Development of human endogenous retrovirus type K- related treatments for human diseases
Abstract
Human endogenous retroviruses (HERVs) constitute approximately 8% of the human genome and have long been regarded as silent passengers within our genomes. However, the reactivation of HERVs has been increasingly linked to a range of human diseases, particularly the HERV-K (HML-2) family. Many studies are dedicated to elucidating the potential role of HERV-K in pathogenicity. While the underlying mechanisms require further investigation, targeting HERV-K transactivation emerges as a promising avenue for treating human diseases, including cancer, autoimmune disorders, neurodegenerative conditions, and infectious diseases. In this review, we summarize recent advancements in the development of HERV-K-targeted therapeutic strategies against various human diseases, including antiretroviral drugs, immunotherapy, and vaccines.
1 INTRODUCTION
Human endogenous retrovirus sequences (HERVs) make up to 8% of the human genome and have resided in the human genome for several million years.1 While the majority of HERVs are dysfunctional due to the accumulation of multiple nonsense mutations, certain HERVs, particularly HERV-Ks, may become activated under specific physiological and pathological conditions. HERV-Ks consist of 11 subgroups (HML-1 to HML-11), with the most studied subgroup in human diseases being HERV-K (HML-2), whose transactivation has been implicated in various human diseases, including cancer, autoimmune disorders, neurodegenerative diseases, and infectious diseases.2, 3
HERV-K transactivation has been linked to the development of various human cancers, such as breast cancer, lung cancer, prostate cancer, hepatocellular carcinoma, melanomas, germ cell tumors, and leukemia.4-10 Numerous studies have identified high levels of HERV-K expressed products in cells, tissues, and blood of patients with cancer.6-10 The transactivation of HERVs may impact the carcinogenesis process by directly expressing viral mRNA, functional proteins, and/or viral particles, or indirectly by activating tumor-associated genes. Consequently, HERV-K transactivation has been considered as a potential prognostic marker for several malignant diseases, such as lung cancer and hepatocellular carcinoma.7, 11
Abnormal expressions of HERV-K viral transcripts have also been detected in autoimmune and neurodegenerative diseases. For instance, increased expression of HERV-K gag, pol, and env transcripts was observed in amyotrophic lateral sclerosis (ALS) brain tissue.12, 13 This expression was specific to ALS, as it could not be found in patients with other neurodegenerative diseases such as Parkinson's or Alzheimer's disease. Significant increases in HERV-K gag activity were observed in patients with rheumatoid arthritis compared to disease controls.14 Additionally, HERV-K18 expression was significantly elevated in peripheral blood from patients with juvenile rheumatoid arthritis compared to controls.15
It is well established that human immunodeficiency virus-1 (HIV-1) infection can increase HERV-K mRNA levels in peripheral blood mononuclear cells (PBMCs).16, 17 Several studies demonstrated that certain members of the HERV-K family, especially HERV-K18, could be transactivated as a superantigen (SAg) by Epstein–Barr virus (EBV) infection, subsequently activating TCRVB13 T cells through MHC-II, which plays a central role in EBV infection and associated diseases.18-20 Other data also suggested that human herpesvirus 6 (HHV-6), either in latent form or during acute infection, directly transactivated HERV-K18 SAg activities.21, 22 Recent data from our group found that Kaposi's sarcoma-associated herpesvirus (KSHV) de novo infection was able to transactivate HERV-K through various cellular signaling pathways and transcription factors, and that HERV-K transactivation played an important role in KSHV pathogenesis and tumorigenesis in vitro and in vivo.23
These findings underscore the therapeutic potential of targeting HERV-K transactivation, although specific drugs or vaccines designed for HERVs are still lacking. In this review article, we summarize recent advances in the development of HERV-K-targeted therapy strategies against various human diseases.
