Preliminary Study of the Association of White-Matter Metabolite Concentrations With Disease Severity in Patients With Huntington’s Disease

The protocol and all study procedures were approved by the University of Iowa Institutional Review Board. All participants provided written informed consent and received financial compensation. Participants had either diagnosed HD (i.e., positive family history in the presence of characteristic motor abnormalities diagnosed as HD by an independent neurologist) or a positive genetic test for the HD gene expansion, with at least some motor abnormalities as defined by the Unified Huntington’s Disease Rating Scale (UHDRS) motor exam (i.e., Diagnostic Confidence Level >1). The subjects were participating in a multisite, prospective, randomized, clinical trial of citalopram or placebo, examining symptomatic treatment for early executive dysfunction in HD. All participants at the University of Iowa site were offered participation in an add-on imaging study, and a subset of six nonclinically depressed subjects with HD who had complaints of reduced cognition completed spectroscopy scans at their baseline visit before receiving drug. Patients were screened for depression with a clinician-administered depression rating scale (Hamilton Rating Scale for Depression [Ham-D]) and interviewed by the study investigator. Patients with active depression were excluded. All participants had a positive test for the HD gene expansion (CAG >39). A battery of measures of functional and motor symptoms (Unified HD Rating Scale [UHDRS]), Total Functional Capacity (TFC), and Total Motor Score [TMS]), as well as neuropsychological tests (Symbol Digit Modalities Test [SDMT], Psychomotor Speed and the Trail-Making Test Part B [TMTB], Executive Functioning) were administered on the same day as the MR scans.

MRI and multivoxel 1H-MRS experiments were performed on a Siemens 3T scanner. T1-MPRAGE images (TR/TI/TE: 2530/800/3.18 msec) were obtained, with a voxel size of 0.8 mm³. T2 turbo spin echo images were obtained for accurate placement of the 1H-MRS grid. 1H-MRS was performed with a phase-encoded version of a point-resolved spectroscopy sequence (PRESS) with and without water suppression (TR/TE: 1500/40 msec, slice thickness: 15 mm; FOV: 220×220 mm; total scan time: 582 sec). The 1H-MRS volume of interest was selected in cortical GM and WM regions superior to the lateral ventricles and parallel to the AC–PC axis.

The 1H-MRS data were processed with the software package LCModel. The T1-MPRAGE images were the basis for tissue classification of GM, WM, and CSF in the 1H-MRS volume of interest. Tissue water was used as a concentration reference and for partial volume correction, as previously described.¹³ The 1H-MRS signals examined included several metabolites whose spectrums overlap and therefore are quantified together. The NAA signal represents N-acetylaspartate and N-acetylaspartylglutamate signals, dominated by N-acetylaspartate. The Cr signal combines creatine and phosphocreatine signals. The Cho signal consists of contributions from glycerophosphocholine, phosphocholine, and, typically, a small amount of free choline. Also analyzed in this experiment was the Ins signal. Statistical analyses were performed with SPSS (http://www.spss.com/).

All participants had mild-to-moderate HD (TFC range: 7–13; TMS mean: 17.2 [20.0]) and evidence of cognitive decline relative to their estimated premorbid IQ, particularly on psychomotor speed (Symbol Digit Modalities Test mean: 1.2 standard deviations (SD) below normative controls). In an exploratory bivariate correlation analysis uncorrected for multiple comparisons, the CAG-repeat length correlated significantly only with WM NAA concentrations (Pearson r = −0.822; p=0.045), and several clinical measures of disease severity consistently demonstrated strong and significant correlations with WM NAA, Cho, and/or Ins (Table 1). In follow-up multivariate regression analyses with patient age as a covariate, the correlations between WM metabolites and disease measures remained strong and significant, except for the correlation of NAA with CAG (p=0.068).

The directions of the correlations of metabolites NAA, Cho, and Ins in WM with subject SDMT score were in the expected directions (i.e., NAA correlated positively with motor and cognitive tests for which higher scores reflected better performance [TCF and standardized SDMT scores]) and negatively with tests for which lower scores reflected better performance (TMS). Since higher Cho and Ins have been shown to reflect pathology, the correlations involving these metabolites were expected to be in the opposite direction. However, none of the measured GM metabolite concentrations correlated with either CAG-repeat length or other disease measures.

The findings of abnormal metabolite concentrations in cortical WM support the previous research that HD disease pathology extends beyond the striatum. DTI has been used to investigate WM microstructure in HD pathology.^7,8,14 Rosas et al. compared preclinical HD subjects with normal-controls and found decreased fractional anisotropy (FA) within the corpus callosum and subcortical structures.⁸ Magnotta et al. extended that research and found a relationship between DTI scalar measures in WM and the proximity to disease diagnosis.⁷ Kloppel et al. used fiber tracking and FA measures to conclude that WM changes may be distributed throughout the brain, and they found a correlation between WM changes, atrophy, microstructural, and cognitive measures.¹⁴ Similarly, there is accumulating evidence of cortical–striatal pathology at the cellular level. Gu and colleagues¹⁵ proposed a pathogenic model of HD stemming from dysfunctional cortical and striatal circuitry. Specifically, they showed that expression of the mutant huntingtin protein in cortical interneurons was critical for cortical pathogenesis and that this inter-neuron dysfunction occurs at an early stage, which, in turn, leads to neuropathogenesis in the cortex and striatum.¹⁵ The findings in our study point to a WM metabolic perturbation in patients with HD related to disease severity and consistent with evidence of WM pathology detected by past morphologic, DTI, and cellular studies.

Our preliminary findings show evidence of cortical WM pathology by use of an imaging modality, multivoxel 1H-MRS, not previously reported, and findings converge with results from other imaging techniques pointing to cortical WM abnormalities in HD. Although no significant correlations with cortical GM metabolite concentrations were observed, these results do not preclude the possibility of altered GM metabolism in the sample relative to subjects without HD. The preliminary findings are limited because of the small sample, the exploratory bivariate correlation uncorrected for multiple comparisons, and lack of control group. A larger, longitudinal study is needed to confirm the role of metabolite concentrations in WM and GM as biomarker for HD disease severity and progression.

This work was previously presented at the 16^th Annual Meeting of the Organization for Human Brain Mapping, Barcelona Spain, 2010.

This research was supported by grants from NIH/NINDS (NS055733-01) and CHDI Foundation (A-2063).