Structure and Expression of Mobile ETnII Retroelements and Their Coding-Competent MusD Relatives in the Mouse

Abstract

ETnII elements are mobile members of the repetitive early transposon family of mouse long terminal repeat (LTR) retroelements and have caused a number of mutations by inserting into genes. ETnII sequences lack retroviral genes, but the recent discovery of related MusD retroviral elements with regions similar to gag , pro , and pol suggests that MusD provides the proteins necessary for ETnII transposition in trans . For this study, we analyzed all ETnII elements in the draft sequence of the C57BL/6J genome and classified them into three subtypes (α, β, and γ) based on structural differences. We then used database searches and quantitative real-time PCR to determine the copy number and expression of ETnII and MusD elements in various mouse strains. In 7.5-day-old embryos of a mouse strain in which two mutations due to ETnII-β insertions have been identified (SELH/Bc), we detected a three- to sixfold higher level of ETnII-β and MusD transcripts than in control strains (C57BL/6J and LM/Bc). The increased ETnII transcription level can in part be attributed to a higher number of ETnII-β elements, but 70% of the MusD transcripts appear to have been derived from one or a few MusD elements that are not detectable in C57BL/6J mice. This element belongs to a young MusD subgroup with intact open reading frames and identical LTRs, suggesting that the overexpressed element(s) in SELH/Bc mice might provide the proteins for the retrotransposition of ETnII and MusD elements. We also show that ETnII is expressed up to 30-fold more than MusD, which could explain why only ETnII, but not MusD, elements have been positively identified as new insertions.