EBV and METHIONINE

[vesta]

Cell Metabolism SCIENCEDIRECT
Volume 34, Issue 9
, 6 September 2022, Pages 1229-1231
Journal home page for Cell Metabolism
Preview
Methionine restriction forces Epstein-Barr virus out of latency
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Sriraksha Bharadwaj Kashyap 1
,
Racheal Mulondo 1
,
Peter J. Mullen 1 2 3

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https://doi.org/10.1016/j.cmet.2022.08.009
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Methionine metabolism controls the B cell EBV epigenome and viral latency
Cell Metabolism, Volume 34, Issue 9, 6 September 2022, Pages 1280-1297.e9
Rui Guo, Jin Hua Liang, Yuchen Zhang, Michael Lutchenkov, Zhixuan Li, Yin Wang, Vicenta Trujillo-Alonso, Rishi Puri, Lisa Giulino-Roth, Benjamin E. Gewurz
View PDF
EBV gene expression is repressed during viral latency to prevent an immune response, but it is not known how metabolism contributes to this silencing. In this issue of Cell Metabolism, Guo et al. describe how methionine restriction reactivates the expression of EBV genes, offering new therapeutic approaches against EBV-driven diseases.

Previous article in issue
Next article in issue
Main text
Epstein-Barr virus (EBV) is one of the most prevalent human viruses, chronically infecting over 95% of the global population. Although most people experience mild or no symptoms, EBV is associated with multiple cancers, including Hodgkin’s and Burkitt’s lymphomas, and is a causative factor in multiple sclerosis. EBV chronically infects B cells, where it maintains a series of latent states to avoid detection by the host immune system. During latency, EBV uses epigenetic factors to downregulate the expression of its own genes, sometimes expressing only one. Previous studies showed that histone and viral DNA methylation are necessary to repress EBV gene expression. This requires the universal methyl donor S-adenosylmethionine (SAM), which is derived from the amino acid methionine. Prior work uncovered that methionine uptake and metabolism are increased during EBV infection (
Wang et al., 2019
).

Viruses rewire host cell metabolism to increase the levels of macromolecules necessary to produce new virus particles, and the metabolic pathways that are altered vary between virus types (
Mullen and Christofk, 2022
). But it is not known whether EBV alters host cell metabolism to silence expression of its own genes. In this issue of Cell Metabolism, Guo et al. show that methionine is required to maintain EBV gene silencing and latency in infected memory B cells (
Guo et al., 2022
).

In mammals, methionine is an essential amino acid that can only be obtained from the diet. Once inside the cell, methionine is metabolized to SAM, which is used to methylate DNA, RNA, metabolites, and histones (
Figure 1
). Methionine can be replenished by the addition of a methyl group from the folate cycle to homocysteine or salvaged using decarboxylated SAM (dcSAM). The methionine cycle is also linked to the transulfuration and polyamine pathways, forming a central hub that links methionine metabolism to proliferation, redox control, transcription, and anabolism (
Sanderson et al., 2019
).


Download: Download high-res image (369KB)
Download: Download full-size image
Figure 1. Dietary nutrients feed into methionine metabolism and promote EBV latency

The methionine and folate cycles are essential cellular processes. A main output is to donate methyl groups from SAM to DNA, RNA, metabolites, and histones. These pathways are upregulated during EBV infection, leading to methylation of viral DNA and repression of EBV gene expression. Reduced EBV gene expression is a hallmark of latent infection, preventing host immune cells from recognizing infected cells. Viral gene repression can be reversed by inhibiting one carbon metabolism using dietary restriction of methionine or small molecule inhibition of SHMT1/2, leading to increased expression of EBV latent genes. Forcing EBV out of latency could provide a new therapeutic strategy against EBV-driven diseases such as cancer and multiple sclerosis.

