BubR1 allelic effects drive phenotypic heterogeneity in MVA progeria syndrome [3 month old RNA-seq]

Mosaic-variegated aneuploidy (MVA) syndrome is a rare childhood disorder characterized by biallelic BUBR1, CEP57, or TRIP13 aberrations, increased chromosome missegregation, and a broad spectrum of clinical features, including various cancers, congenital defects, and progeroid pathologies. To investigate the mechanisms underlying this disorder and its phenotypic heterogeneity, we mimicked the BUBR1L1012P mutation in mice (BubR1L1002P) and combined it with two other MVA variants, BUBR1X753 and BUBR1H, generating a truncated protein and low amounts of wildtype protein, respectively. Whereas, BubR1X753/L1002P and BubR1H/X753 mice die prematurely, BubR1H/L1002P mice are viable and exhibit many MVA features, including cancer predisposition and various progeroid phenotypes, including short lifespan, dwarfism, lipodystrophy, sarcopenia, and low cardiac stress tolerance. Strikingly, although these mice had a similar reduction in total BubR1 and spectrum of MVA phenotypes as BubR1H/H mice, several progeroid pathologies were attenuated in severity, which in skeletal muscle coincided with reduced senescence-associated secretory phenotype (SASP) complexity. Additionally, mice harboring heterozygous BUBR1 MVA variants developed mild MVA pathologies later in life, with alleles conferring unique phenotypic profiles. Together, these data demonstrate that subtle BUBR1 allelic effects contribute to disease heterogeneity in both MVA patients and heterozygous carriers of MVA mutations, independent of aneuploidy rates and BUBR1 protein levels. Skeletal muscle (skM, gastrocnemius) from 3-month-old wildtype and two BubR1 MVA allele carriers, BubR1+/L1002P and BubR1+/X753 were collected and harvested from naturally aged mice, euthanized at 3 months of age. RNA was extracted from tissues and was used for RNA sequencing.镶嵌型可变非整倍体（Mosaic-variegated aneuploidy, MVA）综合征是一种罕见的儿童遗传性疾病，其特征为双等位基因BUBR1、CEP57或TRIP13发生畸变，染色体错分离率升高，且伴随涵盖多种癌症、先天性缺陷与早衰类病理改变的广泛临床表型。为探究该疾病的发病机制及其表型异质性，我们在小鼠体内模拟了人类BUBR1L1012P突变，并构建了对应小鼠突变体BubR1L1002P；同时结合另外两种MVA相关变异体BUBR1X753与BUBR1H，分别得到了表达截短型蛋白与低水平野生型蛋白的模型。其中，BubR1X753/L1002P与BubR1H/X753双突变小鼠会过早死亡，而BubR1H/L1002P双突变小鼠可正常存活，并表现出多种MVA特征，包括癌症易感性以及多种早衰样表型，如寿命缩短、侏儒症、脂肪营养不良、肌肉减少症与心脏应激耐受性下降。值得注意的是，尽管此类小鼠的总BubR1蛋白水平下降幅度与MVA表型谱均与BubR1H/H纯合小鼠相似，但部分早衰类病理的严重程度有所缓解；在骨骼肌组织中，这一现象与衰老相关分泌表型（senescence-associated secretory phenotype, SASP）的复杂度降低相一致。此外，携带杂合型BUBR1 MVA变异体的小鼠会在晚年出现轻度MVA病理改变，且不同等位基因对应独特的表型特征。综合上述实验数据可知，细微的BUBR1等位基因效应会导致MVA患者及MVA突变杂合携带者的疾病异质性，且这一过程独立于染色体非整倍体发生率与BUBR1蛋白水平。我们收集了3月龄野生型小鼠、两种BubR1 MVA等位基因携带者（BubR1+/L1002P与BubR1+/X753）的骨骼肌（skM，腓肠肌）样本，同时收集了自然衰老并于3月龄时实施安乐死的小鼠的该组织样本；从上述组织中提取RNA并用于RNA测序（RNA sequencing）分析。