Contains source data for graphs in Fig 5.

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.

HIRA复合物（HIRA complex）介导不依赖复制的组蛋白H3.3（histone H3.3）沉积，其在小鼠基因调控中的功能仍有待全面阐明。本研究分析了HIRA复合物基因Ubn1与Ubn2的突变，发现雌雄Ubn1突变小鼠均可存活且具备生育能力。与之相反，Ubn2突变会引发胚胎致死，且外显率可变，同时伴随偏向雄性的性别比例偏移。Ubn1与Ubn2联合突变则会导致完全外显的胚胎致死，表现为胚胎在扭尾期前出现可变的发育停滞，且回收得到的雌性胚胎数量减少。与HIRA复合物具有雌性特异性功能的结论一致，对胚胎发生过程中Hira突变的重新分析显示，此前观测到的严重与轻微表型类别分别对应雌性与雄性个体。机制层面的实验证实，小鼠胚胎干细胞中的Ubn1、Ubn2及Hira突变会影响X染色体失活（X inactivation）的起始过程。Xist介导的基因沉默会因Ubn1、Ubn2、Hira突变以及Ubn1与Ubn2联合突变而程度不等地受损。我们鉴定出，诱导Xist表达后，X连锁基因上的组蛋白H3赖氨酸27三甲基化的建立失败是最早的分子缺陷；而Xist介导的赖氨酸27去乙酰化过程在缺失Ubinuclein蛋白后基本不受影响。综上，本研究揭示了X染色体失活起始阶段，组蛋白H3乙酰化赖氨酸27向三甲基化赖氨酸27的表观修饰转变依赖于HIRA复合物的功能。