2 SMALL-MOLECULE COMPOUNDS
2.1 Reverse transcriptase (RT) inhibitors
One recent study reported the crystal structure of HERV-K HML-2 RT as a ternary complex with double-stranded DNA (dsDNA) and dNTP.24 They found that the structure of HERV-K RT closely resembled that of HIV-1 RT, even forming an analogous asymmetric dimer. Notably, while HERV-K RT is a homodimer, HIV-1 RT is a heterodimer with the ribonuclease H (RNase H) domain of the catalytically inactive subunit (p51) proteolytically removed. Common nucleoside RT inhibitors (NRTIs) such as Zidovudine (AZT), Lamivudine (3TC), and Carbovir (CBV) were found to inhibit HERV-K RT activity, while classic non-NRTIs (NNRTIs) such as Nevirapine (NVP) and Efavirenz (EFV) could not. Another study reported similar findings, where NRTIs like AZT, Stavudine (d4T), Didanosine (ddI), and 3TC, as well as the nucleotide RTI inhibitor Tenofovir (TDF), effectively blocked HERV-K RT activity.25 In contrast, HIV-1-specific NNRTIs showed no such effects, except for Etravirine (ETV). Interestingly, the inhibition of HERV-K infectivity by NRTIs appeared to occur either during the reverse transcription step of the viral genome before HERV-K viral particle formation and/or within infected cells. However, Tyagi et al. found that both NRTIs and NNRTIs (e.g., EFV, Etravirine, and NVP) could significantly inhibit HERV-K RT activity.26 Supporting this, another group found that NNRTIs EFV and NVP effectively suppressed HERV-K activation in melanoma cells, antagonizing the expansion of the CD133+ subpopulation with stemness features.27 These discrepancies may stem from differences in agents, systems, and detection assays used in these studies. Nevertheless, targeting HERV-K RT shows promise for inhibiting HERV-K transactivation.
2.2 Protease (PR) inhibitors
An early study by Towler et al., found that although HERV-K PR functions similarly to HIV-1 PR, it displayed high resistance to several clinically useful HIV-1 inhibitors, including Ritonavir, Indinavir, and Saquinavir.28 This suggests that HERV-K PR may complement HIV-1 PR during infection, which has implications for PR inhibitor therapy and drug resistance. To identify compounds that could inhibit protein processing dependent on HERV-K PR, Kuhelj et al., screened a series of cyclic ureas previously shown to inhibit HIV-1 PR.29 Several symmetric bisamides acted as potent inhibitors of both truncated and full-length forms of HERV-K PR in subnanomolar or nanomolar ranges. One of these cyclic ureas, SD146, was found to inhibit the processing of in vitro translated HERV-K gag polyprotein substrate by HERV-K PR.
Deficiency of the tumor suppressor Merlin leads to the development of multiple nervous system tumors such as schwannomas, meningiomas, and ependymomas.30 In a recent study, Maze et al., found that ectopic overexpression of HERV-K Env in normal Schwann cells increased proliferation and upregulated expression of c-Jun and p-ERK1/2, key components of known tumorigenic pathways in schwannoma.31 Interestingly, retroviral PR inhibitors Ritonavir, Atazanavir, and Lopinavir reduced proliferation of schwannoma and meningioma cells through inhibition of HERV-K PR.
2.3 Integrase inhibitors
Recently, Li et al., reported that an integrase strand-transfer inhibitor, Dolutegravir (DTG), effectively inhibited the proliferation of multiple cancer cell lines.32 Furthermore, the antiproliferative potency of DTG correlated positively with the expression levels of HERV-K. Similarly, another group found that integrase inhibitor Raltegravir effectively blocked HERV-K infection and production using a pseudotyped HERV-K virus infection system.26
2.4 Other inhibitors
A recent study found that HERV-K expression positively correlated with MEK-ERK and p16INK4A-CDK4 pathway activities in melanoma cells and tumor tissues.33 Furthermore, inhibitors of MEK (e.g., PD98059) or CDK4 (e.g., 219476), especially in combination, significantly reduced levels of HERV-K Env in melanoma cells. Thus, the authors suggest that triple therapy targeting HERV-K, MEK, and CDK4 may be necessary for more effective and long-lasting therapeutic effects than single or double inhibition of MEK-ERK and CDK4.
3 IMMUNOTHERAPY
3.1 Antibody-based treatment
In a comprehensive study, Wang-Johanning et al., found that the expression of HERV-K Env protein in malignant breast cancer cell lines was significantly higher than in nonmalignant breast cells.34 Moreover, HERV-K expression was detected in the majority of primary breast tumors (66%, 148 of 223 cases), with a higher rate of lymph node metastasis associated with HERV-K-positive compared to HERV-K-negative tumors (43% vs. 23%, p = 0.003). Based on these findings, they developed a series of anti-HERV-K Env monoclonal antibodies (mAbs; 6H5, 4D1, 4E11, 6E11, and 4E6), which inhibited growth and induced apoptosis of breast cancer cells in vitro. Importantly, mice treated with 6H5 mAb showed significantly reduced growth of xenograft tumors compared with mice treated with control immunoglobulin. These findings underscore the clinical relevance and implications of HERV-K transactivation in breast cancer, with mAbs against HERV-K Env protein showing potential as novel immunotherapeutic agents for breast cancer therapy.
