Autonomic dysfunction: A unifying multiple sclerosis theory, linking chronic cerebrospinal venous insufficiency, vitamin D₃, and Epstein-Barr virus

Abstract

Multiple sclerosis (MS) is a disease with multiple etiologies. The most recent theory of the vascular etiology of MS, Chronic Cerebrospinal Venous Insufficiency (CCSVI), suggests that cerebral venous obstruction could lead to cerebral venous reflux, promoting local inflammatory processes.

This review article offers strong evidence that the route of the observed narrowing of cerebral veins arises from autonomic nervous system dysfunction, particularly cardiovascular autonomic dysfunction.

The dysfunction of this system has two major effects: 1) the reduction of mean arterial blood pressure, which has the potential to reduce the cerebral perfusion pressure and the transmural pressure, and 2) the failure of cerebral autoregulation to maintain constant cerebral blood flow in the face of fluctuations in cerebral perfusion pressure. Alterations in cerebral autoregulation could in turn raise the critical closure pressure, indicated to be the cerebral perfusion pressure at which the transmural pressure will be sub-sufficient to overcome the active tension imparted by the smooth muscle layer of the vessel. These two effects of autonomic nervous system dysfunction (reduction in arterial blood pressure and alterations in cerebral autoregulation), when combined with inflammation-induced high levels of nitric oxide in the brain, will lower transmural pressure sufficiently to the point where the threshold for critical closure pressure is reached, leading to venous closure.

In addition, cerebral vessels fail to overcome the closure as a result of low central venous pressure, which is also regulated by autonomic nervous system function. Furthermore, through their neuroregulatory effects, infectious agents such as the Epstein-Barr virus and vitamin D₃ are able to alter the functions of the autonomic nervous system, influencing the rate of CCSVI occurrence.

The absence of CCSVI specificity for MS, observed in recent clinical studies, may stem from a high prevalence of autonomic nervous system dysfunction in control groups which were recruited to these studies. Future studies should investigate CCSVI in relation to cardiovascular autonomic function.

Introduction

Multiple sclerosis (MS) is the most common neurological disease in young adults [1]. The disease is heterogeneous in clinical appearance, and it is divided into different groups based on both clinical and radiographical data [2]. Factors responsible for the different courses of the disease are unknown and the etiology of MS remains uncertain.

One of the most widely accepted hypotheses is that MS is a progressive, autoimmune, demyelinating disease of the CNS where chronic inflammation is central to the axonal injury and loss, leading to deficits of motor, autonomic, and neurocognitive function [3], [4]. This autoimmune response involves T-cells, B cells, macrophages/microglia, and cytokine-mediated effects against auto-antigens such as myelin basic protein (MBP), proteolipid protein, and myelin oligodendrocyte glycoprotein [5].

Immunomodulatory therapies targeted to reduce inflammation have shown clinical benefits in MS patients [6]. First-line immunomodulators include Interferon-beta and Glatiramer acetate, which are used for the treatment of relapsing remitting (RR) MS patients. Both drugs are known to modulate the immune system, reducing the annual rate of relapse and T2 lesions on the MRI scan by 30%. A newer drug, Natalizumab, which is a selective adhesion molecule inhibitor, is reserved for MS patients who do not adequately respond to first-line immunomodulatory therapies [7]. Nevertheless, these modalities are less likely to benefit patients with the chronic progressive (CP) type of the disease [8].

Both genetic and environmental factors have been shown to contribute to susceptibility to MS. Among studied genes for susceptibility to MS, the genes on chromosome 6p21 in the area of the major histocompatibility complex [MHC; histocompatibility leukocyte antigen (HLA) in humans], in particular the alleles -DR and -DQ of the genes, have been shown to have strong correlations accounting for 10%–60% of the genetic risk of MS [9], [10]. The association between these susceptibility genes and autoimmunity is thought to involve their role as antigen-presenting molecules to CD4 + T cells [11]. This deregulation in self-recognition, which has been shown to be related to the T-cell signaling pathways [11], can result in epitope spreading and eventual autoimmunity [12].

However, studies based on the concordance rates of mono-zygotic twins show that the genetic component of MS risk is around 25% [13], suggesting that environmental factors also contribute to MS pathogenesis. In addition, epigenetic mechanisms, including DNA methylation, histone modifications, and micro (mi)RNAs which have the ability to regulate post-transcriptional gene expression through multiple mechanisms, have been shown to account for the observed disconcordance rates of mono-zygotic twins. These epigenetic factors provide a bridge between the external environment, such as infectious agents and UV sunlight, and the individual genetic makeup, determining the initiation and the progression of MS [14]. Nevertheless, a detailed discussion of epigenetic mechanisms is beyond the scope of this review.

