Molecular Pathogenesis
To date, functional investigations indicate that SLC6A3-related DTDS results from loss of transporter function [Kurian et al 2009, Kurian et al 2011b, Hansen et al 2014, Ng et al 2014a]. SLC6A3 encodes the dopamine transporter (DAT) that is expressed predominantly within the substantia nigra (projecting to the striatum) and in the midbrain ventral tegmental area (projecting to the hippocampus, nucleus accumbens, and corticolimbic areas). The transporter has a crucial role in mediating reuptake of dopamine from the synaptic cleft, thereby controlling dopamine homeostasis by regulating the duration and amplitude of synaptic dopaminergic transmission.
A number of nonsense variants, splice site changes, and deletions have been reported in SLC6A3-related DTDS, and it is likely that for these pathogenic variants nonsense-mediated decay or absent/truncated protein are mechanistic factors in disease. Reported missense substitutions result in mutated proteins that impair transporter function through a number of mechanisms including (i) reduced transporter activity, (ii) impaired dopamine recognition and/or binding affinity, (iii) decreased cell surface expression of the transporter, and (iv) abnormal posttranslational protein modification with impaired glycosylation [Kurian et al 2009, Kurian et al 2011b, Hansen et al 2014, Ng et al 2014a]. Abnormal DAT protein folding and transporter oligomerization are also postulated to play a role.
SLC6A3 pathogenic variants therefore impair the normal physiologic recycling of dopamine leading to presynaptic dopamine depletion. Excess dopamine in the synaptic cleft is metabolized to HVA, which can be detected on CSF analysis. High levels of synaptic dopamine may have downstream signaling effects on postsynaptic dopamine receptors, and are also likely to suppress tyrosine hydroxylase activity through action on D2 autoreceptors, thereby inhibiting presynaptic dopamine synthesis [Blackstone 2009].
A DAT knockout mouse model shows a number of features described in humans, including reduced growth, early hyperkinesia, and difficulties with feeding. Over time they develop abnormal clasping and kyphosis with progressive bradykinesia, reminiscent of the parkinsonism-dystonia phenotype in humans [Giros et al 1996].
Gene structure. The protein-coding transcript NM_001044.4 (ENST00000270349) comprises 15 exons in total, 14 of which are coding.
Pathogenic variants. To date, 20 individuals have been identified with biallelic (i.e., homozygous or compound heterozygous) pathogenic variants in SLC6A3.
Pathogenic variants have been identified throughout the entire coding region and flanking splice sites. No mutation hot spots or recurrent/common pathogenic variants have been identified. In these 20 individuals, 41 pathogenic variants including 27 missense changes, one pathogenic nonsense variant, seven splice site variants, and four intra-exon small deletions have been described. Large deletions have been described: a homozygous multiexon deletion [Kurian et al 2011b] and a microdeletion/translocation encompassing SLC6A3 [Kurian, personal communication 2016].
Note that rare missense variants in SLC6A3 have been reported in individuals with bipolar disorder, attention-deficit/hyperactivity disorder, and autism spectrum disorder [Hayden & Nurnberger 2006, Hamilton et al 2013, Bowton et al 2014] leading to the proposal that – through currently unknown mechanisms – this gene may confer risk for multiple complex psychiatric disorders [Bowton et al 2014]. However, parents of individuals with DTDS have not manifested attention-deficit disorder or psychiatric diseases.
Normal gene product. The gene transcript encodes a protein of 620 amino acids (NP_001035.1). The dopamine transporter (DAT) is one of the solute carrier 6 (SLC6) transporters [Bröer & Gether 2012]. Based on the crystal structure of a prokaryotic homolog [Singh et al 2007], DAT comprises 12 transmembrane helices connected by a series of interconnecting extracellular and intracellular loops from the N- to C-terminus. The protein undergoes posttranslational modification prior to being expressed at the cell surface of the presynaptic membrane. Its role in the translocation of dopamine requires tandem transport of sodium and chloride ions across the cell membrane.
Abnormal gene product. Pathogenic variants in the gene are postulated to lead to loss of transporter function and a number of pathogenic variants reported in the literature are pathogenic nonsense variants, splice site variants, or deletions.
Functional studies indicate that for pathogenic missense variants, mutated DAT displays impaired transporter function through a number of mechanisms including (i) reduced transporter activity, (ii) impaired dopamine recognition and binding affinity, (iii) decreased cell surface expression of the transporter, and (iv) abnormal posttranslational protein modification with impaired glycosylation [Kurian et al 2009, Kurian et al 2011b, Hansen et al 2014, Ng et al 2014a].
