HHV-8 Genetic Diversification and Its Impact on Severe Clinical Presentation of Associated Diseases

(See the Major Article by Jary et al, on pages 1320–8.)

Infection with human herpesvirus 8 (HHV-8), is common in certain parts of Africa, the Middle East, and the Mediterranean, but is rare elsewhere, except in persons with HIV [1]. HHV-8 is the etiologic agent of Kaposi sarcoma (and for this reason is also known as Kaposi sarcoma-associated herpesvirus, KSHV), the most common malignancy associated with HIV infection [2]. HHV-8 is also involved in the development of primary effusion lymphoma and multicentric Castleman disease, and recently has been involved in the pathogenesis of the so-called inflammatory cytokine syndrome [3].

HHV-8 is a γ-2 herpesvirus, capable of infecting multiple cell types, including endothelial and epithelial cells, as well as B lymphocytes. Like all herpesviruses, after primary infection, HHV-8 establishes a latent state in which the viral genome is maintained as a covalently closed circular episome with limited viral gene expression [4]. During the viral latent phase, no virion production occurs and only a small subset of genes is expressed [5]. One of these genes encodes the latency associated nuclear antigen (LANA), a predominant regulator of latency, capable of promoting host cell proliferation and survival and, therefore, contributing to the HHV-8–mediated oncogenesis [6]. So far, according to the K1 open reading frame (ORF-K1) gene, encoding a highly variable glycoprotein related to the immunoglobulin receptor family, which maps at the extreme left-hand end of the HHV-8 genome, 7 HHV-8 subtypes have been identified (A, B, C, D, E, F, and Z), each comprising several subclades with a different geographic distribution in human populations [1, 7]. Subtypes A and C circulate predominantly in Europe, North America, and Northern Asia, while subtype D and E are found in Taiwan and in ancient populations of South America, respectively. Both subtypes B and A5 were found in Sub-Saharan Africa, whereas subtypes F and Z reside in Uganda and in Zambia, respectively. Phylogenetic studies performed on the well-conserved ORF 26 (minor capsid gene) allowed the identification of 8 distinctive subtypes designated as A/C, J, K/M, D/E, B, Q, R, and N groups, which are also diversely distributed in different geographical regions, whose distribution in the world parallels that of ORF-K1 variants [7]. Until now, there has been a little information regarding whether and how different subtypes and variants may have different pathogenic and tumorigenic properties, associated with diverse rates of disease progression [2, 8, 9]. The clinical course of Kaposi sarcoma can range from an indolent state to a severe, progressive disease leading to significant morbidity and mortality. Antiretroviral treatment (ART) is essential for HIV-infected Kaposi sarcoma patients but, despite ART, up to a third of Kaposi sarcoma patients have disease recurrence or do not respond to ART alone. Moreover, Kaposi sarcoma also develops in HIV-infected persons with effective response to ART, that is in patients with no detectable HIV load and near-normal CD4 cell levels [2, 10–13].

In this issue of The Journal of Infectious Diseases, Jary and colleagues present new and interesting findings on the role of HHV-8 genetic variability in ORF-K1 protein in modulating HHV-8 oncogenic potential, and raise the concept that different HHV-8 subtypes can be associated with different disease progression [14].

The variable region 1 (VR1) of the HHV-8 ORF-K1 protein is the most studied genetic determinant underlying the differentiation of HHV-8 genotypes. ORF-K1 is a highly glycosylated transmembrane protein, produced during the lytic infection and composed of an extracellular, a transmembrane, and a cytoplasmic domain. The extracellular domain is characterized by the presence of 2 hypervariable regions: the above-mentioned VR1 (encompassing aa 54–93) and VR2 (aa 191–228). Notably, beyond LANA, ORF-K1 can also act as viral oncogene. Indeed, ORF-K1 protein can promote cell proliferation by activating multiple intracellular growth signaling, inhibit apoptosis, and stimulate tumor angiogenesis [15–17]. A previous study has shown that viral genetic determinants in VR1 (and also VR2) are not only important for the differentiation of HHV-8 subtypes but can also help to identify patients with a higher susceptibility to developing Kaposi sarcoma [18].

In particular, the study by Jary et al was aimed at investigating the genetic diversity of HHV-8 in a well-defined cohort of men who have sex with men (MSM) with or without HIV and with or without diseases associated with HHV-8 [14]. Viral genetic diversity was also correlated with the severity of clinical manifestations.

