Does the efficacy of IFN-alpha2 reflect interference with a reactivated dormant virus—human endogenous retrovirus (HERV)?
Since IFN-alpha2 has highly potent antiviral activity, it is tempting to consider if the efficacy of IFN-alpha2 in MPNs reflects that IFN-alpha2 interferes with replication of a virus that is involved in the pathogenesis of MPNs. In this regard, particular attention has been payed to the potential role of human endogenous retrovirus (HERV), which has recently been revived as a potential causative factor for the development of MPNs [
124]. Thus, the story on HERV being involved in MPN pathogenesis is not new. Indeed, HERV-K particles have been reported in megakaryocytes cultured from patients with ET [
155,
156]. In the context of chronic inflammation as a potential trigger and driver of clonal evolution [
124‐
126], it is intriguing to consider if the marked deregulation of inflammation and immune genes in MPNs [
142‐
144]—several of these being deregulated in virus-induced malignancies as well—might be due to chronic inflammation elicited by a virus infection, e.g., reactivation of an endogenous retrovirus [
124]. Thus, a chronic HERV infection of myeloid cells might account for activation of immune cells with deregulation of inflammation and immune genes. The immune attack with apoptosis of virus-infected cells might consequently elicit a sustained compensatory myeloproliferation of non-infected cells. However, ultimately, the immune system fails to clear the virus and from an early stage (ET) the disease progresses during the next 10–20 years in concert with a steady increase in bone marrow fibrosis, reflecting sustained reparative processes in an attempt to heal “the wound that won’t heal” [
124,
157].
How does the mutational and cytogenetic landscape impact the efficacy of interferon-alpha2 in MPNs?
The mutational landscape in MPNs is complex and highly heterogeneous. Thus, in addition to the driver mutations—
JAK2V617F,
CALR, and
MPL—several mutations outside the JAK-STAT pathway have been comprehensively described during the years [
69,
141,
160]. Importantly, disease progression and clonal evolution in the biological continuum from the early cancer stages (ET/PV) along the path towards the advanced myelofibrosis stage have been closely linked to the development of additional subclonal mutations (
ASXL1,
SRSF2,
CBL,
IDH1/IDH2,
TP53, and
SRSF2), being independently associated with leukemic transformation and poor survival [
69,
152,
160]. Thus, despite a low mutation rate, it has been shown that the presence of two or more somatic mutations significantly reduces overall survival and increases the risk for leukemic transformation in patients with MPNs [
152].
Recent studies have suggested that mutations in the epigenetic modifiers—
TET2,
DNMT3A,
ASXL1,
EZH2, and
IDH1/2—may lead to alterations in hematopoietic stem cell (HSC) function [
150,
161‐
164]. Since IFN-alpha2 directly targets the malignant HCS [
13,
14,
26], thereby potentially depleting and eliminating the disease-initiating HSC compartment [
115], such alterations might negatively affect the response to IFN-alpha2. Indeed, in a small series of
JAK2V617F patients, Kiladjian et al. showed that a subset had persistent
TET2-positive clones during IFN-alpha2a treatment despite eradication of the
JAK2 mutations, indicating that IFN-alpha2 is able to reduce or eliminate the
JAK2V617F mutant clone but not the
TET2 mutant clone [
165]. These preliminary data might imply that patients with concurrent
JAK2V617F and
TET2 mutations have a less favorable response to treatment with IFN-alpha2 taking into account that the
TET2 mutation—as the
JAK2V617F mutation—is an “inflammatory mutation,” which gives rise to increased production of IL-6 and thereby an “inflammatory soil” in the bone marrow with potential impairment of IFN signaling and accordingly impaired clinical and molecular response to IFN-alpha2.
Highly interestingly, by serial sequencing of
TET2,
ASXL1,
EZH2,
DNMT3A, and
IDH1/2 in ET and PV patients treated with pegylated IFN-alpha2a, Quintas-Cardama et al. showed that the frequency of mutations in genes outside of
JAK2 was higher in patients failing to achieve a complete molecular remission (CMR) (56%) versus those achieving CMR (30%), although this difference did not reach statistical significance. Furthermore, patients not achieving CMR were more prone to acquire new mutations during therapy [
36]. Of note,
TET2 mutations at therapy onset had a higher
JAK2V617F mutant allele burden and a less significant reduction in
JAK2V617F allele burden compared with
JAK2 mutant/
TET2 wild-type patients [
36]. Surprisingly, in this study,
TET2 mutant alleles were shown to be eradicated by IFN-alpha2a in a subset of patents. However, all together,
TET2 mutant clones most commonly persisted during IFN-alpha2a treatment despite eradication of
JAK2V617F mutant clones [
36]. The authors speculated if the discovery that mutations in
TET2 [
150,
162,
163,
166],
DNMT3A [
161], and
IDH1/2 [
167] elicit an increased self-renewal might actually negatively influence the ability of IFN-alpha2 to reduce or eliminate mutant MPN disease initiating cells, which harbor these mutations and accordingly conferring acquired resistance to IFN-alpha2 [
36]. The authors concluded that IFN-alpha2 induces CMR in a subset of PV or ET patients, and that the molecular signature may impact clinical and molecular responses to IFN-alpha2a [
36]. Larger studies are needed to assess whether mutations in
TET2 and/or other genes that regulate the HSC compartment (such as
DNMT3a and
IDH1/2) result in persistence of malignant clones during IFN-alpha2 therapy and if their persistence indeed impact upon the prognosis of ET and PV patients being treated long-term with IFN-alpha2.