Infectious agents have been suggested to be among non-genetic causes of MS. The most convincing evidence for the association between infectious agents and MS comes from the Faroe Islands MS epidemics after 1940, when British troops in the Second World War occupied the islands, which were known not to have had MS cases before [15].

In addition, the oligoclonal IgG antibodies in the CSF, one of the tests used to diagnose MS, frequently show increased antibody titer for measles, rubella, or varicella zoster [16]. The results of clinical studies show benefits of treating MS patients with the antiviral agent Acyclovir [17] or with its prototype, Valaciclovir [18], [19], further supporting the role of infectious agents in MS pathogenesis.

Infectious agents are able to trigger autoreactive T cell responses against myelin and its components through molecular mimicry [20]. Such a molecular mimicry has been identified for the Epstein-Barr virus (EBV), a double-stranded DNA herpes virus, which has been extensively studied for its relation to MS [21], [22], [23], [24], [25], [26]. The molecular mimicry is imparted by the existing homologies between EBNA1 (an EBV nuclear antigen) and MBP, leading to cross-reactivity between EBNA1 and antibodies against MBP [27].

In addition, due to structural similarities, strong cross-reactivity was observed between a peptide belonging to the EBV DNA polymerase, BALF5, and a peptide from MBP [28]. These specific structures are recognized by the same T-cell receptor in the context of HLA DR2 haplotypes DRB1*1501 and DRB5*0101 [28], two HLA haplotypes which are linked to predisposition to MS [29].

Molecular mimicry could be also achieved indirectly through EBV activating the endogenous human retroviruses. Proteins encoded by endogenous human retroviruses have close homologies to exogenous viral antigens and could elicit potent antiviral immunity leading to cross-reactivity against self antigens [30]. Endogenous human retroviruses have the potential to directly regulate immune responses [30]. As a result, EBV-induced activation of endogenous human retroviruses could impact the immune system in one way or another.

Epidemiological studies show that the age when EBV infection occurs is associated with MS risk. A high prevalence of early EBV infection in under-developed, low socio-economic areas is likely to result in immunization of most children to EBV infection as early as three years of age. In this population, approximately 100% become EBV seropositive within the first decade [31]. These early EBV infections are usually asymptomatic and prevent the likelihood of EBV infection in later life. In contrast, in well-developed countries approximately 50% of children are still EBV seronegative at the end of the first decade of their lives. These individuals are likely to become infected later in life. These delayed infections manifest as infectious mononucleosis [31], which has been shown to increase the relative risk of MS by 2.3 times [21].

By establishing a latent infection in memory B cells, EBV causes B cells immortalization [32] and their clonal expansion in brain lesions and in the CSF [33], [34]. Furthermore, it has been suggested that in addition to genetic susceptibility through HLA alleles, some individuals have genetic defects in their ability to eliminate the EBV-infected B cells, leading to their accumulation in the CNS [32]. The periodic reactivation of infected B cells could then serve as an antigenic stimulation for the immune system triggering CD8 + T cells' activation and promoting a chronic inflammatory state [35], [36], [37].

The EBV infection has been observed in brain-infiltrating B cells and plasma cells in the majority of the MS cases examined [35], and their depletion by Rituximab treatment has shown clinical benefits in MS patients [38]. The use of retroviral integrase inhibitors and vaccination against EBV are additional strategies to control EBV infection [39].

Another etiology of MS is the latitude, which explains the geographic distribution of MS, which is low in equatorial regions, but increasing drastically with latitude in both hemispheres [40]. The latitude in turn is related to the degree of exposure to UV sunlight catalyzing the production of 1,25-dihydroxyvitamin D₃ (Vit D₃) in the skin [41], [42]. Vit D₃ could also be obtained, through food and supplementation, although diet is a poor source of Vit D₃ [43] as compared to UV exposure [44].

Studies show that suboptimal sunlight exposure is an environmental risk factor contributing to MS etiology. The level of exposure to UV light is inversely associated with the risk of mortality from MS [45]. Furthermore, the timing of birth in relation to the level of sun light exposure have been shown to affect the risk of developing MS [46], while high sun light exposure in early childhood conveys protection against MS [41]. The role of Vit D₃ in MS is further supported by longitudinal studies showing an inverse relation between serum Vit D₃ levels and MS disease activity [47].

Similar to EBV, both the genetic and immune regulatory factors are associated with Vit D₃ status. A recent study demonstrated that calcitriol, the hormonally active form of Vit D₃, modulates the expression of HLA-DRB1 *1501 via vitamin D response element in the promoter region of the HLADRB1* 1501 allele [48]. It has been suggested that Vit D deficiency among those individuals carrying the vitamin D-responsive DRB1*1501 allele may lead to a lower expression of the MHC Class II molecule, hence impairing the process of self tolerance during negative selection [49]. Nevertheless, a recent genome-wide association study did not observe a relationship between MS susceptibility genes and Vit D₃ serum concentrations [50].

