# Ubinuclein 2 is essential for mouse development and functions in X chromosome inactivation

## TL;DR

Ubinuclein 2 is crucial for mouse development and helps in silencing one X chromosome in females.

## Contribution

The study reveals a female-specific role of the HIRA complex in X chromosome inactivation and dosage compensation.

## Key findings

- Ubn2 mutations cause embryonic lethality with a male-biased sex ratio.
- HIRA complex is essential for initiating X chromosome inactivation by establishing H3K27me3 marks.
- Combined Ubn1 and Ubn2 mutations lead to complete embryonic lethality and developmental arrest.

## Abstract

The HIRA complex mediates deposition of histone H3.3 independent of replication. Its functions in gene regulation in mice remain to be fully understood. Here we analyze mutations of the HIRA complex genes Ubn1 and Ubn2. We observe that Ubn1 mutant mice of both sexes are viable and fertile. In contrast, mutation of Ubn2 causes embryonic lethality with variable penetrance and skewed sex ratio in favor of males. Combined Ubn1 and Ubn2 mutations cause embryonic lethality with complete penetrance, variable developmental arrest before turning, and reduced recovery of female embryos. Consistent with a female specific function of the HIRA complex, reanalysis of the Hira mutation during embryogenesis reveals that previously observed severe and mild phenotypic classes correspond to female and male sex. Mechanistically, we show that mutations of Ubn1, Ubn2, and Hira in mouse embryonic stem cells affect the initiation of X inactivation. Xist mediated gene silencing is impaired to increasing extent by Ubn1, Ubn2, Hira, and combined Ubn1 and Ubn2 mutations. We identify a failure of establishing histone H3 tri-methyl lysine 27 over X-linked genes after induction of Xist expression as earliest molecular defect, whereas deacetylation of lysine 27 by Xist remains largely unaffected by the loss of Ubinucleins. Our study thereby identifies a switch from histone H3 acetyl to tri-methyl lysine 27 at the initiation of X inactivation that depends on HIRA complex function.

The genome of all eukaryotes is assembled into a nucleosomal structure, which is accomplished by deposition of histone proteins on the DNA. This process involves distinct mechanisms that are dependent or independent of DNA replication. The evolutionary conserved HIRA complex deposits histone H3.3 independent of DNA replication within active gene loci and regulatory regions. Its functions in gene regulation remain to be fully understood in animal development. By analyzing mutations in HIRA complex genes in mice, we discover a female specific function of the HIRA complex in embryogenesis. HIRA complex function is required for establishing gene silencing on the inactive X chromosome and dosage compensation in mice. We identify a switch from acetyl to tri-methyl modified histone H3 lysine 27 at X-linked genes during the initiation of X chromosome inactivation as molecular event that is dependent on HIRA complex function. Our study provides a characterization of the function of HIRA complex genes in mouse development and identifies a novel function in the establishment of facultative heterochromatin on the inactive X chromosome.