbiased manner, enabling their robust analysis and comparison. We describe the complete landscape of epigenetic and epitranscriptomic genes' expression along the course of the disease. We identify signatures of genes significantly dysregulated in association with disease progression, particularly with liver fibrosis development. Most of these epigenetic and epitranscriptomic effectors have not been previously described in human NAFLD, and their altered expression may have pathogenic implications. We also performed a comprehensive analysis of the expression of enzymes involved in the metabolism of the substrates and cofactors of epigenetic and epitranscriptomic effectors. This study provides novel information on NAFLD pathogenesis and may also guide the identification of drug targets to treat this condition and its progression towards hepatocellular carcinoma.

Inhibition of S-adenosylhomocysteine hydrolase induces endothelial senescence via hTERT downregulation

BACKGROUND AND AIMS

It has been established that endothelial senescence plays a critical role in the development of atherosclerosis. Elevated S-adenosylhomocysteine (SAH) level induced by inhibition of S-adenosylhomocysteine hydrolase (SAHH) is one of the risk factors of atherosclerosis; however, the interplay between endothelial senescence and inhibition of SAHH is largely unknown.

METHODS

Human umbilical vein endothelial cells (HUVECs) after serial passage were used. SAHH-specific inhibitor adenosine dialdehyde (ADA) and SAHH siRNA treated HUVECs and SAHH^+/-mice were used to investigate the effect of SAHH inhibition on endothelial senescence.

RESULTS

HUVECs exhibited distinct senescence morphology as HUVECs were passaged, together with a decrease in intracellular SAHH expression and an increase in intracellular SAH levels. SAHH inhibition by ADA or SAHH siRNA elevated SA β-gal activity, arrested proliferation, and increased the expression of p16, p21 and p53 in HUVECs and the aortas of mice. In addition, decreased expression of hTERT and reduced occupancy of H3K4me3 over the hTERT promoter region were observed following SAHH inhibition treatment. To further verify the role of hTERT in the endothelial senescence induced by SAHH inhibition, hTERT was overexpressed with a plasmid vector under CMV promoter. hTERT overexpression rescued the senescence phenotypes in endothelial cells induced by SAHH inhibition.

CONCLUSIONS

SAHH inhibition induces endothelial senescence via downregulation of hTERT expression, which is associated with attenuated histone methylation over the hTERT promoter region.

Methionine cycle in nonalcoholic fatty liver disease and its potential applications

As a chronic metabolic disease affecting epidemic proportions worldwide, the pathogenesis of Nonalcoholic Fatty Liver Disease (NAFLD) is not clear yet. There is also a lack of precise biomarkers and specific medicine for the diagnosis and treatment of NAFLD. Methionine metabolic cycle, which is critical for the maintaining of cellular methylation and redox state, is involved in the pathophysiology of NAFLD. However, the molecular basis and mechanism of methionine metabolism in NAFLD are not completely understood. Here, we mainly focus on specific enzymes that participates in methionine cycle, to reveal their interconnections with NAFLD, in order to recognize the pathogenesis of NAFLD from a new angle and at the same time, explore the clinical characteristics and therapeutic strategies.

A robust gene expression signature for NASH in liver expression data

Non-Alcoholic Fatty Liver Disease (NAFLD) is a progressive liver disease that affects up to 30% of worldwide population, of which up to 25% progress to Non-Alcoholic SteatoHepatitis (NASH), a severe form of the disease that involves inflammation and predisposes the patient to liver cirrhosis. Despite its epidemic proportions, there is no reliable diagnostics that generalizes to global patient population for distinguishing NASH from NAFLD. We performed a comprehensive multicohort analysis of publicly available transcriptome data of liver biopsies from Healthy Controls (HC), NAFLD and NASH patients. Altogether we analyzed 812 samples from 12 different datasets across 7 countries, encompassing real world patient heterogeneity. We used 7 datasets for discovery and 5 datasets were held-out for independent validation. Altogether we identified 130 genes significantly differentially expressed in NASH versus a mixed group of NAFLD and HC. We show that our signature is not driven by one particular group (NAFLD or HC) and reflects true biological signal. Using a forward search we were able to downselect to a parsimonious set of 19 mRNA signature with mean AUROC of 0.98 in discovery and 0.79 in independent validation. Methods for consistent diagnosis of NASH relative to NAFLD are urgently needed. We showed that gene expression data combined with advanced statistical methodology holds the potential to serve basis for development of such diagnostic tests for the unmet clinical need.