SAM-dependent DNA and histone methylation has been shown to regulate host gene expression in healthy and diseased states (
Mentch et al., 2015
), but it is not known how nutrient levels affect the EBV epigenomic state. To investigate this, Guo et al. restricted methionine in several EBV-associated Burkitt lymphoma B cell lines. They found that methionine restriction increased the expression of early, but not late, EBV genes. The authors then knocked out two methionine pathway genes (MAT2A and AHCY), revealing that impaired flow of carbons through the methionine cycle reduced levels of repressive 5mC on DNA at the promotors of the LMP and BZLF1 viral genes. Surprisingly, methionine restriction did not decrease translation in EBV- associated Burkitt lymphoma cells. Previous studies in other systems showed that translation is inhibited during methionine restriction via reduced mTORC1 activity. Cellular methionine levels are sensed by SAMTOR, which inhibits mTORC1 translocation to the lysosome through a GATOR1-dependent mechanism when SAM levels are low (
Gu et al., 2017
). This suggests that nutrient sensing may be impaired during EBV infection, which warrants further investigation.

This study by Guo et al. suggests an intriguing therapeutic possibility for EBV-associated tumors: reducing methionine metabolism could force EBV out of an immune-evading latent state, making infected cells visible for the immune system to target. One approach to reduce methionine metabolism in patients could be to restrict dietary methionine. Dietary restriction of amino acids has been investigated as a therapeutic strategy to reduce tumor growth in other cancer types—for example, reducing dietary serine and glycine in Apc-inactivated intestinal cancers (
Maddocks et al., 2017
) and asparagine in Kras-driven lung and pancreatic cancers (
Krall et al., 2021
)—and has shown promising results. Methionine restriction itself has anti-tumor efficacy in chemotherapy-resistant KRAS-driven colorectal cancer and radiation-resistant Kras-driven soft tissue sarcoma (
Gao et al., 2019
).

In the present study, Guo et al. also investigated whether Burkitt tumors are dependent upon methionine to regulate the EBV epigenome and repress viral gene expression in vivo. They implanted EBV-driven Burkitt tumors in mice, which were then fed a low-methionine or control diet. The low-methionine mice had reduced levels of tumor methionine and SAM, which were coupled with reduced DNA methylation at the EBV promotors Cp, LMPp, and BMRF1p in the same tumors. In addition, the low-methionine mice showed increased expression of EBV genes and proteins and had smaller tumors, which indicates the therapeutic potential of this strategy.

Why might increasing the expression of EBV genes have therapeutic value? The increase in viral gene expression could convert immune-cold EBV-associated tumors into immune-hot tumors that are susceptible to host immune surveillance and immunotherapy. It will be fascinating to determine whether this idea is correct. But can methionine restriction be achieved in humans using a dietary approach? A previous study provided clear evidence that a 3-week 83% reduction in dietary methionine was tolerated in humans and reduced serum methionine and related metabolite levels (
Gao et al., 2019
).

Other metabolic pathways, such as the folate cycle, are also altered during EBV infection (
Wang et al., 2019
). This study by Guo et al. provides in vitro evidence that serine restriction increases viral gene expression and in vivo evidence that inhibiting SHMT1 and SHMT2 with the inhibitor SHIN2 decreases SAM levels and increases the expression of some EBV genes. The question now is whether dietary restriction of serine, glycine, and other nutrients can also elevate EBV gene expression and reduce Burkitt tumor growth.

Characterization of how EBV uses metabolism to control gene expression by rewiring host and virus epigenetics could also provide therapeutic strategies against other EBV-associated diseases. One potential target is multiple sclerosis. A previous study showed that methionine restriction dampens pathogenic T cell responses in a mouse model of this autoimmune disease (
Roy et al., 2020
). Multiple sclerosis also involves clonal expansion of B cells, which produce autoantibodies that target myelin-producing glial cells. Promisingly, this study by Guo et al. shows that the outgrowth of newly EBV-infected B cells requires methionine. Further work will be needed to determine whether methionine restriction prevents the establishment of autoantibody-producing plasma cells in multiple sclerosis. Expanding these concepts into other virus-induced diseases could improve our understanding of disease pathogenesis and lead to new strategies to treat them.

Acknowledgments
P.J.M. acknowledges the support of the American Lung Association and Impetus grants.

Declaration of interests
The authors declare no competing interests.

