Novel biological implication of a strictly monomorphic GCC repeat in the human PRKACB gene

Across human protein-coding genes, PRKACB (Protein Kinase CAMP-Activated Catalytic Subunit Beta) contains one of the longest GCC-repeats, and is predominantly expressed in the brain. Here we studied this STR in 300 human subjects, consisting of late-onset neurocognitive disorder (NCD) (N = 150) and controls (N = 150). We also studied the impact of this STR on the three-dimensional structure of DNA. While the PRKACB GCC-STR was strictly monomorphic at 7-repeats, we detected two 7/8 genotypes only in the NCD group. In comparison to all other lengths, (GCC)7 had the least effect on the three-dimensional structure of DNA, evidenced by minimal divergence between 0 and 7-repeats (divergence score = 0.04) and significant divergence between 0 and 8 repeats (divergence score = 0.50). A similar inert effect to the GCC-repeat was not detected in other classes of STRs such as GA and CA repeats. In conclusion, we report monomorphism of an exceptionally long GCC repeat in the PRKACB gene in human, its inert effect on DNA structure, and divergence in two cases of late-onset NCD. This is the first indication of natural selection for an exceptionally long monomorphic GCC-repeat, which probably evolved to function as an “epigenetic knob”, without changing the regional DNA structure.