Based on the report of Lan et al.,1 we sought to replicate the association between stuttering and single-nucleotide polymorphisms (SNPs) that reside in the dopamine D2 receptor (DRD2) gene in additional populations. The only individual significant association observed by Lan et al. was with the C allele of rs6277, a synonymous SNP (Pro319Pro) within the coding sequence of this gene. In their Han Chinese sample of 112 cases and 112 controls, this allele occurred at a frequency of 0.96 in cases and 0.88 in controls, with a reported P-value of 0.001.

We studied this polymorphism as well as another DRD2 synonymous SNP, rs6275, also studied by Lan et al., in two additional case–control populations that together represented a larger sample. The first was a group of 50 cases and 50 sex-matched controls of mixed ancestry2 from Sao Paulo State, Brazil, while the second was a group of 214 cases and 451 neurologically normal controls of western European origin.3 All subjects provided written informed consent under institutionally approved human subjects research protocols. Speech diagnosis was performed using the Stuttering Severity Instrument, 3rd Edition (SSI-3),4 and as previously described.3 Genotypes were determined by dye terminator sequencing of both strands using an ABI 3730 XL capillary instrument.

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