In regard to patients with early myelofibrosis, Silver and co-workers have most recently described the impact of the mutational landscape on the response to IFN-alpha2 in a phase 2 study of 30 patients with early myelofibrosis [
117,
168], including their initial cohort of 17 patients [
169‐
171]. The authors correlated response to IFN-alpha2 treatment with the mutation profile at the time of diagnosis, including both driver mutations (
JAK2V617F,
CALR, and
MPL) and high risk mutations (HRMs), including
ASXL1,
EZH2,
SRSF2, and
IDH1/2 [
168]. Importantly, patients with these HRM did not respond to IFN-alpha2 therapy, irrespective of spleen size. Of note, the longest surviving patient who was in complete remission for more than 25 years had a molecular profile that included positive
CALR and
TET2 mutation status. This observation is of utmost importance since it dictates that the
TET2 mutation may not consistently imply a poor response to IFN-alpha2 treatment [
117,
168]. The findings by Silver et al. suggest that treatment with IFN-alpha2 in patients with early myelofibrosis may offer a survival benefit, putting in perspective the rationales for early therapeutic intervention with IFN-alpha2 in this patient group [
22,
23,
51,
120] instead of “watchful waiting,” which is recommended in patients with low-risk MF at most MPN centers. The authors argue for early intervention with IFN-alpha2 before the development of the advanced myelofibrosis stage with large splenomegaly and bone marrow failure. At this stage of increasing genomic instability and subclone formation, IFN-alpha2 has only a minor impact, in part due to the presence of HRMs. The observations by Silver et al. substantiate “The Early IFN Intervention Concept” in MPNs [
22‐
25,
51,
120], implying treatment with IFN-alpha2 to be initiated as early after the diagnosis as possible, when the tumor burden is at a minimum, because, at this stage, IFN-alpha2 is likely to have the optimal chance of inducing MRD as defined by normalization of the bone marrow and low
JAK2V617F allele burden sustained even several years after discontinuation of IFN-alpha2 [
22‐
25,
51,
120].
Several studies of smaller series of patients have documented cytogenetic remissions during treatment with IFN-alpha2 [reviewed in 24]. In recent years, larger studies, including the above study by Quintas-Cardama et al., have convincingly confirmed that long-term treatment with IFN-alpha2 may be followed by complete cytogenetic remissions [
36,
37]. Thus, this highly important observation has also been confirmed by Gisslinger et al. using the new formulation of pegylated interferon alpha (peg-proline-IFNa-2b, AOP2014/P1101) [
37]. In addition to high response rates being obtained on both hematologic and molecular levels, (the
JAK2V617F mutational load) peg-proline-IFNa-2b treatment also led to cytogenetic remissions in a subset of their PV patients, even in those with complex cytogenetic findings at treatment onset [
37]. In a previous study, Gisslinger et al. have reported that chromosomal aberrations emerged at the time of IFN-alpha2 resistance in a patient with primary myelofibrosis [
172]. The impact of the mutational and cytogenetic landscape upon the immediate and long-term responses to IFN-alpha2 in MPNs remains to be definitely described in larger studies.
How does the chronic inflammatory state in MPNs impact the efficacy of interferon-alpha2?
Chronic inflammation may impact the efficacy of IFN-alpha2 in MPNs. Thus, it has been shown that inflammatory signaling impedes the effect of IFN-alpha2 [
173]. As previously alluded to, all effects of IFN-alpha2 on cells are elicited through interaction with the type I IFN receptor on the cell surface. This receptor consists of IFNAR1 and IFNAR2c chains. Among the potential mechanisms of refractoriness to IFN-alpha2 is downregulation of IFNAR1. Indeed, low levels of IFNAR1 correlate with poor response to IFN-alpha2 in patients with malignant melanoma [
174]. Highly intriguing, Huang Fu et al. have shown that inflammatory cytokines interleukin 1-alpha (IL1-alpha) and tumor necrosis factor alpha (TNF-alpha)) stimulate IFNAR1 degradation and attenuate IFN-alpha signaling [
173]. In patients with chronic hepatitis C, unresponsiveness to IFN-alpha is common, partly being explained by oxidative stress, impairing IFN-alpha signaling [
175]. Since MPNs are associated with elevated levels of several inflammatory cytokines, including IL1-alpha and TNF-alpha, being produced by the malignant clone itself but also by the stroma cells in the bone marrow, and the highest levels have been reported in patients in the advanced myelofibrosis stage [
126], these data also support the concept of early intervention with IFN-alpha2 when the inflammatory state is less pronounced. The fact that the effects of IFN-alpha are negatively impacted by inflammation may have several implications. First, one may speculate if smoking—exposing a huge systemic inflammatory load—may actually interfere with IFN signaling in MPN patients [
176], implying either a weaker response to IFN-alpha2 or larger doses to be used to obtain adequate IFN responses in terms of inducing CHR. Second, agents with an anti-inflammatory potential in terms of lowering inflammatory cytokines, including IL1-alpha and TNF-alpha, might improve the IFN-alpha2 response. Indeed, the effects of IFN-alpha2 have most recently been shown to be enhanced by combination therapy with the JAK1–2 inhibitor, ruxolitinib, which is potently anti-inflammatory and immunosuppressive as well [
177,
178]. Studies are ongoing to elucidate if statins, which have been suggested as potential useful agents in MPNs due to their anti-proliferative, anti-angiogenic, proapoptotic, and not least anti-inflammatory capabilities [
179,
180], may also enhance the efficacy of IFN-alpha2 in MPNs. Taking into account that patients with MPNs have a 40% increased risk of second cancers [
105], and statins have been shown to reduce cancer-associated mortality by 15% [
181], their role in the treatment of MPNs certainly deserves to be investigated in the future [
179,
180].