Unravelling the link between neurodevelopmental disorders and short tandem CGG-repeat expansions

Novel islands of GGC and GCC repeats coincide with human evolution

BACKGROUND

Because of high mutation rate, overrepresentation in genic regions, and link with various neurological, neurodegenerative, and movement disorders, GGC and GCC short tandem repeats (STRs) are prone to natural selection. Among a number of lacking data, the 3-repeats of these STRs remain widely unexplored.

RESULTS

In a genome-wide search in human, here we mapped GGC and GCC STRs of ≥3-repeats, and found novel islands of up to 45 of those STRs, populating spans of 1 to 2 kb of genomic DNA. RGPD4 and NOC4L harbored the densest (GGC)3 (probability 3.09061E-71) and (GCC)3 (probability 1.72376E-61) islands, respectively, and were human-specific. We also found prime instances of directional incremented density of STRs at specific loci in human versus other species, including the FOXK2 and SKI GGC islands. The genes containing those islands significantly diverged in expression in human versus other species, and the proteins encoded by those genes interact closely in a physical interaction network, consequence of which may be human-specific characteristics such as higher order brain functions.

CONCLUSION

We report novel islands of GGC and GCC STRs of evolutionary relevance to human. The density, and in some instances, periodicity of these islands support them as a novel genomic entity, which need to be further explored in evolutionary, mechanistic, and functional platforms.

Expanding horizons of tandem repeats in biology and medicine: Why 'genomic dark matter' matters

Approximately half of the human genome includes repetitive sequences, and these DNA sequences (as well as their transcribed repetitive RNA and translated amino-acid repeat sequences) are known as the repeatome. Within this repeatome there are a couple of million tandem repeats, dispersed throughout the genome. These tandem repeats have been estimated to constitute ∼8% of the entire human genome. These tandem repeats can be located throughout exons, introns and intergenic regions, thus potentially affecting the structure and function of tandemly repetitive DNA, RNA and protein sequences. Over more than three decades, more than 60 monogenic human disorders have been found to be caused by tandem-repeat mutations. These monogenic tandem-repeat disorders include Huntington's disease, a variety of ataxias, amyotrophic lateral sclerosis and frontotemporal dementia, as well as many other neurodegenerative diseases. Furthermore, tandem-repeat disorders can include fragile X syndrome, related fragile X disorders, as well as other neurological and psychiatric disorders. However, these monogenic tandem-repeat disorders, which were discovered via their dominant or recessive modes of inheritance, may represent the 'tip of the iceberg' with respect to tandem-repeat contributions to human disorders. A previous proposal that tandem repeats may contribute to the 'missing heritability' of various common polygenic human disorders has recently been supported by a variety of new evidence. This includes genome-wide studies that associate tandem-repeat mutations with autism, schizophrenia, Parkinson's disease and various types of cancers. In this article, I will discuss how tandem-repeat mutations and polymorphisms could contribute to a wide range of common disorders, along with some of the many major challenges of tandem-repeat biology and medicine. Finally, I will discuss the potential of tandem repeats to be therapeutically targeted, so as to prevent and treat an expanding range of human disorders.