Candidate genes and the dopamine system : possible implications in complex neurological and psychiatric disease

Abstract

The mesencephalic dopamine (DA) system is strongly involved in the pathogenesis of Parkinsons disease (PD), where symptoms occur after the loss of the vast majority of DA neurons. An involvement of this system also in schizophrenia (SZ) and bipolar disease (BD) is suggested by the fact that DA receptor antagonists are among the most potent drugs in clinical use for the treatment of these conditions. Basic science of the past years has provided several clues for how DA cells develop and under which conditions they thrive. Different approaches include research on neurotrophic factors (providing target-derived support for DA cells), neuropeptides (modulators of neurotransmission), transcription factors (activating a large number of "downstream" genes important for development and maintenance of the cell) and endogenous as well as environmental neurotoxins.

This thesis describes an effort to link basic science knowledge of the type described above to clinical samples employing the candidate gene approach. Analyzing genes encoding the above-mentioned proteins in material from patients suffering from PD, SZ and BD aims at establishing direct links between mutations in such genes and susceptibility for die disorders.

The CALCA locus is one of the first and by now most well-characterized examples of alternative splicing of genes in humans. While the calcitonin protein is localized to C-cells of the thyroid gland, the alternatively spliced protein CGRP is a neuropeptide with multiple functions within the brain. We identified several new polymorphisms in this locus and investigated association within available materials from PD, SZ and BD patients. No significant associations were found with these disorders. However, the report of the sequence variations identified in the gene may still be valuable markers, since CGRP has been strongly implicated both in hypertension and migraine.

NURR1 is a nuclear receptor being absolutely essential for the development of mesencephalic dopamine neurons. Three different unique missense mutations were identified in patients suffering from psychosis (two in SZ, one in BD). All three mutations were located within 78 bp from each other in a region of exon 3 and disturbed NURR1-mediated transcription similarly when modeled in vitro. Since the NURR1 gene is highly conserved between humans and rodents, introduction of the identified mutations in transgenic animals may lead to a valid model for psychosis susceptibility.

The ADH4 gene codes for in alcohol dehydrogenase (ADH) that converts retinol to retinal and also acts upon a wealth of other alcohols and aldehydes, some of which have been implicated in the pathogenesis of PD as endogenous or exogenous neurotoxins. We found significant association between one of several polymorphisms identified in this gene and idiopathic PD. In a follow-up study of a larger material several genes within the ADH cluster including ADH4 were analyzed. The previously found association of ADH4 with PD remained significant, while other polymorphisms did not show significant association. A nonsense mutation in ADH1C was identified in three (of 123) PD patients but none of 127 control individuals.

An effort was also made to characterize the usefulness of available postmortem material for immunohistochemistry Using a plethora of different antibodies, we could confirm the feasability of postmortem studies on the protein level and found a new marker for corpora amylacea, namely nestin. Since bodies made of aggregated proteins play roles in a number of neurodegenerative disorders, we went on to characterize corpora amylacea further with antibodies against alpha- synuclein, PGP9.5 and other (vimentin, NF) neurofilaments.

Expression of different alcohol and aldehyde dehydrogenases was also studied in postmortem human brain tissue using in situ hybridization. High and specific expression of an aldehyde dehydrogenase (ALDH1) was found in human DA cells. Furthermore, ALDH1 mRNA levels were found to be strongly decreased in remaining dopamine neurons of substantia nigra of PD patients as compared to controls. Other DA neurons (of the ventral tegmental area) showed normal levels of expression, indicating a selective decrease of ALDH1 mRNA levels in neurons degenerating in PD.

To summarize, evidence for the involvement of alcohol- and aldehyde dehydrogenases as wed as the nuclear transcription factor NURR1 in the pathogenesis of the diseases of interest was found in the available material. Further studies along these lines using larger materials employing more markers and probes are warranted.