Obesity Prevents S-Adenosylmethionine-Mediated Improvements in Age-Related Peripheral and Hippocampal Outcomes

Background: Age predisposes individuals to a myriad of disorders involving inflammation; this includes stress-related neuropsychiatric disorders such as depression and anxiety, and neurodegenerative diseases. Obesity can further exacerbate these effects in the brain. We investigated whether an inexpensive dietary supplement, s-adenosylmethionine (SAMe), could improve age- and/or obesity-related inflammatory and affective measures in the hippocampus. Methods: Mice were placed on their diets at six weeks of age and then aged to 14 months, receiving SAMe (0.1 g/kg of food) for the final six weeks of the experiment. Prior to tissue collection, mice were tested for anxiety-like behaviors in the open field test and for metabolic outcomes related to type 2 diabetes. Results: SAMe treatment significantly improved outcomes in aged control mice, where fasting glucose decreased, liver glutathione levels increased, and hippocampal microglia morphology improved. SAMe increased transforming growth factor β-1 mRNA in both control mice, potentially accounting for improved microglial outcomes. Obese mice demonstrated increased anxiety-like behavior, where SAMe improved some, but not all, open field measures. Conclusions: In summary, SAMe boosted antioxidant levels, improved diabetic measures, and hippocampal inflammatory and behavioral outcomes in aged mice. The effects of SAMe in obese mice were more subdued, but it could still provide some positive outcomes for obese individuals dealing with anxiety and having difficulty changing their behaviors to improve health outcomes.

Homocysteine: A novel prognostic biomarker in liver transplantation for alpha-fetoprotein- negative hepatocellular carcinoma

BACKGROUND

Precise recipient selection optimizes the prognosis of liver transplantation (LT) for hepatocellular carcinoma (HCC). Alpha-fetoprotein (AFP) is the most commonly used biomarker for diagnosis and prognosis of HCC in the clinical context. As a crucial molecule in methionine cycle, homocysteine (Hcy) level has been proved to be related to HCC progression and metastasis.

OBJECTIVE

We aimed to explore the prognostic capacity of pre-transplant serum Hcy level in LT for HCC.

METHODS

This study retrospectively enrolled 161 HCC patients who had underwent LT from donation after cardiac death (DCD) in the First Affiliated Hospital of Zhejiang University from 2015.01.01 to 2018.09.01. Pre-transplant serum Hcy level was incorporated into statistical analysis together with other clinical parameters and pathological features.

RESULTS

From an overall perspective, significant difference was observed in Hcy level between recurrence (n= 61) and non-recurrence group (n= 100) though subsequent analysis showed unsatisfactory predicting performance. In the whole cohort, multivariate analysis showed that lnAFP (p= 0.010) and Milan criteria (MC, p< 0.001) were independent risk factors of HCC recurrence after LT. MA score based on MC and lnAFP performed well in predicting post-LT tumor recurrence with the AUROC at 0.836 (p< 0.001) and 3-year recurrence-free survival rate at 96.8% (p< 0.001) in the low risk group (n= 69). According to the clinical practice, serum concentration lower than 20 μg/L is considered as normal range of AFP. Elevated pre-transplant serum AFP (> 20 μg/L) predicts high HCC recurrence after LT. We further divided the 161 recipients into AFP- group (n= 77, AFP ⩽ 20 μg/L) and AFP+ group (n= 84, AFP > 20 μg/L). MA score was still well presented in the AFP+ group and the AUROC for tumor recurrence was 0.823 (p< 0.001), whereas the predicting accuracy was reduced in AFP- group (AUROC: 0.754, P< 0.001). After subsequent analysis, we found that elevated pre-transplant Hcy level (> 12.75 μmol/L) predicted increased tumor recurrence risk in AFP- group. The 3-year recurrence-free survival rates were 92.0% and 53.7% (p< 0.001) in low Hcy subgroup (n= 40) and high Hcy subgroup (n= 37) respectively. Multivariate analysis showed that Hcy (p= 0.040) and Milan criteria (p= 0.003) were independent risk factors for post-transplant tumor recurrence in AFP- group. Further combination of Hcy level and Milan criteria identified a subgroup of AFP- recipients with acceptable outcomes even though beyond Milan criteria (3-year recurrence-free survival rate: 77.7%, p< 0.001).

CONCLUSION

As a classic predictor in HCC prognosis, AFP performed well in our study cohort when combined with Milan criteria. Homocysteine was an effective prognostic biomarker in LT for AFP- hepatocellular carcinoma. In recipients exceeding Milan criteria, acceptable post-transplant outcome could be seen in those with low Hcy and AFP level.

