Magnetic resonance imaging techniques to define and monitor tissue damage and repair in multiple sclerosis

Reconciling lesions, relapses and smouldering associated worsening: A unifying model for multiple sclerosis pathogenesis

The failure of relapses and white matter lesions to properly explain long-term disability and progression in multiple sclerosis is compounded by its artificial separation into relapsing remitting, secondary progressive, and primary progressive pigeonholes. The well-known epidemiological disconnection between relapses and long-term disability progression has been rediscovered as "progression independent of relapse activity", i.e. smouldering multiple sclerosis. This smouldering associated worsening proceeds despite early and prolonged use of disease modification therapies, even those that are highly effective at preventing relapses and new/enhancing white matter lesions on MRI. We recognise that smouldering associated worsening and relapse/lesion associated worsening coexist, to varying extents. The extent of cortical demyelination has been shown to correlate significantly with the severity of diffuse injury in normal appearing white matter (post mortem histopathologically (r = 0.55; P = 0.001), and in vivo with MRI (r = -0.6874; P = 0.0006)) and does so independently of white matter lesion burden. Axon loss in the normal appearing white matter explains disability in multiple sclerosis better than focal white matter lesions do. Smouldering associated worsening typically manifests as a length-dependent central axonopathy. We propose a unifying model for multiple sclerosis pathogenesis, wherein accumulation of cortical lesion burden predisposes associated normal appearing white matter to diffuse injury, whilst also intensifying damage within white matter lesions. Our novel two-hit hypothesis implicates cortical disease as a culprit for smouldering multiple sclerosis, abetted by active focal inflammation in the white matter (and vice versa). Substantiation of the two-hit hypothesis would advance the importance of specific therapeutic intervention for (and monitoring of) cortical/meningeal inflammation in people with multiple sclerosis.

Cortical lesion load correlates with diffuse injury of multiple sclerosis normal appearing white matter

BACKGROUND

Degeneration of central nervous system normal appearing white matter (NAWM) underlies disability and progression in multiple sclerosis (MS). Axon loss typifies NAWM degeneration.

OBJECTIVE

The objective of this paper is to assess correlation between cortical lesion load and magnetisation transfer ratio (MTR) of the NAWM in MS. This was in order to test the hypothesis that cortical lesions cause NAWM degeneration.

METHODS

Nineteen patients with MS underwent 7 Tesla magnetisation-prepared-rapid-acquisition-gradient-echo (MPRAGE), and magnetisation transfer ratio (MTR) brain magnetic resonance imaging (MRI). Cortical lesions were identified using MPRAGE and MTR images of cortical ribbons. White matter lesions (WMLs) were segmented using MPRAGE images. WML maps were subtracted from white matter volumes to produce NAWM masks. Pearson correlation was calculated for NAWM MTR vs cortical lesion load, and WML volumes.

RESULTS

Cortical lesion volumes and counts all had significant correlation with NAWM mean MTR. The strongest correlation was with cortical lesion volumes obtained using MTR images (r = -0.6874, p = 0.0006). WML volume had no significant correlation with NAWM mean MTR (r = -0.08706, p = 0.3615).

CONCLUSION

Our findings are consistent with the hypothesis that cortical lesions cause NAWM degeneration. This implicates cortical lesions in the pathogenesis of NAWM axon loss, which underpins long-term disability and progression in MS.

Increased X-linked inhibitor of apoptosis protein (XIAP) expression exacerbates experimental autoimmune encephalomyelitis (EAE)

