Bilallelic germline mutations in MAD1L1 induce a novel syndrome of aneuploidy with high tumor susceptibility

Aneuploidy is a frequent feature of human tumors. Germline mutations leading to aneuploidy are very rare in humans, and their tumor-promoting properties are mostly unknown at the molecular level. We report here novel germline biallelic mutations in MAD1L1, the gene encoding the Spindle Assembly Checkpoint (SAC) protein MAD1, in a 36-year-old female with a dozen of neoplasias, including five malignant tumors. Functional studies in peripheral blood cells demonstrated lack of full-length protein and deficient SAC response, resulting in ~30-40% of aneuploid cells as detected by cytogenetic and single-cell (sc) DNA analysis. scRNA-seq analysis of proband blood cells identified mitochondrial stress accompanied by systemic inflammation with enhanced interferon and NFkB signaling. The inference of chromosomal aberrations from scRNA-seq analysis detected inflammatory signals both in aneuploid and euploid cells, suggesting a non-cell autonomous response to aneuploidy. In addition to random aneuploidies, MAD1L1 mutations resulted in specific clonal expansions of T-cells with chromosome 18 gains and enhanced cytotoxic profile, as well as intermediate B-cells with chromosome 12 gains and transcriptomic signatures characteristic of chronic lymphocytic leukemia cells. These data point to MAD1L1 mutations as the cause of a new variant of mosaic variegated aneuploidy syndrome (MVA) with systemic inflammation and unprecedented tumor susceptibility.

染色体非整倍体（aneuploidy）是人类肿瘤的常见特征。可引发染色体非整倍体的生殖系突变（germline mutations）在人类中极为罕见，其促肿瘤发生的分子机制目前大多尚未阐明。本研究报道了1例36岁女性患者体内检出的新型生殖系双等位基因突变（biallelic mutations），该突变位于MAD1L1基因——该基因编码纺锤体组装检验点（Spindle Assembly Checkpoint，SAC）蛋白MAD1。该患者共罹患十余种赘生物，其中包含5种恶性肿瘤。对该患者外周血细胞开展的功能研究证实，其体内缺乏全长MAD1蛋白，且纺锤体组装检验点应答存在缺陷；通过细胞遗传学分析与单细胞（sc）DNA分析检测发现，约30%~40%的血细胞呈现染色体非整倍体表型。对先证者血细胞进行的单细胞RNA测序（single-cell RNA sequencing，scRNA-seq）分析显示，细胞出现线粒体应激，伴随全身性炎症反应，且干扰素与核因子κB（NF-κB）信号通路的激活水平显著升高。从单细胞RNA测序数据中推断染色体畸变时发现，染色体非整倍体细胞与整倍体细胞中均检测到炎症信号，这提示机体对染色体非整倍体存在非细胞自主性的应答反应。除随机性染色体非整倍体外，MAD1L1基因突变还可诱导特定的T细胞克隆扩增：此类T细胞携带18号染色体拷贝数增加，且细胞毒性功能增强；同时还可出现携带12号染色体拷贝数增加的中间型B细胞克隆扩增，该类B细胞具有慢性淋巴细胞白血病（chronic lymphocytic leukemia）细胞特征性的转录组学特征。上述研究数据表明，MAD1L1基因突变是一种新型镶嵌式斑驳非整倍体综合征（Mosaic Variegated Aneuploidy Syndrome，MVA）变体的致病根源，该综合征伴随全身性炎症与前所未有的肿瘤易感性。