O-GlcNAcylation regulates the methionine cycle to promote pluripotency of stem cells

Methionine metabolism is critical for the maintenance of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) pluripotency. However, little is known about the regulation of the methionine cycle to sustain ESC pluripotency. Here, we show that adenosylhomocysteinase (AHCY), an important enzyme in the methionine cycle, is critical for the maintenance and differentiation of mouse embryonic stem cells (mESCs). We show that mESCs exhibit high levels of methionine metabolism, whereas decreasing methionine metabolism via depletion of AHCY promotes mESCs to differentiate into the three germ layers. AHCY is posttranslationally modified with an O-linked β-N-acetylglucosamine sugar (O-GlcNAcylation), which is rapidly removed upon differentiation. O-GlcNAcylation of threonine 136 on AHCY increases its activity and is important for the maintenance of trimethylation of histone H3 lysine 4 (H3K4me3) to sustain mESC pluripotency. Blocking glycosylation of AHCY decreases the ratio of S-adenosylmethionine versus S-adenosylhomocysteine (SAM/SAH), reduces the level of H3K4me3, and poises mESC for differentiation. In addition, blocking glycosylation of AHCY reduces somatic cell reprogramming. Thus, our findings reveal a critical role of AHCY and a mechanistic understanding of O-glycosylation in regulating ESC pluripotency and differentiation.

Gegen Qinlian Decoction abates nonalcoholic steatohepatitis associated liver injuries via anti-oxidative stress and anti-inflammatory response involved inhibition of toll-like receptor 4 signaling pathways

Gegen Qilian Decoction (GGQLD) is a well-established classic Chinese medicine prescription in treating nonalcoholic steatohepatitis (NASH). However, the molecular mechanism of GGQLD action on NASH is still not clear. This study aimed to assess the anti-NASH effect of GGQLD, and to explore its molecular mechanisms in vivo and in vitro. In HFD-fed rats, GGQLD decreased significantly serum triglyceride (TG), cholesterol (CHO), total bile acid (TBA), low-density lipoprotein (LDL), free fatty acid (FFA) and lipopolysaccharide (LPS) levels, increased levels of differentially expressed proteins (DEPs) Ahcy, Gpx1, Mat1a, GNMT, and reduced the expression of ALDOB. In RAW264.7 macrophages, GGQLD reduced the expression levels of inflammatory factors TNF-α and IL-6 mRNA, and diminished NASH by increasing differentially expressed genes (DEGs) CBS, Mat1a, Hnf4α and Pparα to reduce oxidative stress or lipid metabolism. The results of DEGs verification also showed that GGQLD up-regulated expressions of Hnf4α, Pparα and Cbs genes. In HepG2 cells, GGQLD decreased IL-6 levels and intracellular TG content, and inhibited FFA-induced expression of toll-like receptor 4 (TLR4). In summary, GGQLD abates NASH associated liver injuries via anti-oxidative stress and anti-inflammatory response involved inhibition of TLR4 signal pathways. These findings provide new insights into the anti-NASH therapy by GGQLD.

LINC00662 promotes hepatocellular carcinoma progression via altering genomic methylation profiles

The identification of viability-associated long noncoding RNAs (lncRNAs) is a means of uncovering therapeutic approaches for hepatocellular carcinoma (HCC). In addition, aberrant genome-wide hypomethylation has been implicated in HCC initiation and progression. However, the relationship between lncRNA dysregulation and genome-wide hypomethylation in hepatocarcinogenesis has not been fully elucidated. A novel lncRNA named LINC00662 was previously demonstrated to play a role in gastrointestinal cancer. In this study, we demonstrated that this lncRNA was correlated with survival and exhibited oncogenic properties, both in vitro and in vivo. Moreover, we determined that LINC00662 could lead to genome-wide hypomethylation and alter the genomic methylation profile by synchronously reducing the S-adenosylmethionine (SAM) level and enhancing the S-adenosylhomocysteine (SAH) level. Mechanistically, LINC00662 was determined to regulate the key enzymes influencing SAM and SAH levels, namely, methionine adenosyltransferase 1A (MAT1A) and S-adenosylhomocysteine hydrolase (AHCY), by RNA-RNA and RNA-protein interactions. In addition, we demonstrated that some SAM-dependent HCC-promoting genes could be regulated by LINC00662 by altering the methylation status of their promoters via the LINC00662-coupled axes of MAT1A/SAM and AHCY/SAH. Taken together, the results of this this study indicate that LINC00662 could be a potential biomarker for HCC therapy. More importantly, we proposed a new role of lncRNA in regulating genomic methylation to promote oncogene activation.

