HLA Allele, Four Measures of MS Severity

The HLA-DRB1*1501–DQB1*0602 haplotype is the best-established susceptibility allele in multiple sclerosis (MS). More than half of people with MS in Northern European populations carry it, fine mapping points to the DRB1 gene itself as responsible for a significant portion of that risk, and the effect is dose-dependent, with homozygotes at several-fold higher risk than heterozygotes. What remained open is whether the same variation shapes what happens after diagnosis. Okuda and colleagues took that question to 505 clinically well-characterized patients at the University of California, San Francisco, measuring four things at a single visit: conventional anatomic MRI, high-resolution volumetrics, multi-voxel proton MR spectroscopy of normal-appearing tissue, and standardized cognitive and motor testing. The study grew out of their earlier work in the Optic Neuritis Treatment Trial, where carriers of the allele showed higher numbers of focal brain lesions at first presentation with optic neuritis.

What Was Measured, and Who Was Blinded to What
Of 511 patients enrolled, six were excluded for motion artifact or a missing dual-echo sequence, leaving 505: 88 clinically isolated syndrome, 352 relapsing remitting, 46 secondary progressive, 14 primary progressive, three progressive relapsing and two of uncertain subtype. Mean age was 42.1 years, 68% were women, mean age at disease onset was 33.5 years, median disease duration 7.0 years, median EDSS 1.5, and 69% had treatment exposure. Genotyping split them into 232 carriers (46%) and 273 non-carriers (54%), using an allele-specific TaqMan probe-based 5′ nuclease assay with a β-globin internal positive control in every well. All imaging came from one 3 T scanner, with brain MR performed within two weeks of entry, and spectroscopy acquired over a single 1.5 cm slice just above the corpus callosum at an in-plane resolution of 1.2 × 1.0 cm, keeping only metabolite estimates within 5% Cramer-Rao error. Three formally trained coordinators administered the functional composite, blinded to MS subtype, genotype and radiological outcomes.

Two Differences Visible Before Any Scan
Carriers were 74% women against 63% among non-carriers (P = 0.009), and their mean age at disease onset was younger, 32.4 years against 34.3 (P = 0.025). On everything else the groups matched: age at assessment (P = 0.116), disease duration (P = 0.170), treatment exposure (P = 0.462), EDSS (P = 0.201) and total functional composite score (P = 0.434).

Spectroscopy Reaches Tissue Conventional MRI Calls Normal
Spectroscopic data were acquired for 409 patients, 81% of the cohort, split 185 carriers to 224 non-carriers. The concentration of the metabolite NAA in normal-appearing white matter was lower in carriers, 9.80 ± 1.30 mM against 10.10 ± 1.30 (P = 0.025), while normal-appearing gray matter showed no such difference (P = 0.221). Linear regression incorporating gender, age of disease onset, T2-lesion volume and disease duration kept that reduction significant. NAA is treated as a marker of neural integrity because of its high concentration in neurons, dendrites, axons and synapses in the central nervous system, and the authors point out that earlier studies of normal-appearing white matter used single-voxel spectroscopy with lower spatial resolution than the two-dimensional multi-voxel method applied here, which had kept firm associations out of reach.

Lesion Volume, Brain Volume, and Where the Effect Fades
On conventional imaging, analysis of covariance on Box–Cox transformed T2-lesion volumes gave carriers a 1,127 mm³ greater adjusted lesion volume than all other DRB1 alleles (P = 0.031). Sorting untransformed volumes into quartiles sharpened where that effect lives: carriers with clinically isolated syndrome, relapsing remitting or secondary progressive disease were 1.90 times more likely to fall into the second quartile rather than the first (P = 0.015) and 1.69 times more likely for the third (P = 0.049), with no significant difference for the fourth (P = 0.314), and adding EDSS and treatment exposure as covariates left both the odds ratios and their significance largely unchanged. The authors read the fourth-quartile null as a ceiling rather than an absence: as T2-lesion volume rises there is less remaining normal white matter at risk for demyelination, which could slow the accumulation of new focal lesions and blunt any difference between groups. Normalized brain parenchymal volume followed the same direction, with regression correcting for age of disease onset, gender, disease duration, EDSS and treatment exposure giving carriers a least-squares mean reduction of 14 cm³ (P = 0.023), or 0.9% of total brain volume, with no difference in the normalized scaling factors.

One Cognitive Measure Separates the Groups
Of the three components of the functional composite, only PASAT-3, which measures processing speed and working memory, distinguished the haplotype groups, at P = 0.004 by Wilcoxon two-sample test, with median Z-scores of −0.128 in carriers against 0.266 in non-carriers. The result held after a sensitivity analysis for potential confounders. The 9-Hole Peg Test (P = 0.738) and the 25-Foot Timed Walk (P = 0.119) showed nothing, nor did the composite as a whole. Age, disease duration, EDSS, self-reported quality of life scores, exposure to immunotherapy, the type of therapy and the presence of gadolinium-enhancing lesions were all similar between groups. The authors were deliberate about one alternative explanation in particular, checking fatigue and mood through the self-reported instrument and finding no statistical differences, which they take as evidence that these were not contributing factors to the cognitive result.

The Cascade the Authors Propose, and the Immunology Behind It
Their model runs in one direction, and each step in it is one of the four measures. The allele drives development of more T2-foci; those foci have the potential of transecting axons, producing widespread axonal compromise registered as reduced NAA in normal-appearing white matter; that leads to subsequent degeneration with resultant brain atrophy; and the associated injury shows up as the decline in cognitive performance. The immunology they offer for the first step is specific. The gene products of DRB1*1501 in white populations, and of the closely related DRB1*1503 in blacks, share a structurally related hydrophobic peptide-binding motif allowing tight binding to aromatic amino acids, including the critical aa92 phenylalanine residue of the 89–96 region of myelin basic protein, a sequence known to be immunodominant in these individuals and efficiently presented to T cells. Their proposal is that high numbers of MBP-reactive T cells stochastically favour the development of more focal lesions early in the disease course, and a greater concentration of encephalitogenic T cells within lesions favouring secondary axonal degeneration, with continued epitope spreading and pathogenic T cell responses not restricted to DRB1*1501 modulating the effect over time. On treatment as a possible confounder, the proportions receiving interferon, glatiramer or immune suppression were similar across groups, as was total duration of treatment, and they note that an earlier unconfirmed study suggesting an interaction between the allele and glatiramer response would, if real, have biased their results in the opposite direction. Longitudinal studies combining imaging, genetic and clinical variables with other biomarkers were underway to test and extend these cross-sectional findings, and the authors state that similar results from independent datasets would provide additional confirmation.

Disclaimer: This blog post is based on the cited study and is intended for informational purposes only. It is not intended to provide medical advice. Please consult with a healthcare professional for any health concerns.

Reference:
Okuda, D. T., Srinivasan, R., Oksenberg, J. R., Goodin, D. S., Baranzini, S. E., Beheshtian, A., Waubant, E., Zamvil, S. S., Leppert, D., Qualley, P., Lincoln, R., Gomez, R., Caillier, S., George, M., Wang, J., Nelson, S. J., Cree, B. A. C., Hauser, S. L., & Pelletier, D. (2009). Genotype–phenotype correlations in multiple sclerosis: HLA genes influence disease severity inferred by ¹HMR spectroscopy and MRI measures. Brain, 132(1), 250–259. https://doi.org/10.1093/brain/awn301