Yael Nechemia-Arbely, PhD

Background

The centromere is a central genetic element responsible for accurate chromosome segregation during cell division. Paradoxically, centromeres are neither defined by DNA sequences nor conserved among species. Rather, functional centromeres are specified through the stable acquisition of a non-DNA sequence-dependent “epigenetic mark” (Cleveland et al., 2003). A prime candidate for such an epigenetic mark is CENP-A, a histone H3 variant found exclusively in nucleosomes at all active centromeres (Fachinetti et al., 2013). CENP-A-containing chromatin underlies the mitotic kinetochore, the proteinaceous complex that mediates the physical connection between each chromosome to spindle microtubules.

Failure of normal chromosome delivery have broad medical implications, including infertility and birth defects. Moreover, many human tumors have abnormal numbers of chromosomes, a condition known as aneuploidy, and several types of cancer acquired neocentromeres (epigenetic stable acquisition of a new centromere at a new chromosomal site). Therefore, identification of the initial events driving mammalian centromere assembly, as well as the mechanisms required for centromere maintenance and propagation, is of high biological relevance given its broad effects on infertility and tumor development.

Research

CENP-A containing chromatin is a an octameric nucleosome across the entire cell cycle

DNA replication restricts CENP-A to centromeres and corrects errors in CENP-A deposition

Here, we tackled an unresolved question of high interest in genetics and cell biology: how are centromeres specified, maintained, and propagated across the cell cycle when CENP-A is diluted during DNA replication and replenished half a cell cycle later, at exit of mitosis? Using CENP-A ChIP-sequencing combined with Repli-seq (replication timing sequencing) and genome-wide mapping onto unique centromere reference models, we discovered that while CENP-A is loaded onto centromeres at mitotic exit (Jansen et al., 2007; Nechemia-Arbely et al., 2012), CENP-A is also ectopically loaded outside of the centromeres (Nechemia-Arbely et al., 2019). We identified a DNA replication-dependent error correction mechanism that acts in S-phase to remove ectopically loaded CENP-A while maintaining the positions of centromere-bound CENP-A with high precision throughout DNA replication (Nechemia-Arbely et al., 2019). We find that DNA replication serves as an error correction mechanism that restricts CENP-A to centromeres by removing it from the ectopic sites of deposition at the chromosome arms. Ectopic CENP-A peaks are removed as DNA replication progresses (Fig. 2).
  This error correction mechanism explains how centromere identity is epigenetically maintained and restricted to one position on the chromosome, a finding of high importance for our understanding of evolutionary time-scale of faithful chromosome inheritance (Fig. 3).
CENP-A retention at centromeres is mediated by CENP-C and the CCAN complex it nucleates, revealing a novel role for this centromeric complex in interphase. This error correction mechanism explains how centromere identity is epigenetically maintained and restricted to one position on the chromosome, a finding of high importance for our understanding of evolutionary time-scale of faithful chromosome inheritance (Fig. 3).

Current efforts are aimed towards understanding:

1. the epigenomic landscape of centromeric chromatin
2. consequences of ectopic CENP-A deposition in human cells
3. is the error correction mechanism disrupted in cancer cells?
4. regulation and maintenance of centromeric chromatin across the cell cycle in health and disease