Self-targeting spacers in CRISPR-array: Accidental occurrence or evolutionarily conserved phenomenon

In recent years, a tremendous amount of inquisitiveness among scientists in the clustered regularly interspaced short palindrome repeats (CRISPR)-CRISPR-associated proteins (Cas) has led to many studies to delineate their exact role in prokaryotes. CRISPR-Cas is an adaptive immune system that protects prokaryotes from phages and mobile genetic elements. It incorporates small DNA fragment of the invader in the CRISPR-array and protects the host from future invasion by them. In a few instances, the CRISPR-array also incorporates self-targeting spacers, most likely by accident or leaky incorporation. A significant number of spacers are found to match with the host genes across the species; however, self-targeting spacers have not been investigated in detail in most of the organisms. The presence of self-targeting spacers in the CRISPR-array led to speculation that the CRISPR-Cas system has a lot more to offer than just being the conventional adaptive immune system. It has been implicated in gene regulation and autoimmunity more or less equally. In this review, an attempt has been made to understand self-targeting spacers in the context of gene regulation, autoimmunity, and its avoidance strategies.

Genomic DNA transposition induced by human PGBD5

Transposons are mobile genetic elements that are found in nearly all organisms, including humans. Mobilization of DNA transposons by transposase enzymes can cause genomic rearrangements, but our knowledge of human genes derived from transposases is limited. In this study, we find that the protein encoded by human PGBD5, the most evolutionarily conserved transposable element-derived gene in vertebrates, can induce stereotypical cut-and-paste DNA transposition in human cells. Genomic integration activity of PGBD5 requires distinct aspartic acid residues in its transposase domain, and specific DNA sequences containing inverted terminal repeats with similarity to piggyBac transposons. DNA transposition catalyzed by PGBD5 in human cells occurs genome-wide, with precise transposon excision and preference for insertion at TTAA sites. The apparent conservation of DNA transposition activity by PGBD5 suggests that genomic remodeling contributes to its biological function.