In line with previous studies worldwide [19–21], the authors found that the most common HHV-8 subtypes circulating in their cohort of MSM were C and A. Interestingly, they found that subtype C, and specifically variant C3, was the most prevalent in MSM living in France and tended to be associated with less severe epidemic clinical Kaposi sarcoma. In contrast, among patients with Kaposi sarcoma, subtype A was associated with higher HHV-8 viremia and, in turn, with a more aggressive clinical presentation of the disease. This is in line with a previous study showing the association of subtype A with a more rapid development of Kaposi sarcoma in HIV-positive HHV-8 coinfected patients [8]. Similarly, another study has pointed out an association of subtype A5 with more extensive forms of the disease [22]. These findings suggest an accurate determination of HHV-8 genotype is crucial to identify patients with a higher risk of disease progression.

An intriguing issue, deserving further investigation, is represented by the apparent synergy between HIV and HHV-8 in promoting cell transformation. Indeed, HIV-1 can promote the onset of Kaposi sarcoma by releasing specific secretory proteins in the bloodstream of coinfected patients. Among them, the HIV-1 regulatory proteins Tat and Nef play an important role [23–25]. In particular, Tat is a multifunctional protein known to regulate not only viral but also cellular gene expression. Importantly, Tat has been shown to induce the reactivation of HHV-8 replication after the establishment of a latent phase [26, 27], to promote the growth of HHV-8–infected endothelial cells, and angiogenesis in cooperation with ORF-K1 protein [28–30], thus playing a crucial role in the initiation and progression of Kaposi sarcoma in the setting of HIV infection. This raises the concept of a potential synergism between HIV-1 and HHV-8 in modulating the onset and severity of Kaposi sarcoma. Further studies are necessary in order to verify the impact of genetic variability (and potential coevolution) of Tat and ORF-K1 in modulating (positively or negatively) HHV-8 oncogenic properties.

Another interesting finding in the study by Jary et al was the identification of a new HHV-8 variant closely related to the first subtype F described in Uganda [14]. This new F variant was identified by population-based sequencing and confirmed by whole-genome sequencing on various biological samples. Based on this finding, the authors proposed the subdivision of subtype F into two variants: F1 variant circulating in Africa (particularly in Uganda) and F2 variant so far circulating in a restricted population of white MSM. From an evolutionary point of view, it is conceivable that this new variant has a recent origin and has started to spread in humans recently. This paves the way for further phylogenetic studies aimed at providing insights into the origin of this new HHV-8 variant and its spreading capability in MSM and persons with HIV.

Notably, among the 5 patients in whom this new variant was detected, 4 were immunocompromised and had severe forms of Kaposi sarcoma, again supporting the role of viral genetic determinants in mechanisms underlying HHV-8 pathogenetic potential. This point deserves emphasis considering that, as previously mentioned, Kaposi sarcoma still continues to be clinically relevant in the era of modern ART. Virologically, this can be explained by the fact that the production of the regulatory proteins Tat and Nef cannot be inhibited by antiretroviral drugs and, thus, these proteins can continue to exert their pathogenetic potential on HHV-8 despite prolonged virus suppression. This hypothesis raises the possibility that beyond immunosuppression, specific virologic mechanisms can modulate the onset and progression of Kaposi sarcoma and other HHV-8–associated diseases. Further studies are necessary to verify this hypothesis that suggests the need to set up different approaches to effectively prevent and treat HHV-8 associated cancers in the modern ART era.

In conclusions, despite being a DNA virus, HHV-8 has a degree of genetic variability that allows the virus to undergo continuing genetic diversification. This may lead to the generation of new variants associated with more aggressive clinical manifestations. The seeming synergism between HHV-8 variants (both classical and novel) and HIV, together with host and other cofactors, are intriguing points that deserve further attention to better clarify mechanisms (beyond immunosuppression) underlying HHV-8 pathogenesis in the era of modern ART. The findings also support the need to set up diagnostic assays for proper HHV-8 genotyping and quantification of viral load to optimize the management of HIV-positive HHV-8 coinfected patients.

Note

Potential conflicts of interest. All authors: No reported conflicts of interest. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

References

Author notes