Lars Nguyen

Introduction: Transposons are DNA elements that can change their location within the genome hence being able to induce mutation or reverse mutation. Long interspersed nuclear element-1 (LINE-1 or L1) are class I retrotransposons¹ and are typically silenced in somatic cells by methylation, staying as heterochromatin form. Endogenous activation of LINE-1 increases chromatin accessibility, which means LINE-1 could play a role in development by regulating chromatin accessibility in stem cells and early embryos⁴. Active transposable elements, such as LINE-1, can also introduce different regulatory motifs in promoter and enhancer regions which could alter gene expression or even insert new enhancer sites increasing expression or changing promoter specificity^2,3,5. In this study I will focus on the nuclear exosome targeting (NEXT) complex, a potential post-transcriptional regulator of LINE-1, and what kind of consequences would follow when defective. The NEXT is an RNA exosome cofactor in the nucleoplasm. It is required for the exosomal degradation and surveillance of many nuclear RNA substrates^4,6 and plays a substantial role in telomerase maturation⁷. The NEXT complex which consists of the RNA helicase hMTR4/SKIV212, Zn-knuckle protein Zcchc8, and the RNA recognition motif-containing protein RBM7. Methods and Results: In Zcchc8 knockout (KO) mice experiments, where the Zcchc8 gene is altered by CRISPR/Cas9 technology Zcchc8 protein is truncated^4,7. The mice were then interbred to produce heterozygous mutant mice, producing both heterozygous and homozygous null progeny with no decreased or no detectable Zcchc8 protein. Telomerase RNA analysis was performed using RNA immunoprecipitation and brain structures were examined using CT scans. Higher levels of LINE-1 transcripts⁴ and shortened telomerase RNA⁷ were observed. The KO mice also showed fertility defects and much lower survival rate during the first 3 months after birth⁴. KO mice also had decreased skull area at birth and development of macrocephaly after birth⁷. The development of progressive worse pulmonary fibrosis was observed along with neurodevelopmental delays and deficiency in affected Zcchc8 deficient mice⁷. Conclusion: Zcchc8 mediated mechanism plays an important role in the post-transcriptional regulation of LINE-1 in early embryos and embryonic stem cells, consequently being a factor in pluripotency and early development⁴. LINE-1 activity is substantially regulated by the nuclear exosome targeting complex, both in regulating RNA activity and potentially silencing genes⁶. Functional NEXT and Zcchc8 are required for proper telomerase maturation and dysfunctional NEXT could lead to RNA dysregulation disorders. Possible future treatment involves targeting upstream or downstream of the telomerase pathway to restore its function⁸.

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