GNbAC1 or Temelimab is a humanized mAb targeting HERV Env protein, which is expressed in MS lesions, is proinflammatory, and inhibits oligodendrocyte precursor cell differentiation. Curtin et al. reported the results of the first-in-humans, Phase I, randomized, double-blind, placebo-controlled study of GNbAC1 in healthy subjects (total 33 male subjects).35 Each subject received a single dose of IV GNbAC1 at 0.0025, 0.025, 0.15, 0.6, 2, or 6 mg/kg or an inactive vehicle (placebo), infused over 1 h. GNbAC1 was well tolerated after dosing in all subjects and in each dose cohort, with only minor and nonspecific adverse events (AEs) recorded, and no serious AEs reported. The mean elimination half-life ranged between 19 and 26 days, with therapeutically efficient concentrations maintained over a 4-week period at doses of 2 and 6 mg/kg. No emergence of anti-GNbAC1 antibodies was detected after dosing in any subject over the entire observation period. Subsequently, in a Phase IIa trial, Derfuss et al., reported an open-label extension of up to 12 months of clinical study testing GNbAC1 in 10 MS patients at doses of 2 and 6 mg/kg.36 GNbAC1 was well tolerated, with no safety issues observed in the MS patients, and no anti-GNbAC1 antibodies detected during the entire observation period. In a recent Phase IIb trial, total 270 MS patients were randomized to receive monthly intravenous Temelimab (6, 12, or 18 mg/kg) or placebo for 24 weeks; at Week 24 placebo-treated participants were re-randomized to treatment groups.37 At Week 48, participants treated with 18 mg/kg Temelimab had fewer new T1-hypointense lesions (p = 0.014) and showed consistent, however statistically nonsignificant, reductions in brain atrophy and magnetization transfer ratio decrease, as compared with the placebo/comparator group. The authors concluded that Temelimab failed to show an effect on features of acute inflammation but demonstrated preliminary radiological signs of possible anti-neurodegenerative effects, which supporting the development of such humanized mAb for progressive MS.
3.2 CAR-T-based treatment
In an intriguing study, Zhou et al. generated a chimeric antigen receptor (CAR) specific for HERV-K Env protein (K-CAR) using anti-HERV-K mAb.38 K-CAR T cells from PBMCs of nine breast cancer patients and 12 normal donors were able to inhibit the growth of, and exhibit significant cytotoxicity toward, breast cancer cells but not normal breast cells. As expected, the antitumor effects in cancer cells were significantly reduced when control T cells were used, or the expression of HERV-K was knocked down by shRNA. Treatment with K-CAR significantly reduced tumor growth and tumor weight in breast cancer xenograft models. Importantly, K-CAR treatment prevented tumor metastasis to other organs, indicating promising potential for such immunotherapy of breast cancer. The same group also reported that HERV-K-specific T cells generated from autologous dendritic cells pulsed with HERV-K Env antigens displayed high cytotoxic T lymphocyte (CTL) lysis of autologous tumor cells from ovarian cancer patients.39
4 HERV-K VACCINES
Kraus et al. developed a recombinant vaccinia virus (MVA-HKenv) expressing the HERV-K Env protein, utilizing the modified vaccinia virus Ankara (MVA) platform.40 To assess its vaccination efficacy, they established a murine renal carcinoma cell (Renca) xenograft model: Renca cells were genetically modified to express either Escherichia coli beta-galactosidase (RLZ cells) or the HERV-K ENV gene (RLZ-HKenv cells), which were subsequently intravenously injected into syngeneic BALB/c mice to induce pulmonary metastases. Their findings revealed that a single vaccination of tumor-bearing mice with MVA-HKenv significantly reduced the number of pulmonary RLZ-HKenv tumor nodules compared to vaccination with wild-type MVA. Moreover, prophylactic vaccination of mice with MVA-HKenv prevented the formation of RLZ-HKenv tumor nodules, while wild-type MVA-vaccinated animals succumbed to metastasis. In a subsequent study, the same group engineered a recombinant vaccinia virus (MVA-HKcon) expressing the HERV-K gag protein, which exhibited similar anticancer activities in the Renca xenograft model (instead, RLZ-HKGag cells were injected to establish the model).41 Collectively, these data demonstrate that HERV-K proteins may serve as valuable targets for vaccine development, offering new treatment avenues for cancer patients.
