Neurophysiology in Huntington's disease: an update

Auditory mismatch negativity in pre-manifest and manifest Huntington's disease

AIM

The aim of this study was to investigate the characteristics of the electrophysiological brain response elicited in a passive acoustic oddball paradigm, i.e. mismatch negativity (MMN), in patients with Huntington's disease (HD) in the premanifest (pHD) and manifest (mHD) phases. In this regard, we correlated the results of event-related potentials (ERP) with disease characteristics.

METHODS

This was an observational cross-sectional MMN study. In addition to the MMN recording of the passive oddball task, all subjects with first-degree inheritance for HD underwent genetic testing for mutant HTT, the Huntington's Disease Rating Scale, the Total Functional Capacity Scale, the Problem Behaviors Assessment short form, and the Mini-Mental State Examination.

RESULTS

We found that global field power (GFP) was reduced in the MMN time window in mHD patients compared to pHD and normal controls (NC). In the pHD group, MMN amplitude was only slightly and not significantly increased compared to mHD, while pHD patients showed increased theta coherence between trials compared to mHD. In the entire sample of HD gene carriers, the main MMN traits were not correlated with motor performance, cognitive impairment and functional disability.

CONCLUSION

These results suggest an initial and subtle deterioration of pre-attentive mechanisms in the presymptomatic phase of HD, with an increasing phase shift in the MMN time frame. This result could indicate initial functional changes with a possible compensatory effect.

SIGNIFICANCE

An initial and slight decrease in MMN associated with increased phase coherence in the corresponding EEG frequencies could indicate an early functional involvement of pre-attentive resources that could precede the clinical expression of HD.

Cortical inhibitory deficits in Huntington's disease are not influenced by gender

Huntington's disease (HD) affects GABA-mediated inhibitory circuitry in the cortex. As there is evidence that sex hormones affect GABAergic function, we investigated whether gender modulates GABA-related pathophysiological changes in HD. Fifteen premanifest HD, 11 symptomatic HD and 16 healthy control participants were assessed with paired-pulse transcranial magnetic stimulation applied to the primary motor cortex. Cortical inhibition was significantly reduced in symptomatic HD, compared with premanifest HD and controls. There was reduced inhibition in females overall, but no Group-by-Sex interaction. These findings suggest that sex hormones do not exert a direct influence on the mechanisms underpinning cortical inhibitory deficits in HD.

A GABBR2 gene variant modifies pathophysiology in Huntington's disease

Striatal degeneration in Huntington's disease (HD) causes changes in cortico-subcortical pathways. Transcranial magnetic stimulation (TMS) is a valuable tool for assessing pathophysiology within these pathways, yet has had limited application in HD. As cortico-subcortical pathways are largely mediated by GABA and dopamine receptor genes, understanding how these genes modulate neurophysiology in HD may provide new insights into how underlying pathology maps onto clinical phenotype. Twenty-nine participants with HD underwent motor cortex stimulation, while corticospinal excitability, cortical inhibition and intracortical facilitation were indexed via peripheral electromyography. Single-nucleotide polymorphism mapping was performed across six genes that are known to modulate cortico-subcortical pathways (GABRA2, GABBR1, GABBR2, DRD1, DRD2, DRD4). Genetic associations with six TMS measures and age at onset were investigated. Our hierarchical multiple regression analysis, controlling for CAG and age, revealed that a GABBR2 variant, predicted to be disease-causative, was significantly associated with corticospinal excitability at corrected levels. A subsequent uncorrected exploratory analysis revealed associations between GABBR2, GABRA2 and DRD2 variants with TMS measures of corticospinal excitability and cortical inhibition in HD, as well as with age at onset. Our findings support the notion that uncovering genetic associations with pathophysiological measures and age at onset is an important way forward in terms of generating meaningful biomarkers with diagnostic and prognostic sensitivity, and identifying novel human-validated targets for future clinical trials.