References
Gao et al., 2019
X. Gao, S.M. Sanderson, Z. Dai, M.A. Reid, D.E. Cooper, M. Lu, J.P. Richie Jr., A. Ciccarella, A. Calcagnotto, P.G. Mikhael, et al.
Dietary methionine influences therapy in mouse cancer models and alters human metabolism
Nature, 572 (2019), pp. 397-401,
10.1038/s41586-019-1437-3
View at publisher
View in Scopus
Google Scholar
Gu et al., 2017
X. Gu, J.M. Orozco, R.A. Saxton, K.J. Condon, G.Y. Liu, P.A. Krawczyk, S.M. Scaria, J.W. Harper, S.P. Gygi, D.M. Sabatini
SAMTOR is an S-adenosylmethionine sensor for the mTORC1 pathway
Science, 358 (2017), pp. 813-818,
10.1126/science.aao3265
View at publisher
This article is free to access.
View in Scopus
Google Scholar
Guo et al., 2022
R. Guo, J.H. Liang, Y. Zhang, M. Lutchenkov, Z. Li, Y. Wang, V. Trujillo-Alonso, R. Puri, L. Giulino-Roth, B.E. Gewurz
Methionine Metabolism Controls the B-cell EBV Epigenome and Viral Latency
Cell Metab., 34 (2022), pp. 1280-1297
View at publisher
This article is free to access.
View in Scopus
Google Scholar
Krall et al., 2021
A.S. Krall, P.J. Mullen, F. Surjono, M. Momcilovic, E.W. Schmid, C.J. Halbrook, A. Thambundit, S.D. Mittelman, C.A. Lyssiotis, D.B. Shackelford, et al.
Asparagine couples mitochondrial respiration to ATF4 activity and tumor growth
Cell Metab, 33 (2021), pp. 1013-1026.e6,
10.1016/j.cmet.2021.02.001
View PDF
View article
View in Scopus
Google Scholar
Maddocks et al., 2017
O.D.K. Maddocks, D. Athineos, E.C. Cheung, P. Lee, T. Zhang, N.J.F. van den Broek, G.M. Mackay, C.F. Labuschagne, D. Gay, F. Kruiswijk, et al.
Modulating the therapeutic response of tumours to dietary serine and glycine starvation
Nature, 544 (2017), pp. 372-376,
10.1038/nature22056
View at publisher
View in Scopus
Google Scholar
Mentch et al., 2015
S. Mentch, M. Mehrmohamadi, L. Huang, X. Liu, D. Gupta, D. Mattocks, P. Gómez Padilla, G. Ables, M. Bamman, A. Thalacker-Mercer, et al.
Histone Methylation Dynamics and Gene Regulation Occur through the Sensing of One-Carbon Metabolism
Cell Metab., 22 (2015), pp. 861-873,
10.1016/j.cmet.2015.08.024
View PDF
View article
View in Scopus
Google Scholar
Mullen and Christofk, 2022
P.J. Mullen, H.R. Christofk
The metabolic relationship between viral infection and cancer
Annu. Rev. Cancer Biol., 6 (2022), pp. 1-15,
10.1146/annurev-cancerbio-070120-090423
View at publisher
View in Scopus
Google Scholar
Roy et al., 2020
D.G. Roy, J. Chen, V. Mamane, E.H. Ma, B.M. Muhire, R.D. Sheldon, T. Shorstova, R. Koning, R.M. Johnson, E. Esaulova, et al.
Methionine Metabolism Shapes T Helper Cell Responses through Regulation of Epigenetic Reprogramming
Cell Metab., 31 (2020), pp. 250-266.e9,
10.1016/j.cmet.2020.01.006
View PDF
View article
View in Scopus
Google Scholar
Sanderson et al., 2019
S.M. Sanderson, X. Gao, Z. Dai, J.W. Locasale
Methionine metabolism in health and cancer: a nexus of diet and precision medicine
Nat. Rev. Cancer, 19 (2019), pp. 625-637,
10.1038/s41568-019-0187-8
View at publisher
View in Scopus
Google Scholar
Wang et al., 2019
L.W. Wang, H. Shen, L. Nobre, I. Ersing, J.A. Paulo, S. Trudeau, Z. Wang, N.A. Smith, Y. Ma, B. Reinstadler, et al.
Epstein-Barr-Virus-Induced One-Carbon Metabolism Drives B Cell Transformation
Cell Metab., 30 (2019), pp. 539-555.e11,
10.1016/j.cmet.2019.06.003
View PDF
View article
View in Scopus
Google Scholar
Cited by (0)Cell Metabolism
Volume 34, Issue 9
, 6 September 2022, Pages 1229-1231
Journal home page for Cell Metabolism
Preview
Methionine restriction forces Epstein-Barr virus out of latency
Author links open overlay panel
Sriraksha Bharadwaj Kashyap 1
,
Racheal Mulondo 1
,
Peter J. Mullen 1 2 3

