MDC1 directs chromosome-wide silencing of the sex chromosomes in male germ cells

Abstract

Chromosome-wide inactivation is an epigenetic signature of sex chromosomes. The mechanism by which the chromosome-wide domain is recognized and gene silencing is induced remains unclear. Here we identify an essential mechanism underlying the recognition of the chromosome-wide domain in the male germline. We show that mediator of DNA damage checkpoint 1 (MDC1), a binding partner of phosphorylated histone H2AX (gH2AX), defines the chromosome-wide domain, initiates meiotic sex chromosome inactivation (MSCI), and leads to XY body formation. Importantly, MSCI consists of two genetically separable steps. The first step is the MDC1-independent recognition of the unsynapsed axis by DNA damage response (DDR) factors such as ataxia telangiectasia and Rad3-related (ATR), TOPBP1, and gH2AX. The second step is the MDC1-dependent chromosome-wide spreading of DDR factors to the entire chromatin. Furthermore, we demonstrate that, in somatic cells, MDC1-dependent amplification of the gH2AX signal occurs following replicative stress and is associated with transcriptional silencing. We propose that a common DDR pathway underlies both MSCI and the response of somatic cells to replicative stress. These results establish that the DDR pathway centered on MDC1 triggers epigenetic silencing of sex chromosomes in germ cells.

Parental epigenetic control of embryogenesis: a balance between inheritance and reprogramming?

“…The establishment of MSCI requires phosphorylation of the histone variant H2AX by the kinase ATR to form γH2AX [101]. While ATR is initially targeted to chromatin by the tumor suppressor BRCA1 [101], spreading of γH2AX along the sex chromosomes occurs in an MDC1-dependent manner [102]. Several additional histone variants and modifications have been proposed to regulate MSCI.…”

The X and Y chromosome in meiosis: how and why they keep silent

“…13 A recent series of experiments by Royo et al 5 has shown that pachytene arrest in male mice can be induced by inappropriate expression of sex-linked genes, and they identified a single Y-linked gene, named Zfy, whose expression is sufficient to induce apoptosis of pachytene spermatocytes. Thus, in the Mdc1 knockout mice analyzed by Ichijima et al, 4 it is to be expected that the failure to achieve MSCI also explains the strict pachytene arrest that is observed in these mice (Figure 1). Whether the presence of autosomal asynapsed chromatin and/or persistent DNA damage can also trigger activation of a pachytene checkpoint is not clear.…”

“…6 The phosphorylation of H2AX at the XY body is mediated by the checkpoint kinase ATR that can be visualized along the unsynapsed axes of X and Y from late zygotene onwards. 8 In their recent paper, Ichijima et al 4 reveal that the phosphorylation of H2AX occurs in two phases; first, it is restricted to the chromosomal axes, then the area extends, and H2AX also becomes phosphorylated in the surrounding chromatin loops. This second phase, when cH2AX spreads, depends on a protein named mediator of DNA damage checkpoint 1 (MDC1), and this function is required for transcriptional silencing of the sex chromosomes ( Figure 1).…”

“…During meiotic prophase, progression of chromosome pairing and synapsis can be followed by immunofluorescent staining of SYCP3 (red), a component of the axial/lateral elements of the synaptonemal complex. In the enlarged area, the recent findings of Ichijima et al 4 and Royo et al 5 are summarized: mediator of DNA damage checkpoint 1 (MDC1) mediates spreading of cH2AX (green), phosphorylated by ATR (yellow) from the unsynapsed chromosomal axes (phase I) into the surrounding chromatin (phase II). Spreading of cH2AX and ATR is required to achieve meiotic sex chromosome inactivation (MSCI) and MSCI failure induces apoptosis (MSCI checkpoint) of pachytene spermatocytes, caused by misexpression of one or more sex-linked genes.…”

The great escape

“…1). 26,27 MSCI is strictly required for male fertility. When MSCI is disrupted, germ cells are arrested at meiotic prophase and are completely eliminated.…”