5 CONCLUSION
Despite recent advances in the development of strategies targeting HERV-K transactivation, most of these therapies remain at the preclinical stage (summarized in Table 1). We anticipate that more clinical trials targeting HERVs, similar to the GNbAC1 trial, which has shown encouraging results, will be approved for various diseases. Notably, while all currently approved Federal Drug Administration (FDA) antiretroviral drugs were designed against HIV-1, not HERVs, it is essential to recognize that HERVs, including HERV-K, possess their own reverse transcriptase, protease, and integrase, with functions somewhat different from those of HIV-1. This underscores the necessity for designing more specific antiretroviral drugs tailored to HERVs.
| Therapeutic agents | Therapeutic activities | References | |
|---|---|---|---|
| Small molecule compounds | The NRTIs, AZT, 3TC, and CBV | Inhibit HERV-K RT activity | [24] |
| The NRTIs, AZT, d4T, ddI, and 3TC, and the nucleotide RTI inhibitor TDF | Inhibit HERV-K RT activity | [25] | |
| The NRTIs, abacavir and AZT; NNRTIs, EFV, etravirine, and NVP | Inhibit HERV-K RT activity, block HERV-K infection and production | [26] | |
| The NNRTIs, EFV and NVP | Restrain the activation of HERV-K in melanoma cells | [27] | |
| The cyclic ureas, such as SD146 | Inhibit HERV-K PR activity | [29] | |
| Retroviral PR inhibitors, ritonavir, atazanavir, and lopinavir | Reduce proliferation of schwannoma and meningioma cells, through inhibition of HERV-K PR | [31] | |
| The integrase inhibitor DTG | Inhibit the proliferation of cancer cells, through repression of HERV-K expression | [32] | |
| The integrase inhibitor Raltegravir | Block HERV-K infection and production | [26] | |
| The inhibitor of MEK, PD98059 or CDK4, 219476 | Reduce the levels of HERV-K Env in melanoma cells | [33] | |
| Immunotherapy | Anti-HERV-K Env mAbs | Inhibit growth and induce apoptosis of breast cancer cells in vitro, reduce tumor growth in xenograft models | [34] |
| GNbAC1 or Temelimab, a humanized mAb targeting HERV Env protein | Get the proof of safe doses in healthy subjects and MS patients (Phase I/IIa), Temelimab failed to show an effect on features of acute inflammation but demonstrated preliminary radiological signs of possible antineurodegenerative effects (Phase IIb) | [35-37] | |
| CAR-T cells targeting HERV-K Env protein | Inhibit growth of breast cancer cells in vitro and in vivo, display high cytotoxic T lymphocyte (CTL) lysis of autologous tumor cells from ovarian cancer patients | [38, 39] | |
| Vaccines | HERV-K Env- or gag-based vaccine | Therapeutic and/or prophylactic vaccination reduced the formation of pulmonary metastases in murine renal carcinoma xenograft models | [40, 41] |
- Abbreviations: 3TC, lamivudine; AZT, zidovudine; CBV, carbovir; d4T, stavudine; ddI, didanosine; DTG, dolutegravir; EFV, efavirenz; MS, multiple sclerosis; NNRTIs, non-NRTIs; NRTIs, nucleoside RT inhibitors; NVP, nevirapine; PR, protease; RT, reverse transcriptase; TDF, tenofovir.
Regarding the development of immunotherapy and vaccines, further assessment of in vivo efficacy, especially regarding the existence of potential nonresponders, is imperative. Additionally, it is crucial to identify specific HERV loci associated with particular diseases, particularly polymorphic HERV-K loci, which may influence both the viral expression profile and the regulation of host genes in different individuals.3
AUTHOR CONTRIBUTIONS
Zhiqiang Qin developed the concept for this review. Lu Dai, Jiaojiao Fan, and Zhiqiang Qin performed literature search. Lu Dai, Jiaojiao Fan, and Zhiqiang Qin wrote, edited, and approved the manuscript.
ACKNOWLEDGMENTS
This work was supported by NIH R03DE031978, the Arkansas Bioscience Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000. Funding sources had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
Open Research
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request. All the data shown in this paper are available from the corresponding authors upon reasonable request.
