Abstract
Purpose :
MLL1 (KMT2A) and MLL2 (KMT2B) are closely related members in the mixed-lineage leukemia (MLL) family of histone methyltransferases. They form a core complex for epigenetic regulation of gene expression. The previous study showed MLL1 is essential for retinal neurogenesis and horizontal cell integrity. This research aimed to unveil MLL1/MLL2 collaboration, whether by distinct pathways or through redundancy, in the retinal development and maintenance.
Methods :
Cre/loxP-mediated Mll2 and Mll1/Mll2 deletions were specifically targeted in retinal progenitors (Chx10-Cre), developing rods (Nrl-Cre) and cones (HRGP-Cre), respectively. Electroretinography (ERG) detected functional deficits caused by Mll1 and Mll2 deficiency. Hematoxylin and eosin (H&E) staining of retinal cross-sections showed morphological defects at various postnatal (P) ages. EdU labeling and immunostaining of cell-type specific markers reported changes in cell composition between mutant and control retinae. Quantitative PCR (qPCR) determined gene expression differences of cell type-specific markers, phototransduction cascade components and transcriptional factors.
Results :
Mll2 knockout (KO) in progenitors produced similar retinal thinning and functional decline to Mll1KO retinae at all ages. Mll1/Mll2 double-KO mice showed severe flaws, including null ERG responses, retinal thinning at P14 and P30, reactive gliosis at 1 month old (MO), followed by rapid degeneration within 2MO. Progenitor cell proliferation was moderately affected in all mutants. In addition, double-KO retinae had decreased numbers of M-cones, horizontal and amacrine neurons. qPCR results confirmed the cellular changes in mutants and explained the ERG deficits. In contrast, rod-specific double-KO mice showed normal retinal morphology and function, suggesting that Mll1/Mll2 are not functionally required for rod development and survival. Cone-specific double-KO mice had M-cone number reduced by 50% and decreased cone ERG responses.
Conclusions :
Our results identified the redundant roles of MLL1 and MLL2 in the retinal development and maintenance of M-cones. Mll1/Mll2 deficiency results in retinal degeneration and reactive gliosis as well as gene silencing at young adulthood. Future efforts are underway to determine the underlying molecular mechanisms.
This is a 2021 ARVO Annual Meeting abstract.