The present molecular study is consistent with previously reported association between HERV-W expression and SZ,6, 51
but also unravels the existence and specificities of an analogous association between BD and HERV-W env
Moreover, transcriptional levels of HERV-W env RNA from patients with BD here appear significantly higher than among patients with SZ, with additional qualitative differences in nucleotide sequences. This is obviously not attributable to treatment as most patients were treated and no correlation between a given treatment and results was evidenced. In particular, valproic acid had no impact as previously suggested,74
as excluding patients treated with this mood stabilizer yielded similarly significant differences with HC. It therefore cannot account for the quantitative and qualitative molecular findings in patients with BD or with SZ.
The significance of an elevated HERV-W env RNA expression in SZ and BD groups was also confirmed by comparison with a subgroup (C-) excluding controls with elevated C-reactive protein, a nonspecific marker of inflammation76
indicative of risk for various diseases in apparently healthy individuals.77
The influence of disease state could also be debated as previous studies on RNA only included patients with SZ during first or acute psychotic episode, or patients with BD during acute mood episode. For the first time, our present populations of patients with BD, who were mostly euthymic, and of patients with SZ, most of whom were not in an acute phase, evidence that this HERV-W expression is not simply linked to, or a mere consequence of, active episodes of these diseases. Importantly, it can now be assumed that elevated HERV-W/MSRV RNA expression appears constitutive in these patients.
Nonetheless, this does not preclude more elevated RNA expression that might peak during active and highly symptomatic periods of patients with BD or SZ. Moreover, only a subset of patients (with first onset or symptomatically active) have been shown to be positive for HERV-W env
mRNA expression in independent studies,22, 24, 25, 50
which is also the case with the present study on stable patients (62% −28/45 of positives for SZ; 56% −51/91 of positives for BD). Therefore, eventual transiently ‘negative' RNA expression cannot be excluded during the clinical course of these illnesses. Alternatively, this association with HERV-W may only be relevant in a subgroup of patients with SZ and BD, which would point to different pathogenic associations in ‘HERV-W negative' groups. BD and SZ disease entities are only defined by clinical criteria and might comprise subgroups with different etiopathogenic factors, but future longitudinal follow-up studies are now needed to address such important questions.
The unexpected low DNA copy number of HERV-W env in genomes of patients with SZ or BD (as compared with HC) is also accompanied by qualitative differences both in the distribution of copy numbers and in the nucleotide sequences within the probe region. In addition, despite similar DNA copy numbers, patients with SZ or BD show differing distributions of RNA transcription levels and of nucleotide sequences.
Altogether, these data confirm a consistent association with SZ and BD of this MSRV-type of HERV-W family that was first isolated in MS.33
However, the patterns of association between MS or SZ/BD appear quite different for the DNA, as well as for nucleotide sequence features of both RNA and DNA.
Results of the present study could, therefore, suggest that differences in HERV-W panel (number and nature of DNA copies) may preexist in the genome of newborns diagnosed later in their life with SZ or BD. An atypical transcriptional potency could result from inherited HERV-W variants with low copy number. Alternatively, de novo
acquired modifications of such ‘mobile genetic elements' cannot be totally excluded, as reported in other conditions.21
In animals, analogous situations are encountered with families of Endogenous Retroviruses (ERV), in which pathogenic ERV copies are unevenly distributed in species genomes. The presence of particular and/or numerous ERV ‘defective' copies can be protective against pathogenic ERV strains, while their absence may be detrimental.16, 51, 52
The hypothesis of de novo
acquisition of genetic alterations could also be consistent with numerous studies associating intra-uterine/perinatal infections or inflammatory events as potential triggers in the development of BD or SZ later in life.51
Genetic modifications could affect the HERV-W copy numbers as insertion/deletion phenomena have been reported in association with HERVs,56
but could also affect the detection of modified HERV-W copies by the present PCR primers and probe. This alternative explanation may thus shed light on the well-described genetic rearrangements in SZ or BD,16
with small to large deletions and/or copy number variations in the genome of affected individuals, as HERV rearrangements are also known to involve neighboring genes.56
Although the mechanisms are not yet elucidated, activation of HERV-W elements epigenetically silenced in differentiated cells was put forward.49
Here, the only environmental agent showing some degree of association with enhanced HERV-W RNA expression is T. gondii
. It nonetheless required large numbers with the merged SZ and BD groups to reach statistical significance and appeared relevant in a subgroup of patients. The fact that this parasite often had increased prevalence in cohorts of patients with BD or SZ78
would then be consistent with a role as a pathogenic co-factor, which can be replaced by other co-factors displaying analogous effects. Alternatively, it might have a role in association with particular symptomatology, as suggested by a recent study.9
Interestingly, T. gondii
is known to transactivate HERVs, including HERV-W elements, when infecting tumor cells,62, 79
which would also fit with a co-factor triggering HERV-W activation with subsequent genetic and inflammatory effects related to HERV-W envelope. This points to an indirect role of such infectious agents through ‘epigenetically susceptible' HERV elements, as it is known that tumor cells have extensive DNA hypomethylation,80, 81
which occurs physiologically in embryonic cells.82
Thus, T. gondii
could induce a targeted activation of HERV-W elements creating a risk for SZ or BD in individuals carrying an HERV-W ‘pathogenic element'. Such HERV-W elements appear here to fit with the MSRV subtype detected under the form of circulating retrovirus-like particles in a low proportion (8.9±6.2%) of studied Caucasian populations.83
Thus, requiring the coincidence of both triggering and responding entities for the genesis of HERV-W pathogenicity, environmental pathogens other than T. gondii
may not yield significant results with our present numbers and patient selection, due to their variability as co-factors and to their less frequent prevalence in our cohorts.
We thus hypothesize that, at a particular vulnerable developmental stage (for example, when the panel of HERV-W genes is hypomethylated and/or with low epigenetic control), an environmental trigger such as an infection favors cell lineage-specific genetic modifications in these elements (retrotransposition, gene rearrangements, and so on) establishing altered/variable patterns of neurodevelopment. Later in life, each of these patterns would respond differently to subsequent environmental triggers and be translated into distinct clinical phenotypes such as SZ, BD. Quite different nature and conditions of interactions with HERV-W elements (for example, no association ever found with T. gondii
nor with perinatal infection), as well as temporal differences in triggering events during lifetime, could then lead to MS.49, 51
The emerging concept involving HERVs in human medicine21
highlights the importance such gene–environment interactions in a number of multifactorial diseases with poorly understood etiology,84