We did not detect any significant association between the investigated COMT or DAT1 polymorphisms and borderline symptoms in either the US young adult sample or the Hungarian psychiatric patient population, although the DAT1 9-repeat allele has been linked not only to BPD [7] but also to impulsivity [54]. The latter workgroup also showed higher reward-related ventral striatum reactivity (which was associated with self-reported impulsivity) in subjects with at least one DRD4 7-repeat allele and in DRD2 -141C Del carriers. They argued for a connection between greater ventral striatum reactivity and functional dopaminergic variants that result in decreased postsynaptic dopamine receptor density. However, there is no clear-cut evidence concerning the functionality of these dopaminergic polymorphisms. The reduced DRD4 expression of the 7-repeat allele [27] has not been replicated. The in vitro reporter gene experiment of the DRD2 -141C Del allele showed lower expression [55], whereas a SPECT study showed higher striatal dopamine receptor density for the -141C Del variant [21]. Finally, several studies indicated that the 40 bp VNTR in the DAT1 gene 3' untranslated region affects gene expression, but the results are controversial concerning both the gene expression analyses [56,57] and the SPECT studies [58-60].
