VNB Cng. VNB Cng

Cryptococcal meningitis (CM) accounts for nearly 50% of AIDS-related mortality in sub-Saharan Africa (Armstrong-James et al. 2014). Its main aetiological agent, Cryptococcus neoformans var. grubii (Cng), is subdivided into three molecular type, namely VNI, VNII and VNB. The latter remains mostly uncharacterised, although recent studies suggests that Cng VNB infection is more severe (Beale et al. 2015). The present study recovers environmental isolates of this molecular type from southern Africa and explores its relationship with clinical VNB isolates. The population structure was partitioned into four genetically different clusters. Then, a RNA-Seq-based analysis compared environmental and clinical VNB transcriptomes in response to a heat shock highlighting 130 differentially-expressed genes in clinical isolates. In vitro phenotypic experiments supported the hypothesis (p < 0.03) that clinical Cryptococcus neoformans isolates acquired a selective advantage leading to an increased thermotolerance affecting their success inside the human host. Gene network analyses emphasised the role of the cell wall integrity pathway in response to the heat shock as well as other related biological mechanisms including cellular response to oxidative stress, fatty acid-beta oxidation, Golgi apparatus-derived secretory pathway and protein biosynthesis. Selection analyses within the cryptococcal genome identified 19 genes under positive selection which were also upregulated only among clinical VNB isolates. Finally, the study argues that changes in ploidy is a critical asset for the establishment of VNB infection with 70% of clinical isolates presenting with a Chromosomal Copy Number Variations (CCNV) potentially explaining the critical pathogenicity of this highly genetically-divers molecular type.

隐球菌脑膜炎（Cryptococcal meningitis, CM）在撒哈拉以南非洲地区占艾滋病相关死亡病例的近50%（Armstrong-James等，2014）。其主要致病原为新型隐球菌格鲁比变种（Cryptococcus neoformans var. grubii, Cng），该菌种可分为三个分子型别，即VNI、VNII与VNB。其中VNB型别目前仍多未被充分表征，但近期研究表明Cng VNB感染的病情更为严重（Beale等，2015）。本研究从南非地区分离得到该分子型别的环境菌株，并探讨其与临床VNB菌株的关联。群体结构分析将其划分为四个遗传背景各异的聚类簇。随后，基于RNA测序（RNA-Seq）的分析比较了环境与临床VNB菌株在热应激下的转录组，共筛选得到130个在临床菌株中差异表达的基因。体外表型实验验证了这一假说（p<0.03）：临床隐球菌分离株获得了选择性生存优势，使其耐热性提升，进而增强其在人体宿主内的定植能力。基因共表达网络分析强调了细胞壁完整性通路在热应激应答中的关键作用，同时还揭示了其他相关生物学机制，包括细胞氧化应激应答、脂肪酸β氧化、高尔基体衍生分泌通路以及蛋白质生物合成过程。对隐球菌基因组的选择压力分析共鉴定得到19个处于正向选择下的基因，且这些基因仅在临床VNB菌株中呈上调表达。最后，本研究提出，倍性改变是VNB型别感染建立的关键影响因素：70%的临床分离株存在染色体拷贝数变异（Chromosomal Copy Number Variations, CCNV），这或可解释该高度遗传多样性分子型别的强致病性。