Dysregulated apoptotic signaling has been implicated in most forms of cancer and many autoimmune diseases, such as multiple sclerosis (MS). We have previously shown that the anti-apoptotic protein X-linked inhibitor of apoptosis (XIAP) is elevated in T cells from mice with experimental autoimmune encephalomyelitis (EAE). In MS and EAE, the failure of autoimmune cells to undergo apoptosis is thought to exacerbate clinical symptoms and contribute to disease progression and CNS tissue damage. Antisense-mediated knockdown of XIAP, in vivo, increases the susceptibility of effector T cells to apoptosis, thus attenuating CNS inflammation and thereby alleviating the clinical signs of EAE. We report for the first time, generation of transgenic mice whereby the ubiquitin promoter drives expression of XIAP (ubXIAP), resulting in increased XIAP expression in a variety of tissues, including cells comprising the immune system. Transgenic ubXIAP mice and wild-type (WT) littermates were immunized with myelin oligodendrocyte glycoprotein (MOG35-55) in complete Freund's adjuvant and monitored daily for clinical symptoms of EAE over a 21-day period. The severity of EAE was increased in ubXIAP mice relative to WT-littermates, suggesting that XIAP overexpression enhanced the resistance of T cells to apoptosis. Consistent with this finding, T cells derived from MOG35-55-immunized ubXIAP mice and cultured in the presence of antigen were more resistant to etoposide-mediated apoptosis compared to WT-littermates. This work identifies XIAP is an important apoptotic regulator in EAE and a potential pharmacological target for treating autoimmune diseases such as MS.

Failure to develop multiple sclerosis in patients with neurologic symptoms without objective evidence

BACKGROUND

Many patients referred to multiple sclerosis (MS) centers with symptoms suggestive of MS are found to have normal neurologic examinations, normal or non-specific brain magnetic resonance imaging (MRI) scan findings, and normal cerebrospinal fluid (CSF). Persistent symptoms often lead to multiple consultations and repeated diagnostic investigations. We performed a study to evaluate the diagnostic utility of repeated evaluations in patients with normal initial assessments and persistent neurologic symptoms.

METHODS

143 patients were evaluated initially and 109 returned for a second evaluation after a mean interval of 4.4 years.

RESULTS

All 143 patients had normal initial examinations, brain MRI scans, screening blood tests, and CSF studies. Spinal cord imaging was normal in all patients tested (cervical cord, n = 126; 88.1%; thoracic cord, n = 58; 40.6%). Evoked potential studies were abnormal in a small percentage of patients: visual evoked potentials, VEP (8.1%), somatosensory evoked potentials, SSEP (4.9%), and brainstem auditory evoked potentials, BAEP (2.8%). All follow-up patients (n = 109) had normal examinations and MRI scans. Repeat CSF studies (n = 35; 32.1%) and spinal cord imaging (cervical cord n = 57; 52.3%; thoracic cord n = 32; 29.4%) were normal in all follow-up patients tested. No patients at initial presentation or at follow-up fulfilled diagnostic criteria for MS.

CONCLUSIONS

PATIENTS

and clinicians may be reassured that persistent neurologic symptoms in the absence of objective clinical evidence do not lead to the development of MS. Costly serial investigations should be carefully considered, particularly in the presence of normal neurologic examination at follow-up.

[Myelin and nuclear magnetic resonance]

MRI is one of the most important tools for the investigation of white matter diseases of the central nervous system. Other techniques based on the magnetic resonance phenomena (magnetization transfer imaging, diffusion imaging, magnetic resonance spectroscopy) have joined MRI to better caracterize certain diseases, understand their pathophysiology and follow their evolution.

[Future of non conventional MR techniques in MS]

Although conventional magnetic resonance imaging (MRI) is used for diagnosing multiple sclerosis (MS) and monitoring disease activity and course, the correlation between conventional MRI data and clinical findings remains weak. This "clinical-MRI paradox" could be partly due to the lack of MRI specificity related to the heterogeneous pathological substrates of MS and to its inability to quantify the extent of damage in the normal-appearing tissue. Recently, non-conventional MRI techniques, including magnetization transfer MRI, diffusion tensor MRI, and proton MR spectroscopy have been applied to improve our understanding of the pathophysiology of MS. These techniques may provide information about structural and biochemical changes occurring within and outside macroscopic MS lesions (inflammation, demyelination, axonal loss), in particular in the normal-appearing white and grey matter. These techniques could also significantly improve our ability to monitor inflammatory demyelination and axonal injury. In the same way, functional MRI gives us the potential substrate to assess the mechanisms of adaptive cortical reorganization, which may limit the irreversible consequences of MS tissue injury.