Metabolic impact of partial hepatectomy in the non-alcoholic steatohepatitis animal model of methionine-choline deficient diet

In the liver, obesity is often manifested by the clinical disorder of the Non-Alcoholic Fatty Liver Disease (NAFLD). A proportion of NAFLD patients develop hepatic inflammation, known as Non-Alcoholic Steatohepatitis (NASH), which can end up in cirrhosis, or Hepatocellular Carcinoma (HCC). In this scenario, partial hepatectomy (PH) is an alternative to promote liver regeneration. However, as liver regeneration is impaired in NASH patients, more knowledge about its metabolic condition is needed to improve the regenerative response of the liver in this pathological condition. Although extensively employed, the panoply of molecular alterations involved in the regenerative response of the liver after partial hepatectomy PH is far from being fully characterized. Metabolic fingerprinting (metabolomics) is a powerful tool to help in the elucidation of complex metabolic networks, by means of a blind, naïve approach to study which metabolic nodes (metabolites) show the biggest variations between conditions. The objective of the present study was to gain deeper knowledge about the metabolic processes involved in the NASH animal model, and particularly in the effect of PH by using metabolomics. For achieving such information, twelve 8-week-old male C57BL/6 J mice, fed commercial chow (control diet) or methionine and choline-Deficient diet (MCD) for three weeks were subjected to PH and sacrificed 2 weeks later. Livers were removed and submitted to metabolic profiling analysis through RP-LC/MS (qTOF), GC/MS (qTOF) and CE/MS(TOF). More than 3000 different features were detected and repeated measurements one-way ANOVA analysis was performed to unveil significant features. MCD diet induced changes (p < 0.05) in 46% of the detected features, whereas PH provoked significant changes in 85% of them. Most of the changes were detected through LC/MS and were associated to lipid metabolism. However, changes of metabolites virtually related to other metabolic routes (amino acids, carbohydrates, nucleotides) were found altered and detected by CE/MS and GC/MS. The changes associated to PH show a similar trend regardless of the diet, but in the context of the diet deficient in methionine and choline we have found results that point to a different ratio glycolysis/tricarboxylic acid cycle. Moreover, in the NASH model, the regeneration of the liver structures occurs at the expense of an increased phosphatidylethanolamines/phosphatidylcholines ratio.

Mitochondrial Dysfunction in the Transition from NASH to HCC

The liver constantly adapts to meet energy requirements of the whole body. Despite its remarkable adaptative capacity, prolonged exposure of liver cells to harmful environmental cues (such as diets rich in fat, sugar, and cholesterol) results in the development of chronic liver diseases (including non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH)) that can progress to hepatocellular carcinoma (HCC). The pathogenesis of these diseases is extremely complex, multifactorial, and poorly understood. Emerging evidence suggests that mitochondrial dysfunction or maladaptation contributes to detrimental effects on hepatocyte bioenergetics, reactive oxygen species (ROS) homeostasis, endoplasmic reticulum (ER) stress, inflammation, and cell death leading to NASH and HCC. The present review highlights the potential contribution of altered mitochondria function to NASH-related HCC and discusses how agents targeting this organelle could provide interesting treatment strategies for these diseases.

Study on the mechanism underlying Al-induced hepatotoxicity based on the identification of the Al-binding proteins in liver

Aluminum (Al) is the most abundant metal element in the earth's crust, and is implicated in the pathogenesis of liver lesions. However, the mechanisms underlying Al³⁺-induced hepatotoxicity are still largely elusive. Based on analysis with native gel electrophoresis, Al³⁺ plus 8-hydroxyquinoline staining and LC-MS/MS, the proteins with high Al³⁺ affinity were identified to be carbamoyl-phosphate synthase, adenosylhomocysteinase, heat shock protein 90-alpha, carbonic anhydrase 3, serum albumin and calreticulin. These proteins are involved in physiological processes such as the urea cycle, redox reactions, apoptosis and so on. Then we established an Al³⁺-treated rat model for biochemical tests, morphology observation and Ca²⁺ homeostasis analysis, in order to evaluate the extent of oxidative damage, hepatic histopathology and specific indicators of Al³⁺-related proteins in liver. Our findings indicated the high-affinity interactions with Al³⁺ perturbed the normal function of the above proteins, which could account for the mechanism underlying Al³⁺-induced hepatotoxicity.

