Early energy deficit in Huntington disease: identification of a plasma biomarker traceable during disease progression

Abstract

Huntington disease (HD) is a fatal neurodegenerative disorder, with no effective treatment. The pathogenic mechanisms underlying HD has not been elucidated, but weight loss, associated with chorea and cognitive decline, is a characteristic feature of the disease that is accessible to investigation. We, therefore, performed a multiparametric study exploring body weight and the mechanisms of its loss in 32 presymptomatic carriers and HD patients in the early stages of the disease, compared to 21 controls. We combined this study with a multivariate statistical analysis of plasma components quantified by proton nuclear magnetic resonance ((1)H NMR) spectroscopy. We report evidence of an early hypermetabolic state in HD. Weight loss was observed in the HD group even in presymptomatic carriers, although their caloric intake was higher than that of controls. Inflammatory processes and primary hormonal dysfunction were excluded. (1)H NMR spectroscopy on plasma did, however, distinguish HD patients at different stages of the disease and presymptomatic carriers from controls. This distinction was attributable to low levels of the branched chain amino acids (BCAA), valine, leucine and isoleucine. BCAA levels were correlated with weight loss and, importantly, with disease progression and abnormal triplet repeat expansion size in the HD1 gene. Levels of IGF1, which is regulated by BCAA, were also significantly lower in the HD group. Therefore, early weight loss in HD is associated with a systemic metabolic defect, and BCAA levels may be used as a biomarker, indicative of disease onset and early progression. The decreased plasma levels of BCAA may correspond to a critical need for Krebs cycle energy substrates in the brain that increased metabolism in the periphery is trying to provide.

Figure 1. Partial least square (PLS) analyses of NMR spectra of plasma samples from HD patients with no or little signs of the disease and controls.

Three groups of presymptomatic, early and mildly affected HD patients were constituted on the basis of their UHDRS scores, as described in the methods. The first and second components in the X space (NMR spectrum) are denoted PC and PC respectively. PLS score plots (PC/PC[2]) of pair-wise compared groups show the greater variation within the NMR spectrum according to a priori classification with UHDRS. There is a clear separation between presymptomatic and early HD patients (a), as well as between early and mildly affected HD patients (b). Despite some overlap, presymptomatic mutation carriers can also be distinguished from controls (c).

Figure 2. Differences in the relative concentrations of branched chain amino acids in plasma are responsible for the differences among the HD groups.

PLS-contribution plots allow comparison of plasma metabolic profiles in early affected HD patients and presymptomatic carriers. NMR variables that have the greatest weight (w*₁; scaled in units of standard deviation), therefore contributing most to the separation between HD groups, are decreased concentrations (>2SD) of metabolites located between 0.85 and 1.0 ppm: valine, leucine and isoleucine. A similar contribution plot was obtained when plasma metabolic profiles from mildly affected HD patients were compared to early HD patients (data not shown).

Figure 3. The levels of branched chain amino acids are significantly different in HD patients and controls.

The concentrations of valine, leucine and isoleucine in plasma were determined by ion exchange chromatography. Comparisons of means (ANOVA) were made between men or women with HD and their respective controls. In men, there was a significant decrease of valine, leucine and isoleucine in the HD group. In women, similar results are observed for leucine and isoleucine.