Selection pressure on human STR loci and its relevance in repeat expansion disease
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Short Tandem Repeats (STRs) comprise repeats of one to several base pairs. Because of the high mutability due to strand slippage during DNA synthesis, rapid evolutionary change in the number of repeating units directly shapes the range of repeat-number variation according to selection pressure. However, the remaining questions include: Why are STRs causing repeat expansion diseases maintained in the human population; and why are these limited to neurodegenerative diseases? By evaluating the genome-wide selection pressure on STRs using the database we constructed, we identified two different patterns of relationship in repeat-number polymorphisms between DNA and amino-acid sequences, although both patterns are evolutionary consequences of avoiding the formation of harmful long STRs. First, a mixture of degenerate codons is represented in poly-proline (poly-P) repeats. Second, long poly-glutamine (poly-Q) repeats are favored at the protein level; however, at the DNA level, STRs encoding long poly-Qs are frequently divided by synonymous SNPs. Furthermore, significant enrichments of apoptosis and neurodevelopment were biological processes found specifically in genes encoding poly-Qs with repeat polymorphism. This suggests the existence of a specific molecular function for polymorphic and/or long poly-Q stretches. Given that the poly-Qs causing expansion diseases were longer than other poly-Qs, even in healthy subjects, our results indicate that the evolutionary benefits of long and/or polymorphic poly-Q stretches outweigh the risks of long CAG repeats predisposing to pathological hyper-expansions. Molecular pathways in neurodevelopment requiring long and polymorphic poly-Q stretches may provide a clue to understanding why poly-Q expansion diseases are limited to neurodegenerative diseases.
KeywordsSTR polymorphism Single amino-acid repeat Human evolution Triplet-repeat expansion disease Database for human polymorphism (VarySysDB)
Short tandem repeat
Simple amino acids repeat
Coding sequence region
Coding trinucleotide short tandem repeat
The international nucleotide sequence databases collaboration
Human-gene diversity of life-style related diseases/gene diversity database system
Annotation data set for All Human Genes version 2
H-InvDB gene cluster defined by mapping of transcripts on genome sequence
Percentage of G or C at the third codon
We are grateful to Hidetoshi Inoko for support to use H-GOLD/GDBS data, Yasuyuki Fujii, Katsuhiko Murakami, Yoshiharu Sato and Jun-ichi Takeda for providing gene structure and annotation data, Ryuzo Matsumoto and Yosuke Hayakawa for useful suggestion on computer programming, and other former member of the H-Invitational 2 consortium, Genome Information Integration Project (GIIP), the Integrated Database and Systems Biology Team of BIRC, AIST for their helpful support. This research was financially supported by the Ministry of Economy, Trade and Industry of Japan (METI) and the Japan Biological Informatics Consortium (JBIC). Also, this work is partly supported by the Grants-in-Aid for Scientific Research (C) to MKS (JSPS Grant Numbers 24510271 and 21510205), and the Saito Gratitude Foundation to MKS.
Compliance with ethical standards
Conflict of interest
All authors declare no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
This article does not contain any studies with human participants.
Updated data on STRs and SARs within known human transcriptome sequences will be continuously provided in the VarySysDB database (http://h-invitational.jp/varygene/home.htm). The original data of STRs and SARs in human exonic region are available at the web site of the first author, MKS (http://www.fujita-hu.ac.jp/~mshimada/sub/STR_SAR_Data.html) and a web-based data sharing system provided by the research map (http://researchmap.jp/shimada-mk/%E8%B3%87%E6%96%99%E5%85%AC%E9%96%8B/).
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