Our stereological study of post-mortem HD brains reveals substantial neuronal loss in the putamen and mild neuronal loss in STN. The loss of neurons correlates significantly with motor impairment but not with chorea. This observation supports the hypothesis that motor impairment in HD is caused by neuronal loss, and is consistent with the hypothesis that chorea may be a manifestation of neuronal dysfunction rather than of neuronal loss in early stages of HD 3.
Recently, MRI studies have reported significant reduction of striatal volume in HD including premanifest and moderate HD brains 27, 28
. The PREDICT-HD and TRACK-HD longitudinal analyses of striatal atrophy demonstrate steady changes from premanifest gene carrier to severe HD29, 30
. Neuroimaging studies in patients with HD have also suggested an early atrophy in striatum and extrastriatal regions 31
. The correlation between motor manifestations and striatal neuronal degeneration in HD, demonstrated by our investigation, therefore provides a novel perspective that complements imaging studies relevant to the pathogenesis of motor dysfunction in HD.
In our study, the reduction in overall volume of the putamen appears strongly correlated with motor impairment (r= −0.708, p=0.004), and both the loss and atrophy of DARPP 32 neurons contribute to putaminal atrophy in HD (See results). The pattern of DARPP 32 neuronal loss paralleled the pattern of putaminal atrophy (). In addition to our quantitative study, qualitative examination of DARPP32 immunolabeling revealed a spectrum of morphologic abnormalities in the remaining putaminal neurons. These changes were characterized by perikaryal shrinkage and loss of stained dendritic processes in early stages of HD, whereas axonal-dendritic processes were almost completely destroyed in VS III and IV cases (). The remaining DARPP 32 neurons in later stages of the disease appeared isolated as they lost their connection with their neighboring nerve cells. Together, the massive loss of both DARPP 32 neurons and their neurites could result in disruption of local circuitry within the striatum and also in its efferent connections and thereby lead to motor dysfunction in HD patients. Our observations are consistent with the Golgi cytoarchitectonic study by Graveland et al
. Given the better visualization of dendrites and spine structures with Golgi, it could be helpful to combine Golgi impregnation with DARPP32 immunohistochemistry to study MSNs in HD.
The mechanisms underlying chorea are not fully understood, but both the striatum and STN have been implicated. It has been proposed that chorea might reflect a progressive period of neuronal dysfunction before cell death or an imbalance between different neuronal populations such as enkephalin vs substance P 15, 33–35
. Indeed, our study documents striking and massive degeneration of DARPP 32 neurons in the putamen, and the remaining atrophic DARPP32 neurons with variable cell sizes in the putamen of HD patients may represent neurons in different stages of neuronal degeneration. Interestingly, we observed a strong trend for correlation between larger volume of remaining DARRP32 positive neurons and a higher chorea score (r=0.527, p=0.053), consistent with the notion that chorea is perhaps the manifestation of neurons that are dysfunctional or in the early stages of degeneration.
Studies from both human and non-human primates indicate that chorea can be produced by destruction of neurons in STN 36, 37
. Depending on their size and location, lesions in STN can induce hypokinetic or hyperkinetic movement disorders (hemiballismus and choreoathetosis) 38, 39
. There are no previous stereological studies of the STN in HD, but Lange et al
reported a 25% loss of STN neurons using a non-stereological method 40
. Using stereology, we identified a mild (20%, p<0.05) but significant neuronal loss in STN in HDs that correlated significantly with motor impairment score but not with chorea (). However, four HD patients (1 VS II and 3 VS III) had significant loss of neurons in the putamen but not in the STN. This observation suggests that the pathological changes in STN trail the changes in putamen. Furthermore, three of these four patients did have marked motor impairment. Thus, striatal atrophy appears sufficient to cause motor impairment even in the absence of neuronal loss in the STN. Our results further indicate that the loss of STN neurons appears in relatively later stage of HD, compared to striatal MSNs, which are more vulnerable in HDs 3, 4
In our current study, we did not evaluate neuronal loss in the globus pallidus or degeneration of subtypes of striatal projection neurons, (enkephalin- and substance P-containing neurons), and striatal interneurons in HD 33–35
. Therefore, it may be interesting for future studies to identify the neurochemical nature of the remaining neurons in the putamen.
An important caveat in the interpretation of our results is the long average time interval between the last clinical evaluation and death. In a previous study of 100 postmortem cases, we found no correlation between Vonsattel grades of motor impairment score and interval between last neurological exam and death. We interpret this to indicate that clinical features of HD are fairly stable at end stage of disease, making clinical-pathological correlations feasible even given relatively infrequent exams in the period preceding death9
. Therefore, in the current study, even though there was a substantial interval between the last neurological examination and autopsy in some cases, we were still able to detect the significant correlation between DARPP 32 neuronal loss and MIS.
In this study in which the 14 HD cases exhibited a Vonsattel neuropathological severity ranging from grade II to grade IV (), we only observed a trend for inverse correlation between DARPP32 neuronal loss and Vonsattel grades (r= −0.464, p=0.091). HD patients who had longer CAG repeats had a tendency to exhibit more severe DARPP32 neuronal loss (r= −0.283, p=0.37); but there was no correlation between age of onset (motor symptoms onset) and DARPP32 neuronal loss (r=0.012, p=0.973).
In conclusion, our findings indicate that neuronal loss and atrophy of the putamen correlate significantly with motor impairment but not with chorea in HD. In advanced HD, there is also a modest loss of neurons in the STN, but this loss does not appear as necessary for the development of motor impairment. Our observations support the hypothesis that motor impairment in HD is caused by neuronal loss, and are consistent with the notion that chorea may be a manifestation of neuronal dysfunction rather than of neuronal loss in early stages of HD.