The goal of this study was to assess the associations of CCSVI with HLA DR*1501, a genetic variation that has been consistently linked to MS in familial and association studies. We found that the frequency of CCSVI positivity and HLA DRB1*1501 positivity were both increased in MS compared to controls. However, the frequency of CCSVI positivity was also increased in progressive forms of MS compared to the non-progressive forms of MS.
We reasoned that because HLA DRB1*1501 was well established as a genetic factor associated with the risk of developing MS, it would provide a reference relative to which the role of CCSVI could be evaluated. The goals were therefore to critically assess the associations of CCSVI with MS and MS progression vis-à-vis HLA DRB1*1501. We did not obtain evidence to support a role for statistical interactions between HLA DRB1*1501 and CCSVI status, which suggests that there is no synergistic association between HLA DRB1*1501 and CCSVI with MS. This is evidenced in non-progressive forms of MS because the relative proportions were the similar across the HLA DRB1*1501 negative-CCSVI negative, HLA DRB1*1501 positive -CCSVI negative, HLA DRB1*1501 negative-CCSVI positive, and HLA DRB1*1501 positive-CCSVI positive combinations. There was a higher relative frequency of the HLA DRB1*1501 positive-CCSVI positive combination compared to the HLA DRB1*1501 negative-CCSVI negative combination in progressive MS but this was not significant. The greater relative frequency of the HLA DRB1*1501 negative-CCSVI negative combination compared to the HLA DRB1*1501 positive-CCSVI positive combination in the control group could be interpreted as indicating that the absence of CCSVI is protective.
Although the association between susceptibility to MS and HLA-DRB1*1501
is well established, its relationship to disease characteristics and/or disease progression is controversial. Several studies have linked the DR2 haplotype to disease progression 
especially if extreme cases (benign vs. malignant) are compared 
but there is also evidence that a negative status for DRB1*1501
may be associated with a worse prognosis 
. Our results however, did not provide support for a protective role for DRB1*1501
negative status in progressive MS status.
Interestingly, despite the lower prevalence of CCSVI in our sample compared to the results previously reported 
, the odds ratio for the association of CCSVI with MS was 4.52 compared to the odds ratio of 2.33 for the association of HLA DRB1*1501
with MS. Additionally, CCSVI positivity appeared associated with progressive forms of MS but we did not obtain evidence that HLA DRB1*1501
positivity was associated with progressive forms of MS in our sample. The exact reasons for the associations between CCSVI and progressive forms of MS are not known: only prospective longitudinal studies can address whether the associations are the result of CCSVI modifying disease progression or alternatively, because CCSVI is secondary to the underlying inflammatory/degenerative disease processes.
A potential criticism of our methodology is the use of ECD, which is sometimes viewed as technically demanding and strongly operator dependent. We used a single machine for all subjects and the one operator received extensive training in assessing CCSVI in MS; the operator's intra-rater reproducibility was Kappa 0.75 agreement with 89.3% in a scan-rescan test 
. The operator was blinded to the subjects' clinical diagnosis and we included patients with OND because the obvious presence of disabilities in some patients adversely impacts the effectiveness of blinding 
. Catheter venography and magnetic resonance venography are alternative imaging modalities capable of providing greater anatomical detail than ECD. However, these techniques are difficult to apply for the large sample sizes required for genetic analyses, e.g., the CV is an invasive exam and value of MRV for diagnosis of CCSVI is limited 
. ECD provides qualitatively different functional assessments of flow velocity changes in response to postural adjustments that are complementary to, but not possible with the other imaging methods.
Other than the report of Ferlini et al. 
, who conducted preliminary analysis of copy number variations associated with CCSVI in a group of 15 MS patients, no information is available on the role of genetic factors in CCSVI. These authors reported that CCSVI was associated of copy number variations in the HLA
region for a small group of 15 MS patients 
. In other diseases with venous pathophysiologies, a role for gender, and environmental and genetic factors is suggested. Female gender, older age, and pregnancy are risk factors for chronic venous diseases 
and women have greater frequency of variant hepatic veins 
. Women have also been reported to have a smaller internal jugular vein size than men (1.48 for men vs. 1.27 in women) 
. Venous malformations may have genetic contributions and a “double-hit” mechanism has been invoked to explain incomplete penetrance and variability 
. The R849W substitution in the angiopoietin receptor Tie2 
, an endothelial receptor tyrosine kinase, has been linked to familial venous malformations and results in variable thickness or lack of smooth-muscle cells in the veins of patient lesions. Interestingly Tie2 activates Stat1, which is also critical in interferon signaling. We did not observe, age, gender or disease duration differences in the occurrence of CCSVI (results not shown) in MS. However, a more detailed analysis of candidate gender-dimorphic factors, e.g., vein diameters and autoimmune factors, is warranted as these could strongly interact with changes in cerebral venous outflow.
HLA DRB1*1501 has been consistently linked to MS susceptibility in genetic studies. We did not find evidence for associations between CCSVI diagnosis and HLA DRB1*1501 status for MS patients.