The BDNF Allele That Tracks With More Gray Matter in MS

Alongside demyelination and plaque formation, multiple sclerosis (MS) carries a neurodegenerative component, with progressive neuronal loss following the initial inflammatory process. Brain-derived neurotrophic factor sits close to that biology. It promotes neuronal growth and repair, is abundantly expressed in the adult brain, is also produced by immune cells, and both the protein and its TrkB receptor have been found in active MS lesions. The BDNF variant rs6265 is a G-to-A substitution that swaps valine for methionine at codon 66 of the pro-protein, and it interferes with trafficking within the cell and with activity-dependent secretion, so that the heterodimers formed when both isoforms are present are secretion-impaired relative to the valine homodimers. In healthy subjects the Met66 allele has been tied to reduced hippocampal gray matter volume and poorer episodic memory. Zivadinov and colleagues asked what the same allele does in MS, genotyping and imaging 209 treated patients and adding neuropsychological testing in 108 of them.

The Cohort, and How the Genotypes Were Called
The 209 consecutive patients were 161 women and 48 men, mean age 45.2 ± 9.1 years, median EDSS 2.5, with disease duration of 13.4 ± 8.7 years from first symptoms: 167 relapsing remitting, 40 secondary progressive and two primary progressive. All were on disease-modifying therapy, for 5.3 ± 3.4 years on average, with 174 on interferon-β and 22 on glatiramer, and anyone who had a relapse or received corticosteroids in the preceding 30 days was excluded. Genotypes came out at 140 Val66Val, 62 Val66Met and seven Met66Met, giving allele frequencies of 81.8% for G and 18.2% for A, consistent with Hardy–Weinberg equilibrium and similar to published healthy control frequencies, which is the authors' basis for saying the polymorphism does not determine susceptibility to MS. Calling was done by allele discrimination with fluorescent probes, and then checked: nine samples, three each of G/G, G/A and A/A, were PCR-amplified and sequenced in both directions, agreement with the discrimination assay was 100%, and three sequenced reference samples were included in every run. Imaging ran on a 1.5 T scanner, with analysis blinded both to clinical characteristics and to genotype.

No Effect on Susceptibility, Onset or Disability
Age of MS onset was 31.6 ± 9.4 years in the Val66Val group and 32.0 ± 9.3 years in the Met66 group (P = 0.76). In regression analysis correcting for gender, presence or absence of progressive MS, age, disease duration and treatment duration (F = 19.5, P < 0.001, adjusted R² = 0.35), EDSS was not associated with the Met66 allele, at 3.1 ± 1.8 with it and 3.2 ± 1.9 without (P = 0.96). The authors note this is consistent with other reports, and the same covariate set carries through every subsequent model in the paper.

Gray Matter Volume Runs the Other Way
Normalized gray matter volume gave the clearest result, with the Met66 allele positively associated at t = 2.45, P = 0.015 and a standardized β of 0.15 (model F = 12.2, P < 0.001, adjusted R² = 0.24), meaning higher and better preserved gray matter in Met66 carriers. T2 lesion volume moved in the matching direction, negatively associated with the allele at t = −2.06, P = 0.041 and β = −0.14 (model F = 4.72, P < 0.001). Normalized brain volume showed a trend in the same direction, t = 1.82, P = 0.070, β = 0.11 (model F = 13.1, P < 0.001, adjusted R² = 0.26). The raw group means run the same way: gray matter volume of 766 ± 62 ml in Met allele carriers against 744 ± 67 ml in Val66Val, and T2 lesion volume of 10.1 ± 14.1 ml against 13.6 ± 15.3 ml. Associations with T1 lesion volume, normalized white matter volume and mean parenchyma diffusivity were not significant.