Show more

Add to Mendeley

Share

Cite
https://doi.org/10.1016/j.cmet.2022.08.009
Get rights and content
Under an Elsevier
user license
open archive
Refers to
Methionine metabolism controls the B cell EBV epigenome and viral latency
Cell Metabolism, Volume 34, Issue 9, 6 September 2022, Pages 1280-1297.e9
Rui Guo, Jin Hua Liang, Yuchen Zhang, Michael Lutchenkov, Zhixuan Li, Yin Wang, Vicenta Trujillo-Alonso, Rishi Puri, Lisa Giulino-Roth, Benjamin E. Gewurz
View PDF
EBV gene expression is repressed during viral latency to prevent an immune response, but it is not known how metabolism contributes to this silencing. In this issue of Cell Metabolism, Guo et al. describe how methionine restriction reactivates the expression of EBV genes, offering new therapeutic approaches against EBV-driven diseases.

Previous article in issue
Next article in issue
Main text
Epstein-Barr virus (EBV) is one of the most prevalent human viruses, chronically infecting over 95% of the global population. Although most people experience mild or no symptoms, EBV is associated with multiple cancers, including Hodgkin’s and Burkitt’s lymphomas, and is a causative factor in multiple sclerosis. EBV chronically infects B cells, where it maintains a series of latent states to avoid detection by the host immune system. During latency, EBV uses epigenetic factors to downregulate the expression of its own genes, sometimes expressing only one. Previous studies showed that histone and viral DNA methylation are necessary to repress EBV gene expression. This requires the universal methyl donor S-adenosylmethionine (SAM), which is derived from the amino acid methionine. Prior work uncovered that methionine uptake and metabolism are increased during EBV infection (
Wang et al., 2019
).

Viruses rewire host cell metabolism to increase the levels of macromolecules necessary to produce new virus particles, and the metabolic pathways that are altered vary between virus types (
Mullen and Christofk, 2022
). But it is not known whether EBV alters host cell metabolism to silence expression of its own genes. In this issue of Cell Metabolism, Guo et al. show that methionine is required to maintain EBV gene silencing and latency in infected memory B cells (
Guo et al., 2022
).

In mammals, methionine is an essential amino acid that can only be obtained from the diet. Once inside the cell, methionine is metabolized to SAM, which is used to methylate DNA, RNA, metabolites, and histones (
Figure 1
). Methionine can be replenished by the addition of a methyl group from the folate cycle to homocysteine or salvaged using decarboxylated SAM (dcSAM). The methionine cycle is also linked to the transulfuration and polyamine pathways, forming a central hub that links methionine metabolism to proliferation, redox control, transcription, and anabolism (
Sanderson et al., 2019
).


Download: Download high-res image (369KB)
Download: Download full-size image
Figure 1. Dietary nutrients feed into methionine metabolism and promote EBV latency

The methionine and folate cycles are essential cellular processes. A main output is to donate methyl groups from SAM to DNA, RNA, metabolites, and histones. These pathways are upregulated during EBV infection, leading to methylation of viral DNA and repression of EBV gene expression. Reduced EBV gene expression is a hallmark of latent infection, preventing host immune cells from recognizing infected cells. Viral gene repression can be reversed by inhibiting one carbon metabolism using dietary restriction of methionine or small molecule inhibition of SHMT1/2, leading to increased expression of EBV latent genes. Forcing EBV out of latency could provide a new therapeutic strategy against EBV-driven diseases such as cancer and multiple sclerosis.

