Here we reported clinical and neuropathologic features in a series of patients with genetically confirmed symptomatic HD (three cases) or possessing the HD mutation (one case) who also manifested clinical and/or pathologic features of ALS. Neuropathological and biochemical evaluation of the spinal cord and brain from two patients confirmed the coexistence of pathological features of HD and ALS. Our findings suggest that the HD pathogenic process can involve motor neurons while also indicating that, in these two HD cases, TDP-43-associated processes contribute to motor neuron degeneration, as in typical ALS.
Pathologic findings of HD and ALS coexisted in both examined cases. Both showed obvious upper and lower motor neuron loss with Bunina bodies and ubiquitin-ir skein-like inclusions in remaining lower motor neurons, which are characteristic of ALS [15
], as well as neuronal loss and gliosis in the neostriatum together with polyglutamine inclusion-containing neurons, which are hallmarks of HD [9
]. A single reported autopsy case with HD and familial ALS likewise had these characteristics of ALS and HD, but also showed degeneration of posterior columns and dorsal spinocerebellar tracts, which was not present in our cases [31
As shown in , TDP-43-ir NCIs were found in many regions beyond the affected lower motor nuclei and motor cortex. This pattern represents a broader distribution than seen in classical sporadic ALS, more reminiscent of a “type 2 ALS” pattern of TDP-43 pathology [22
]. The pattern, however, also displayed features atypical of type 2 ALS. For example, TDP-43-ir NCIs were absent from the dentate gyrus in case 1 and Ammon’s horn in both cases. In these regions, TDP-43-ir NCIs were seen in over 85 % of patients with type 2 ALS [22
]. In case 2, only a small number of TDP-43-ir NCIs were seen in the amygdaloid nucleus, where numerous NCIs appear in typical type 2 ALS [7
]. In both cases, 1C2-ir neurons were also few in these regions. Unlike type 2 ALS [22
] our cases lacked prominent TDP-43 pathology in temporal cortex. Instead, TDP-43-ir NCIs were abundant in the parietal cortex in case 1 and frontal cortex in case 2, where more frequent 1C2-ir neurons were also observed than in the temporal cortex. Thus, the distribution of TDP-43 in our HD/ALS cases resembles a classical ALS distribution (type 1 [22
]) with a superimposed, atypical cortical distribution that may somewhat mirror the distribution of 1C2-ir pathology. However, the frequencies of the two different inclusions were not always consistent: abundant 1C2-ir neurons but rare or a few TDP-43-ir NCIs in some regions and vise-versa. Therefore, we could not draw a firm conclusion from the present data that the TDP-43 pathology would be directly affected by mutant huntingtin.
In neurons of the motor cortex and lower motor neurons, we also detected expanded polyglutamine inclusions and aggregation foci which suggest ongoing huntingtin aggregation [10
]. Although 1C2-ir NIIs in anterior horn cells have been described in HD [11
] we confirmed their presence and, using the polyglutamine recruitment technique, found evidence for active polyglutamine aggregation in motor neurons. Thus, we conclude that TDP-43 dependent and huntingtin-dependent pathogenic processes likely occur simultaneously in motor neurons in such cases.
We found neurons coexisting TDP-43-ir NCIs and 1C2/EM48-ir NIIs or NCIs, but they localized separately in all individual neurons. Thus, we failed to observe co-localization of mutant huntingtin protein and TDP-43 in NCIs, NIIs or neuropil inclusions in two cases in this study. We also immunohistochemically examined the frontal cortex and neostriatum of other 11 patients with HD who did not show clinicopathological features associated with ALS (10 male, 1 female, age; 57.82 [27–82] years, CAG repeat; [43–47] in available 7 patients) and the spinal cord of 7 of the patients with an anti-pTDP-43 antibody. We observed only a small number of TDP-43-ir neuropil inclusions similar to mutant huntingtin neuropil inclusions in the frontal deeper cortical layers of 8 patients and much fewer in the neostriatum but no inclusion in the spinal cord. In contrast, others reported frequent colocalization of these two proteins in neuropil inclusions and dystrophic neurites in the cortex and neostriatum of HD cases [34
]. This discrepancy may reflect differences in the cases examined or in methods of tissue preparation.
Motor neuron degeneration is recognized in other polyglutamine diseases: for example, spinobulbar muscular atrophy is a form of motor neuron degeneration [36
], and three patients with spinocerebellar ataxia type 2 (SCA2) and one with SCA6 have been described with concomitant ALS [5
]. Once TDP-43 was discovered to be a key component in ALS, TDP-43-ir NCIs were detected in neurons from various areas of SCA2 disease brain [4
] and in lower motor neurons and axons in patients with Machado-Joseph disease (also known as SCA3) [35
]. Interestingly, in the reported cases, co-localization of TDP-43 and mutant polyglutamine proteins has never been observed, and coexistence of these proteins in the same neurons is not frequent [38
]. Thus, the features appear to be similar to those of our two cases. In addition, expanded CAG repeats in the SCA2 disease gene are associated with increased risk for ALS [4
]. Although the pathophysiological mechanism remains unclear, intermediate expansion in the SCA2 disease protein ATXN2 has been suggested to perturb neuronal proteostasis thereby favoring TDP-43 mislocalization [4
]. Since two studies examining CAG length in the HD disease gene in sporadic ALS and controls have concluded that, unlike the SCA2 gene, the HD disease gene is not a risk factor for ALS [17
], we can not exclude the possibility that our patients suffer from both HD and ALS by chance alone. It is certain that abundant TDP-43-ir NCIs in our two cases is not a common feature of HD cases. Yet in a rare subset of HD patients, mutant huntingtin may predispose motor neurons to TDP-43 associated pathomechanisms and modify TDP-43 pathologic features.
Five patients with genetically confirmed HD who developed ALS-like features have previously been reported [14
]. A review of the clinical features in these patients and our patients () suggests several common features. First, although the sequence and interval between the onset of HD and ALS varied, the CAG repeat expansions in the HD gene were relatively short in all patients, typically resulting in mid- to later-life symptom onset. Consistent with this, HD onset occurred after at least the mid-thirties in all cases, and one patient had not yet developed overt symptoms of HD when ALS symptoms began at age 56. Second, the onset of ALS occurred after the mid-fifties in all but one patient. These observations suggest that age-dependent changes may promote the deleterious effect of mutant huntingtin on motor neurons. Similarly, in Machado-Joseph disease, affected individuals who are older and have the smallest disease-causing expansions are most prone to develop motor neuron degeneration [26
Case reports of genetically confirmed HD and ALS
In summary, we suggest the possibility that a rare subset of older HD patients is prone to develop features of ALS with an atypical TDP-43 distribution that resemble to that of aggregated mutant huntingtin. Age-dependent neuronal dysfunction induced by mutant polyglutamine protein expression rarely may contribute to TDP-43 associated pathomechanisms, and a small proportion of the patients would develop phenotypic appearance of motor neuron diseases in their later-life.