DNA damage response protein TOPBP1 regulates X chromosome silencing in the mammalian germ line

Elias Elinati,Helen R Russell,Obah A Ojarikre,Mahesh Sangrithi,Takayuki Hirota,Dirk G De Rooij,Peter J Mckinnon,James M A Turner

doi:10.1073/pnas.1712530114

Abstract

Meiotic synapsis and recombination between homologs permits the formation of cross-overs that are essential for generating chromosomally balanced sperm and eggs. In mammals, surveillance mechanisms eliminate meiotic cells with defective synapsis, thereby minimizing transmission of aneuploidy. One such surveillance mechanism is meiotic silencing, the inactivation of genes located on asynapsed chromosomes, via ATR-dependent serine-139 phosphorylation of histone H2AFX (γH2AFX). Stimulation of ATR activity requires direct interaction with an ATR activation domain (AAD)-containing partner. However, which partner facilitates the meiotic silencing properties of ATR is unknown. Focusing on the best-characterized example of meiotic silencing, meiotic sex chromosome inactivation, we reveal this AAD-containing partner to be the DNA damage and checkpoint protein TOPBP1. Conditional TOPBP1 deletion during pachynema causes germ cell elimination associated with defective X chromosome gene silencing and sex chromosome condensation. TOPBP1 is essential for localization to the X chromosome of silencing "sensors," including BRCA1, and effectors, including ATR, γH2AFX, and canonical repressive histone marks. We present evidence that persistent DNA double-strand breaks act as silencing initiation sites. Our study identifies TOPBP1 as a critical factor in meiotic sex chromosome silencing.

Highlights

Meiotic synapsis and recombination between homologs permits the formation of cross-overs that are essential for generating chromosomally balanced sperm and eggs
By using combined antibody staining for TOPBP1 and SYCP3 on spermatocyte spreads, we confirmed previous reports that TOPBP1 localizes to the XY bivalent and is absent at the pseudoautosomal region (PAR) [38, 39] (Fig. S1C; legend provides quantification)
Aside from its well-established roles in DNA replication, DNA repair, and checkpoint control, TOPBP1 can interact with chromatin-remodeling complexes and transcription factors to influence gene expression [61]

Summary

Introduction

Meiotic synapsis and recombination between homologs permits the formation of cross-overs that are essential for generating chromosomally balanced sperm and eggs. Meiotic silencing is suggested to restrict double-strand break (DSB) formation [7], prevent nonhomologous recombination [4, 6, 15], and enable DSB repair [16] at sites of asynapsis It may serve a checkpoint function to eliminate germ cells with asynapsed chromosomes, thereby preventing aneuploidy in progeny [10]. Asynapsis signaling is transmitted to “effectors,” which reside in the associated chromatin loops and cause longrange gene silencing Based on their localization to the inactive XY bivalent, many potential sensors and effectors have been identified [4,5,6, 15, 18]. ATR, facilitated by MDC1, translocates along chromatin loops, effecting gene silencing via phosphorylation of H2AFX at serine 139 (γH2AFX)

Methods

Results

Conclusion