Vit D₃ has been also shown to modulate immune function, including cell proliferation, differentiation, and apoptosis [51]. The immunomodulatory effects of Vit D₃ are further indicated by studies showing that an impaired Vit D status increases the risk of viral infection in both children and adults [52], [53], further implying a role for an increased EVB infection in vitamin D-deficient individuals [54], [55]. The immunomodulatory effects of Vit D₃ suggest that early exposure to UV radiation and/or Vit D₃ supplementation could prevent the disease in individuals at risk of developing MS [56].

A new additional theory on the genesis of MS is the Chronic Cerebrospinal Venous Insufficiency (CCSVI) [57], [58], which was recently proposed by Paolo Zamboni. One could not foresee the controversies that Zamboni's vascular theory generated in the MS community, the public, and the media following his invitation by the author to the Grand Round lecture presentation at the Baird MS center, Buffalo, NY.

Zamboni's vascular hypothesis states that MS is caused by the obstruction at different vein levels namely the internal jugular veins (IJVs), azygos veins, vertebral veins, and lumbar venous plexus [57], [58]. IJVs are the main route of drainage for both the superficial system and the deep system [59]. The blockade of IJVs potentially leads to the opening of collateral routes for drainage [57], [60]. Nevertheless, the rate of the venous drainage is thought to be suboptimal, leading to cerebral venous hypertension and disruption of the blood brain barrier (BBB) integrity. The suboptimal drainage can also result in venous reflux and iron deposition within the brain parenchyma with the potential of initiating local inflammatory responses [61], [62].

Using venography, and transcranial and extracranial Doppler sonography, Zamboni et al. [63] identified five different venous anomalies and four different hemodynamic patterns of extracranial–extravertebral venous outflow to define CCSVI. Based on two or more positive venous hemodynamic abnormalities, the investigators observed CCSVI in 100% of MS patients and 0% of control subjects [63]. In addition, in a retrospective study, the same investigators reported a correlation between CCSVI pathology and MS disease course [64].

Subsequent clinical studies [65], [66] also reported the occurrence of CCSVI in MS patients, but the rate of CCSVI was significantly lower among MS patients, and higher among control subjects, than initially reported by Zamboni et al. [63]. In addition, the sensitivity to detect IJV's narrowing varied depending on the method of measurement and was higher when measured by magnetic resonance venography as compared to extracranial Doppler sonography [67].

Applying various techniques such as sonography and magnetic resonance angiography, other investigators [68], [69], [70], [71], [72], [73] could not replicate Zamboni's previous findings. In addition, no differences between MS patients and control cases were observed [68], [71], [72], and no correlation between CCSVI and MS risk or MS severity was documented [70]. These observations in turn strengthened the proposed assumption that CCSVI findings may reflect anatomical variations in venous drainage, with no association with MS clinical pathology [72].

In order to treat venous malformations, percutaneous transluminal angioplasty (PTA) of affected extracranial venous stenoses, which was later termed “Liberation Treatment,” was carried out by Zamboni and colleagues, showing a significant improvement in 35 of 65 treated MS patients [74]. However, it was argued [75] that the observed clinical improvement may have not necessarily been the results of the procedure performed, but was the consequence of methodological bias and confounding factors, including patients' characteristics, the absence of a control group for comparison, and substandard methods of MRI data collection.

Among evidence supporting the vascular etiology of MS is the fact that MS demyelinating lesions tend to be perivenular [76]. However, detection of a central vein within white matter lesions is not a specific finding for MS, since it has also been found in other white matter lesion etiologies, including in patients with microangiopathic white matter lesions [77]. Other arguments against CCSVI include the reported normal level of CSF ferritin [78], normal level of cerebral venous pressure [79], and the absence of a significant association between CCSVI and HLA DRB1*1501 status [80]. In addition, the endovascular intervention, undertaken by Zamboni and colleagues was associated with a relatively high level of restenosis of IJVs within 18 months follow up [74], suggesting that factors outside the cerebral venous system may contribute to the venous obstruction in the first place.

Based on a recent work by the author and earlier results of other investigators, this review provides strong evidence that the underlying basis for CCSVI may involve autonomic nervous system (ANS) dysfunction in general and cardiovascular sympathetic dysfunction in particular. The intention of this review is not to support or reject the CCSVI hypothesis, but rather to present a plausible biological and physiological explanation for the results observed in the related clinical studies. This overview suggests that CCSVI is not the cause of MS, but rather the result of ANS dysfunction, often present in MS patients. In addition, the infectious etiology and the Vit D etiology may affect the occurrence of CCSVI indirectly, by modulating the ANS functional activities. This review therefore provides a foundation upon which future CCSVI studies should be conducted and analyzed.