Retinal development anomalies and cone photoreceptors degeneration upon Bmi1 deficiency

Abstract

Retinal development occurs through the sequential but overlapping generation of six neuronal and one glial cell types. Of these, rod and cone photoreceptors represent the functional unit of light detection and phototransduction and are frequently affected in retinal degenerative diseases. During mouse development, the Polycomb group protein Bmi1 is expressed in immature retinal progenitors and differentiated retinal neurons, including cones. We show here that Bmi1 is required to prevent post-natal degeneration of cone photoreceptors and bipolar neurons, and that inactivation of Chk2 or p53 could improve but not overcome cone degeneration in Bmi1−/− mice. The retinal phenotype of Bmi1−/− mice was also characterized by loss of heterochromatin, activation of tandem-repeats, oxidative stress, and Rip3-associated necroptosis. In the human retina, BMI1 was preferentially expressed in cones at heterochromatic foci. BMI1 inactivation in human embryonic stem cells was compatible with retinal induction but impaired cone terminal differentiation. Despite this developmental arrest, BMI1-deficient cones recapitulated several anomalies observed in Bmi1−/− photoreceptors such as loss of heterochromatin, activation of tandem-repeats and p53 induction, revealing partly conserved biological functions between mouse and man.