Investigation of the transcription elongation factor Ell2 in lens development

Abstract

Purpose : The relationship between post-transcriptional regulatory factors and those involved in transcriptional control of lens development is not well understood. We recently showed that deficiency of the post-transcriptional regulator Celf1 causes congenital cataract. To understand the molecular basis of these defects in Celf1 lens-specific conditional knockout mouse (Celf1^cKO) lens, we performed high-throughput RNA-seq and identified Ell2 (Elongation factor for RNA Polymerase II 2) to be mis-expressed upon Celf1 deletion. Ell2 is a “transcription elongation” factor that interacts with RNA polymerase II and increases its elongation rate along template DNA. Moreover, Ell2 participates in post-transcriptional regulatory events such as mRNA splicing and poly(A) site selection. Ell2 exhibits high iSyTE lens-enriched expression, further supporting its role in lens development. To examine its function in the lens, we generated a new Ell2 lens-specific conditional KO mouse model and characterized its ocular defects.

Methods : Celf1 RNA-immunoprecipitation (RIP) was performed on postnatal P15 wild type mouse lens. Germline knockout mice (Ell2^-/-) were generated by crossing mice carrying CMVCre and Ell2 floxed alleles. Lens-specific conditional KO mice (Ell2^cKO) were generated by crossing mice carrying Pax6GFPCre and Ell2 floxed alleles.

Results : RNA-seq data showed that Ell2 mRNA was abnormally overexpressed in Celf1^cKO lens, suggesting that it is a downstream target of Celf1. This was validated by RT-qPCR. RIP assays on wild type mouse lens showed that Ell2 mRNA was enriched in Celf1-IP, indicative of its close association with Celf1 protein, in turn suggesting that Ell2 mRNA is a potential direct target of Celf1 protein. This indicates that post-transcriptional regulatory mechanisms may be present to ensure optimal levels of Ell2 in the lens, in turn suggesting that Ell2 may have a key function in lens development. In support of this hypothesis, phenotypic characterization of Ell2^-/- and Ell2^cKO mice show small eye defects at age 2 months.

Conclusions : Although lens development involves elevated transcription of key genes, factors regulating the activity of the RNA Polymerse II elongation complex in the lens remain uncharacterized. This study identifies the transcription elongation protein Ell2 as a novel downstream target of the cataract-linked gene Celf1 and as a new factor linked to eye defects.

This is a 2020 ARVO Annual Meeting abstract.