Magnetization Transfer Separates Volume From Microscopic Damage
Magnetization transfer ratio measurements were available for 145 patients, and the subsets with and without them were demographically similar. Mean MTR for normal-appearing gray matter, normal-appearing brain tissue and T2 lesions did not differ significantly between the genotype groups, which the authors read as showing that the rs6265 genotype does not alter the processes involved in microscopic damage to gray matter and brain parenchyma. The combined measure told a different story. The product of mean normal-appearing gray matter MTR and gray matter volume was positively associated with the Met66 allele (t = 2.03, P = 0.045, β = 0.18; model F = 4.68, P < 0.001), with the corresponding product for normal-appearing brain tissue and brain parenchymal volume showing a trend (t = 1.50, P = 0.14, β = 0.14). When gray matter volume was modelled with additional correction for both MTR measures (F = 12.0, P < 0.001), the Met66 allele kept a significant independent positive effect at t = 2.72, P = 0.008, β = 0.20.

One Cognitive Test Moves With the Volume
The 108 patients with neuropsychological testing matched the rest of the cohort on age, sex ratio, age of onset, treatment duration and EDSS, and on Met66 carriage (31.5% against 34.7%, P = 0.67), though their disease duration ran 2.2 years longer (14.6 ± 8.9 against 12.1 ± 8.4 years, P = 0.044). Scores were expressed as Z-scores against controls evaluated at the same centre. Of the full battery, the PASAT, which measures processing speed and divided attention in the auditory modality, was the one to separate the genotypes, favouring the Met66 group at −0.32 ± 1.2 against −0.72 ± 1.2 (t = 1.92, P = 0.057, β = 0.21), with no differences on any remaining variable. Partial correlations between the cognitive scores and normalized gray matter volume, corrected for age, were significant across most of the battery, led by the Symbol Digit Modalities Test at r = 0.47 and the PASAT at r = 0.38. The authors set the 0.4 Z-score difference against the 0.15 to 0.2 units the Polman group set as a threshold for clinical significance on this test, and point out that the PASAT relies heavily on bilateral dorsolateral prefrontal cortex connecting to posterior association cortices, a region where reduction of gray matter is associated with the Met66 allele in healthy subjects.

How the Authors Read a Result That Runs Opposite to Healthy-Subject Data
The authors call the finding unexpected, given reports that normal individuals carrying the Met66 allele have poorer episodic memory and reduced hippocampal engagement during memory tasks on functional MRI, and they propose that associations arising from interactions between a polymorphism and a disease state, particularly an inflammatory or autoimmune one, can differ from those seen in healthy individuals. They cite support for that reading from systemic lupus erythematosus, where 59 patients without CNS involvement showed better cognitive performance in psychomotor speed and motor domains with the Met66 allele, an effect maintained after controlling for prednisone dosage. Against their result stands Liguori and colleagues, who found the Met66 allele associated with lower gray matter volume in 50 Italian relapsing remitting patients (P = 0.005), and the authors give two possible reasons for the difference: the smaller sample there, and that the Liguori cohort was untreated while every patient here had been on disease-modifying therapy for several years. That distinction connects to emerging evidence that these therapies help preserve gray matter volume, with nerve growth factor and BDNF as candidates for mediating interferon-β benefits on gray matter atrophy and glatiramer possibly acting by increasing BDNF production. They also state the caution that applies to any volume readout, that increases in brain volume on MRI need not mean better function because of the confounding effects of edema and inflammation, while noting that their principal findings were gray matter changes that tracked with the cognitive measure. In MS lesions BDNF is expressed in infiltrating T-cells and macrophages as well as in neurons and activated astrocytes, TrkB is expressed on neurons around the edges of active lesions, and oligodendrocytes and their precursors express and upregulate the p75 neurotrophin receptor, with those two receptors mediating cell survival and apoptosis respectively. The role of BDNF in gray matter pathology in MS is not well characterized, and the paper closes on the Met66 allele genotype possibly providing a protective effect on the gray matter compartment in MS, as a topic requiring further research.

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:
Zivadinov, R., Weinstock-Guttman, B., Benedict, R., Tamaño-Blanco, M., Hussein, S., Abdelrahman, N., Durfee, J., & Ramanathan, M. (2007). Preservation of gray matter volume in multiple sclerosis patients with the Met allele of the rs6265 (Val66Met) SNP of brain-derived neurotrophic factor. Human Molecular Genetics, 16(22), 2659–2668. https://doi.org/10.1093/hmg/ddm189