SAM-dependent DNA and histone methylation has been shown to regulate host gene expression in healthy and diseased states (
Mentch et al., 2015
), but it is not known how nutrient levels affect the EBV epigenomic state. To investigate this, Guo et al. restricted methionine in several EBV-associated Burkitt lymphoma B cell lines. They found that methionine restriction increased the expression of early, but not late, EBV genes. The authors then knocked out two methionine pathway genes (MAT2A and AHCY), revealing that impaired flow of carbons through the methionine cycle reduced levels of repressive 5mC on DNA at the promotors of the LMP and BZLF1 viral genes. Surprisingly, methionine restriction did not decrease translation in EBV- associated Burkitt lymphoma cells. Previous studies in other systems showed that translation is inhibited during methionine restriction via reduced mTORC1 activity. Cellular methionine levels are sensed by SAMTOR, which inhibits mTORC1 translocation to the lysosome through a GATOR1-dependent mechanism when SAM levels are low (
Gu et al., 2017
). This suggests that nutrient sensing may be impaired during EBV infection, which warrants further investigation.

This study by Guo et al. suggests an intriguing therapeutic possibility for EBV-associated tumors: reducing methionine metabolism could force EBV out of an immune-evading latent state, making infected cells visible for the immune system to target. One approach to reduce methionine metabolism in patients could be to restrict dietary methionine. Dietary restriction of amino acids has been investigated as a therapeutic strategy to reduce tumor growth in other cancer types—for example, reducing dietary serine and glycine in Apc-inactivated intestinal cancers (
Maddocks et al., 2017
) and asparagine in Kras-driven lung and pancreatic cancers (
Krall et al., 2021
)—and has shown promising results. Methionine restriction itself has anti-tumor efficacy in chemotherapy-resistant KRAS-driven colorectal cancer and radiation-resistant Kras-driven soft tissue sarcoma (
Gao et al., 2019
).

In the present study, Guo et al. also investigated whether Burkitt tumors are dependent upon methionine to regulate the EBV epigenome and repress viral gene expression in vivo. They implanted EBV-driven Burkitt tumors in mice, which were then fed a low-methionine or control diet. The low-methionine mice had reduced levels of tumor methionine and SAM, which were coupled with reduced DNA methylation at the EBV promotors Cp, LMPp, and BMRF1p in the same tumors. In addition, the low-methionine mice showed increased expression of EBV genes and proteins and had smaller tumors, which indicates the therapeutic potential of this strategy.

Why might increasing the expression of EBV genes have therapeutic value? The increase in viral gene expression could convert immune-cold EBV-associated tumors into immune-hot tumors that are susceptible to host immune surveillance and immunotherapy. It will be fascinating to determine whether this idea is correct. But can methionine restriction be achieved in humans using a dietary approach? A previous study provided clear evidence that a 3-week 83% reduction in dietary methionine was tolerated in humans and reduced serum methionine and related metabolite levels (
Gao et al., 2019
).

Other metabolic pathways, such as the folate cycle, are also altered during EBV infection (
Wang et al., 2019
). This study by Guo et al. provides in vitro evidence that serine restriction increases viral gene expression and in vivo evidence that inhibiting SHMT1 and SHMT2 with the inhibitor SHIN2 decreases SAM levels and increases the expression of some EBV genes. The question now is whether dietary restriction of serine, glycine, and other nutrients can also elevate EBV gene expression and reduce Burkitt tumor growth.

Characterization of how EBV uses metabolism to control gene expression by rewiring host and virus epigenetics could also provide therapeutic strategies against other EBV-associated diseases. One potential target is multiple sclerosis. A previous study showed that methionine restriction dampens pathogenic T cell responses in a mouse model of this autoimmune disease (
Roy et al., 2020
). Multiple sclerosis also involves clonal expansion of B cells, which produce autoantibodies that target myelin-producing glial cells. Promisingly, this study by Guo et al. shows that the outgrowth of newly EBV-infected B cells requires methionine. Further work will be needed to determine whether methionine restriction prevents the establishment of autoantibody-producing plasma cells in multiple sclerosis. Expanding these concepts into other virus-induced diseases could improve our understanding of disease pathogenesis and lead to new strategies to treat them.

Acknowledgments
P.J.M. acknowledges the support of the American Lung Association and Impetus grants.

Declaration of interests
The authors declare no competing interests.

