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Although the role of the striatum in language processing is still largely unclear, a number of recent proposals have outlined its specific contribution. Different studies report evidence converging to a picture where the striatum may be involved in those aspects of rule-application requiring non-automatized behaviour. This is the main characteristic of the earliest phases of language acquisition that require the online detection of distant dependencies and the creation of syntactic categories by means of rule learning. Learning of sequences and categorization processes in non-language domains has been known to require striatal recruitment. Thus, we hypothesized that the striatum should play a prominent role in the extraction of rules in learning a language. We studied 13 pre-symptomatic gene-carriers and 22 early stage patients of Huntington's disease (pre-HD), both characterized by a progressive degeneration of the striatum and 21 late stage patients Huntington's disease (18 stage II, two stage III and one stage IV) where cortical degeneration accompanies striatal degeneration. When presented with a simplified artificial language where words and rules could be extracted, early stage Huntington's disease patients (stage I) were impaired in the learning test, demonstrating a greater impairment in rule than word learning compared to the 20 age- and education-matched controls. Huntington's disease patients at later stages were impaired both on word and rule learning. While spared in their overall performance, gene-carriers having learned a set of abstract artificial language rules were then impaired in the transfer of those rules to similar artificial language structures. The correlation analyses among several neuropsychological tests assessing executive function showed that rule learning correlated with tests requiring working memory and attentional control, while word learning correlated with a test involving episodic memory. These learning impairments significantly correlated with the bicaudate ratio. The overall results support striatal involvement in rule extraction from speech and suggest that language acquisition requires several aspects of memory and executive functions for word and rule learning.

The role of sub-cortical structures in language processing, and more specifically of the striatum, remains controversial. In line with psycholinguistic models stating that language processing implies both the recovery of lexical information and the application of combinatorial rules, the striatum has been claimed to be involved either in the former component or in the latter. The present study reconciles these conflicting views by showing the striatum's involvement in both language processes, depending on distinct striatal sub-regions. Using PET scanning in a model of striatal disorders, namely Huntington's disease (HD), we correlated metabolic data of 31 early stage HD patients regarding different striatal sub-regions with behavioural scores on three rule/lexicon tasks drawn from word morphology, syntax and from a non-linguistic domain, namely arithmetic. Behavioural results reflected impairment on both processing aspects, while deficits predominated on rule application. Both correlated with the left striatum but involved distinct striatal sub-regions. We suggest that the left striatum encompasses linguistic and arithmetic circuits, which differ with respect to their anatomical and functional specification, comprising ventrally located regions dedicated to rule computations and more dorsal portions pertaining to lexical devices.

The role of the basal ganglia, and more specifically of the striatum, in language is still debated. Recent studies have proposed that linguistic abilities involve two distinct types of processes: the retrieving of stored information, implicating temporal lobe areas, and the application of combinatorial rules, implicating fronto-striatal circuits. Studies of patients with focal lesions and neurodegenerative diseases have suggested a role for the striatum in morphological rule application, but functional imaging studies found that the left caudate was involved in syntactic processing and not morphological processing. In the present study, we tested the view that the basal ganglia are involved in rule application and not in lexical retrieving in a model of striatal dysfunction, namely Huntington's disease at early stages. We assessed the rule-lexicon dichotomy in the linguistic domain with morphology (conjugation of non-verbs and verbs) and syntax (sentence comprehension) and in a non-linguistic domain with arithmetic operations (subtraction and multiplication). Thirty Huntington's disease patients (15 at stage I and 15 at stage II) and 20 controls matched for their age and cultural level were included in this study. Huntington's disease patients were also assessed using the Unified Huntington's Disease Rating Scale (UHDRS) and MRI. We found that early Huntington's disease patients were impaired in rule application in the linguistic and non-linguistic domains (morphology, syntax and subtraction), whereas they were broadly spared with lexical processing. The pattern of performance was similar in patients at stage I and stage II, except that stage II patients were more impaired in all tasks assessing rules and had in addition a very slight impairment in the lexical condition of conjugation. Finally, syntactic rule abilities correlated with all markers of the disease evolution including bicaudate ratio and performance in executive function, whereas there was no correlation with arithmetic and morphological abilities. Together, this suggests that the striatum is involved in rule processing more than in lexical processing and that it extends to linguistic and non-linguistic domains. These results are discussed in terms of domain-specific versus domain-general processes of rule application.

Fetal neural transplants have become clinically relevant over the past 15 years for two major neurodegenerative diseases, namely Parkinson's disease and Huntington's disease. It is therefore timely to consider how this neurosurgical procedure can integrate the therapeutic armamentarium, what can be expected of it, and what cannot. We use here the example of Huntington's disease to show what fetal neural transplants may uniquely offer for that disease. Up to very recent times, Huntington's disease has been one special example of those neurodegenerative diseases against which neurologists feel totally helpless. This has all changed today and, although results are essentially still to come, one can foresee the mobilization of very large scientific and medical forces against this disease, with definite steps forward in terms of physiopathology and a better view of the therapeutic challenges. While defining the role that fetal neural transplantation may play in meeting these challenges, we also try to show rationales and developments for all types of treatments attempted or suggested so far, as well as their limits and, when relevant, informative failures. The date of writing this review needs to be noted, because the rapid accumulation of data on molecular mechanisms of Huntington's disease pathogenesis and the increasing numbers of clinical trials do not allow much time for the ink of a review to dry.

Huntington's disease is a hereditary disease in which degeneration of neurons in the striatum leads to motor and cognitive deficits. Foetal striatal allografts reverse these deficits in phenotypic models of Huntington's disease developed in primates. A recent open-label pilot study has shown some clinical improvement or stabilization in three out of five Huntington's disease patients who received bilateral striatal grafts of foetal neurons. We show here that the clinical changes in these three patients were associated with a reduction of the striatal and cortical hypometabolism, demonstrating that grafts were able to restore the function of striato-cortical loops. Conversely, in the two patients not improved by the grafts, striatal and cortical hypometabolism progressed over the 2-year follow-up. Finally, detailed anatomical-functional analysis of the grafted striata, enabled by the 3D fusion of MRI and metabolic images, revealed considerable heterogeneity in the anatomic and metabolic profiles of grafted tissue, both within and between Huntington's disease patients. Our results demonstrate the usefulness of PET measurements of brain glucose metabolism in understanding the effects of foetal grafts in patients with Huntington's disease.

Huntington disease is caused by a dominantly transmitted CAG repeat expansion mutation that is believed to confer a toxic gain of function on the mutant protein. Huntington disease patients with two mutant alleles are very rare. In other poly(CAG) diseases such as the dominant ataxias, inheritance of two mutant alleles causes a phenotype more severe than in heterozygotes. In this multicentre study, we sought differences in the disease features between eight homozygotes and 75 heterozygotes for the Huntington disease mutation. We identified subjects homozygous for the Huntington disease mutation by DNA testing and compared their clinical features (age at onset, symptom presentation, disease severity and disease progression) with those of a group of heterozygotes, who were assessed longitudinally. The age at onset of symptoms in the homozygote cases was within the range expected for heterozygotes with the same CAG repeat lengths, whereas homozygotes had a more severe clinical course. The observation of a more rapid decline in motor, cognitive and behavioural symptoms in homozygotes was consistent with the extent of neurodegeneration as available at imaging in three patients, and at the post-mortem neuropathological report in one case. Our analysis suggests that although homozygosity for the Huntington disease mutation does not lower the age at onset of symptoms, it affects the phenotype and the rate of disease progression. These data, once confirmed in a larger series of patients, point to the possibility that the mechanisms underlying age at onset and disease progression in Huntington disease may differ.