Dysregulation of Epigenetic Mechanisms of Gene Expression in the Pathologies of Hyperhomocysteinemia

Hyperhomocysteinemia (HHcy) exerts a wide range of biological effects and is associated with a number of diseases, including cardiovascular disease, dementia, neural tube defects, and cancer. Although mechanisms of HHcy toxicity are not fully uncovered, there has been a significant progress in their understanding. The picture emerging from the studies of homocysteine (Hcy) metabolism and pathophysiology is a complex one, as Hcy and its metabolites affect biomolecules and processes in a tissue- and sex-specific manner. Because of their connection to one carbon metabolism and editing mechanisms in protein biosynthesis, Hcy and its metabolites impair epigenetic control of gene expression mediated by DNA methylation, histone modifications, and non-coding RNA, which underlies the pathology of human disease. In this review we summarize the recent evidence showing that epigenetic dysregulation of gene expression, mediated by changes in DNA methylation and histone N-homocysteinylation, is a pathogenic consequence of HHcy in many human diseases. These findings provide new insights into the mechanisms of human disease induced by Hcy and its metabolites, and suggest therapeutic targets for the prevention and/or treatment.

Prediction of response to anti-cancer drugs becomes robust via network integration of molecular data

Despite the widening range of high-throughput platforms and exponential growth of generated data volume, the validation of biomarkers discovered from large-scale data remains a challenging field. In order to tackle cancer heterogeneity and comply with the data dimensionality, a number of network and pathway approaches were invented but rarely systematically applied to this task. We propose a new method, called NEAmarker, for finding sensitive and robust biomarkers at the pathway level. scores from network enrichment analysis transform the original space of altered genes into a lower-dimensional space of pathways. These dimensions are then correlated with phenotype variables. The method was first tested using in vitro data from three anti-cancer drug screens and then on clinical data of The Cancer Genome Atlas. It proved superior to the single-gene and alternative enrichment analyses in terms of (1) universal applicability to different data types with a possibility of cross-platform integration, (2) consistency of the discovered correlates between independent drug screens, and (3) ability to explain differential survival of treated patients. Our new screen of anti-cancer compounds validated the performance of multivariate models of drug sensitivity. The previously proposed methods of enrichment analysis could achieve comparable levels of performance in certain tests. However, only our method could discover predictors of both in vitro response and patient survival given administration of the same drug.

Mitochondrial role in the neonatal predisposition to developing nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a global epidemic in obese children and adults, and the onset might have fetal origins. A growing body of evidence supports the role of developmental programming, whereby the maternal environment affects fetal and infant development, altering the risk profile for disease later in life. Human and nonhuman primate studies of maternal obesity demonstrate that risk factors for pediatric obesity and NAFLD begin in utero. The pathologic mechanisms for NAFLD are multifactorial but have centered on altered mitochondrial function/dysfunction that might precede insulin resistance. Compared with the adult liver, the fetal liver has fewer mitochondria, low activity of the fatty acid metabolic enzyme carnitine palmitoyl-CoA transferase-1, and little or no gluconeogenesis. Exposure to excess maternal fuels during fetal life uniquely alters hepatic fatty acid oxidation, tricarboxylic acid cycle activity, de novo lipogenesis, and mitochondrial health. These events promote increased oxidative stress and excess triglyceride storage, and, together with altered immune function and epigenetic changes, they prime the fetal liver for NAFLD and might drive the risk for nonalcoholic steatohepatitis in the next generation.

Mouse models for investigating the underlying mechanisms of nonalcoholic steatohepatitis-derived hepatocellular carcinoma

As the incidence of hepatocellular carcinoma (HCC) caused by infection with the hepatotropic viruses hepatitis B and hepatitis C decreases, greater attention has become focused on HCC caused by nonalcoholic steatohepatitis (NASH), an advanced form of nonalcoholic fatty liver disease which has shown increasing prevalence in correspondence with the overall increase in metabolic syndrome over the recent decades. Several clinical population studies have shown a positive relationship between NASH and HCC, while also providing initial insights into the underlying mechanisms of HCC development from NASH. Research into the pathological progression of NASH to HCC has advanced by use of several beneficial rodent models. In this review, we summarize the established mouse models for preclinical research of NASH-associated HCC and discuss the underlying hepatic mechanisms of NASH-related tumorigenesis identified to date that could lead to new targets for treatment and prevention.