References
Gao et al., 2019
X. Gao, S.M. Sanderson, Z. Dai, M.A. Reid, D.E. Cooper, M. Lu, J.P. Richie Jr., A. Ciccarella, A. Calcagnotto, P.G. Mikhael, et al.
Dietary methionine influences therapy in mouse cancer models and alters human metabolism
Nature, 572 (2019), pp. 397-401,
10.1038/s41586-019-1437-3
View at publisher
View in Scopus
Google Scholar
Gu et al., 2017
X. Gu, J.M. Orozco, R.A. Saxton, K.J. Condon, G.Y. Liu, P.A. Krawczyk, S.M. Scaria, J.W. Harper, S.P. Gygi, D.M. Sabatini
SAMTOR is an S-adenosylmethionine sensor for the mTORC1 pathway
Science, 358 (2017), pp. 813-818,
10.1126/science.aao3265
View at publisher
This article is free to access.
View in Scopus
Google Scholar
Guo et al., 2022
R. Guo, J.H. Liang, Y. Zhang, M. Lutchenkov, Z. Li, Y. Wang, V. Trujillo-Alonso, R. Puri, L. Giulino-Roth, B.E. Gewurz
Methionine Metabolism Controls the B-cell EBV Epigenome and Viral Latency
Cell Metab., 34 (2022), pp. 1280-1297
View at publisher
This article is free to access.
View in Scopus
Google Scholar
Krall et al., 2021
A.S. Krall, P.J. Mullen, F. Surjono, M. Momcilovic, E.W. Schmid, C.J. Halbrook, A. Thambundit, S.D. Mittelman, C.A. Lyssiotis, D.B. Shackelford, et al.
Asparagine couples mitochondrial respiration to ATF4 activity and tumor growth
Cell Metab, 33 (2021), pp. 1013-1026.e6,
10.1016/j.cmet.2021.02.001
View PDF
View article
View in Scopus
Google Scholar
Maddocks et al., 2017
O.D.K. Maddocks, D. Athineos, E.C. Cheung, P. Lee, T. Zhang, N.J.F. van den Broek, G.M. Mackay, C.F. Labuschagne, D. Gay, F. Kruiswijk, et al.
Modulating the therapeutic response of tumours to dietary serine and glycine starvation
Nature, 544 (2017), pp. 372-376,
10.1038/nature22056
View at publisher
View in Scopus
Google Scholar
Mentch et al., 2015
S. Mentch, M. Mehrmohamadi, L. Huang, X. Liu, D. Gupta, D. Mattocks, P. Gómez Padilla, G. Ables, M. Bamman, A. Thalacker-Mercer, et al.
Histone Methylation Dynamics and Gene Regulation Occur through the Sensing of One-Carbon Metabolism
Cell Metab., 22 (2015), pp. 861-873,
10.1016/j.cmet.2015.08.024
View PDF
View article
View in Scopus
Google Scholar
Mullen and Christofk, 2022
P.J. Mullen, H.R. Christofk
The metabolic relationship between viral infection and cancer
Annu. Rev. Cancer Biol., 6 (2022), pp. 1-15,
10.1146/annurev-cancerbio-070120-090423
View at publisher
View in Scopus
Google Scholar
Roy et al., 2020
D.G. Roy, J. Chen, V. Mamane, E.H. Ma, B.M. Muhire, R.D. Sheldon, T. Shorstova, R. Koning, R.M. Johnson, E. Esaulova, et al.
Methionine Metabolism Shapes T Helper Cell Responses through Regulation of Epigenetic Reprogramming
Cell Metab., 31 (2020), pp. 250-266.e9,
10.1016/j.cmet.2020.01.006
View PDF
View article
View in Scopus
Google Scholar
Sanderson et al., 2019
S.M. Sanderson, X. Gao, Z. Dai, J.W. Locasale
Methionine metabolism in health and cancer: a nexus of diet and precision medicine
Nat. Rev. Cancer, 19 (2019), pp. 625-637,
10.1038/s41568-019-0187-8
View at publisher
View in Scopus
Google Scholar
Wang et al., 2019
L.W. Wang, H. Shen, L. Nobre, I. Ersing, J.A. Paulo, S. Trudeau, Z. Wang, N.A. Smith, Y. Ma, B. Reinstadler, et al.
Epstein-Barr-Virus-Induced One-Carbon Metabolism Drives B Cell Transformation
Cell Metab., 30 (2019), pp. 539-555.e11,
10.1016/j.cmet.2019.06.003
View PDF
View article
View in Scopus
Google Scholar
Cited by (0)Cell Metabolism
Volume 34, Issue 9
, 6 September 2022, Pages 1229-1231
Journal home page for Cell Metabolism
Preview
Methionine restriction forces Epstein-Barr virus out of latency
Author links open overlay panel
Sriraksha Bharadwaj Kashyap 1
,
Racheal Mulondo 1
,
Peter J. Mullen 1 2 3

Show more

Add to Mendeley

Share

Cite
https://doi.org/10.1016/j.cmet.2022.08.009
Get rights and content
Under an Elsevier
user license
open archive
Refers to
Methionine metabolism controls the B cell EBV epigenome and viral latency
Cell Metabolism, Volume 34, Issue 9, 6 September 2022, Pages 1280-1297.e9
Rui Guo, Jin Hua Liang, Yuchen Zhang, Michael Lutchenkov, Zhixuan Li, Yin Wang, Vicenta Trujillo-Alonso, Rishi Puri, Lisa Giulino-Roth, Benjamin E. Gewurz
View PDF
EBV gene expression is repressed during viral latency to prevent an immune response, but it is not known how metabolism contributes to this silencing. In this issue of Cell Metabolism, Guo et al. describe how methionine restriction reactivates the expression of EBV genes, offering new therapeutic approaches against EBV-driven diseases.

Previous article in issue
Next article in issue
Main text
Epstein-Barr virus (EBV) is one of the most prevalent human viruses, chronically infecting over 95% of the global population. Although most people experience mild or no symptoms, EBV is associated with multiple cancers, including Hodgkin’s and Burkitt’s lymphomas, and is a causative factor in multiple sclerosis. EBV chronically infects B cells, where it maintains a series of latent states to avoid detection by the host immune system. During latency, EBV uses epigenetic factors to downregulate the expression of its own genes, sometimes expressing only one. Previous studies showed that histone and viral DNA methylation are necessary to repress EBV gene expression. This requires the universal methyl donor S-adenosylmethionine (SAM), which is derived from the amino acid methionine. Prior work uncovered that methionine uptake and metabolism are increased during EBV infection (
Wang et al., 2019
).

Viruses rewire host cell metabolism to increase the levels of macromolecules necessary to produce new virus particles, and the metabolic pathways that are altered vary between virus types (
Mullen and Christofk, 2022
). But it is not known whether EBV alters host cell metabolism to silence expression of its own genes. In this issue of Cell Metabolism, Guo et al. show that methionine is required to maintain EBV gene silencing and latency in infected memory B cells (
Guo et al., 2022
).

In mammals, methionine is an essential amino acid that can only be obtained from the diet. Once inside the cell, methionine is metabolized to SAM, which is used to methylate DNA, RNA, metabolites, and histones (
Figure 1
). Methionine can be replenished by the addition of a methyl group from the folate cycle to homocysteine or salvaged using decarboxylated SAM (dcSAM). The methionine cycle is also linked to the transulfuration and polyamine pathways, forming a central hub that links methionine metabolism to proliferation, redox control, transcription, and anabolism (
Sanderson et al., 2019
).


Download: Download high-res image (369KB)
Download: Download full-size image
Figure 1. Dietary nutrients feed into methionine metabolism and promote EBV latency

The methionine and folate cycles are essential cellular processes. A main output is to donate methyl groups from SAM to DNA, RNA, metabolites, and histones. These pathways are upregulated during EBV infection, leading to methylation of viral DNA and repression of EBV gene expression. Reduced EBV gene expression is a hallmark of latent infection, preventing host immune cells from recognizing infected cells. Viral gene repression can be reversed by inhibiting one carbon metabolism using dietary restriction of methionine or small molecule inhibition of SHMT1/2, leading to increased expression of EBV latent genes. Forcing EBV out of latency could provide a new therapeutic strategy against EBV-driven diseases such as cancer and multiple sclerosis.

SAM-dependent DNA and histone methylation has been shown to regulate host gene expression in healthy and diseased states (
Mentch et al., 2015
), but it is not known how nutrient levels affect the EBV epigenomic state. To investigate this, Guo et al. restricted methionine in several EBV-associated Burkitt lymphoma B cell lines. They found that methionine restriction increased the expression of early, but not late, EBV genes. The authors then knocked out two methionine pathway genes (MAT2A and AHCY), revealing that impaired flow of carbons through the methionine cycle reduced levels of repressive 5mC on DNA at the promotors of the LMP and BZLF1 viral genes. Surprisingly, methionine restriction did not decrease translation in EBV- associated Burkitt lymphoma cells. Previous studies in other systems showed that translation is inhibited during methionine restriction via reduced mTORC1 activity. Cellular methionine levels are sensed by SAMTOR, which inhibits mTORC1 translocation to the lysosome through a GATOR1-dependent mechanism when SAM levels are low (
Gu et al., 2017
). This suggests that nutrient sensing may be impaired during EBV infection, which warrants further investigation.

This study by Guo et al. suggests an intriguing therapeutic possibility for EBV-associated tumors: reducing methionine metabolism could force EBV out of an immune-evading latent state, making infected cells visible for the immune system to target. One approach to reduce methionine metabolism in patients could be to restrict dietary methionine. Dietary restriction of amino acids has been investigated as a therapeutic strategy to reduce tumor growth in other cancer types—for example, reducing dietary serine and glycine in Apc-inactivated intestinal cancers (
Maddocks et al., 2017
) and asparagine in Kras-driven lung and pancreatic cancers (
Krall et al., 2021
)—and has shown promising results. Methionine restriction itself has anti-tumor efficacy in chemotherapy-resistant KRAS-driven colorectal cancer and radiation-resistant Kras-driven soft tissue sarcoma (
Gao et al., 2019
).

In the present study, Guo et al. also investigated whether Burkitt tumors are dependent upon methionine to regulate the EBV epigenome and repress viral gene expression in vivo. They implanted EBV-driven Burkitt tumors in mice, which were then fed a low-methionine or control diet. The low-methionine mice had reduced levels of tumor methionine and SAM, which were coupled with reduced DNA methylation at the EBV promotors Cp, LMPp, and BMRF1p in the same tumors. In addition, the low-methionine mice showed increased expression of EBV genes and proteins and had smaller tumors, which indicates the therapeutic potential of this strategy.

Why might increasing the expression of EBV genes have therapeutic value? The increase in viral gene expression could convert immune-cold EBV-associated tumors into immune-hot tumors that are susceptible to host immune surveillance and immunotherapy. It will be fascinating to determine whether this idea is correct. But can methionine restriction be achieved in humans using a dietary approach? A previous study provided clear evidence that a 3-week 83% reduction in dietary methionine was tolerated in humans and reduced serum methionine and related metabolite levels (
Gao et al., 2019
).

Other metabolic pathways, such as the folate cycle, are also altered during EBV infection (
Wang et al., 2019
). This study by Guo et al. provides in vitro evidence that serine restriction increases viral gene expression and in vivo evidence that inhibiting SHMT1 and SHMT2 with the inhibitor SHIN2 decreases SAM levels and increases the expression of some EBV genes. The question now is whether dietary restriction of serine, glycine, and other nutrients can also elevate EBV gene expression and reduce Burkitt tumor growth.

Characterization of how EBV uses metabolism to control gene expression by rewiring host and virus epigenetics could also provide therapeutic strategies against other EBV-associated diseases. One potential target is multiple sclerosis. A previous study showed that methionine restriction dampens pathogenic T cell responses in a mouse model of this autoimmune disease (
Roy et al., 2020
). Multiple sclerosis also involves clonal expansion of B cells, which produce autoantibodies that target myelin-producing glial cells. Promisingly, this study by Guo et al. shows that the outgrowth of newly EBV-infected B cells requires methionine. Further work will be needed to determine whether methionine restriction prevents the establishment of autoantibody-producing plasma cells in multiple sclerosis. Expanding these concepts into other virus-induced diseases could improve our understanding of disease pathogenesis and lead to new strategies to treat them.

Acknowledgments
P.J.M. acknowledges the support of the American Lung Association and Impetus grants.

Declaration of interests
The authors declare no competing interests.

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