Spontaneous point mutations in the capsule synthesis locus leading to structural and functional changes of the capsule in serogroup A meningococcal populations

Whole genome sequencing analysis of 100 <i>Neisseria meningitidis</i> serogroup A isolates has revealed that the <i>csaABCD-ctrABCD-ctrEF</i> capsule polysaccharide synthesis locus represents a spontaneous point mutation hotspot. Structural and functional properties of the capsule of 11 carriage and two disease isolates with non-synonymous point mutations or stop codons in capsule synthesis genes were analyzed for their capsular polysaccharide expression, recognition by antibodies and sensitivity to bactericidal killing. Eight of eleven carriage isolates presenting capsule locus mutations expressed no or reduced amounts of capsule. One isolate with a stop codon in the O-acetyltransferase gene expressed non-O-acetylated polysaccharide, and was not recognized by anti-capsule antibodies. Capsule and O-acetylation deficient mutants were resistant to complement deposition and killing mediated by anti-capsular antibodies, but not by anti-lipopolysaccharide antibodies. Two capsule polymerase mutants, one carriage and one case isolate, showed capsule over-expression and increased resistance against bactericidal activity of both capsule- and lipopolysaccharide-specific antibodies. Meningococci have developed multiple strategies for changing capsule expression and structure, which is relevant both for colonization and virulence. Here we show that point mutations in the capsule synthesis genes substantially contribute to the repertoire of genetic mechanisms in natural populations leading to variability in capsule expression.

对100株血清群A脑膜炎奈瑟菌（Neisseria meningitidis）分离株开展全基因组测序分析后发现，csaABCD-ctrABCD-ctrEF荚膜多糖合成基因座（capsule polysaccharide synthesis locus）属于自发点突变热点区域。本研究针对11株定植分离株（carriage isolates）与2株致病分离株的荚膜结构与功能特性展开分析，这些菌株的荚膜合成基因均存在非同义点突变（non-synonymous point mutations）或终止密码子（stop codons），检测指标涵盖荚膜多糖表达水平、抗体识别能力以及杀菌敏感性。11株携带荚膜基因座突变的定植分离株中，有8株未表达荚膜或仅表达少量荚膜多糖。1株在O-乙酰转移酶基因中携带终止密码子的分离株，其荚膜多糖未发生O-乙酰化修饰，且无法被抗荚膜抗体（anti-capsule antibodies）识别。荚膜及O-乙酰化缺陷突变株可抵御抗荚膜抗体介导的补体沉积（complement deposition）与杀菌作用，但无法抵御抗脂多糖（lipopolysaccharide）抗体的效应。2株荚膜聚合酶（capsule polymerase）突变株（1株为定植分离株，1株为致病分离株）表现出荚膜高表达表型，且对荚膜特异性与脂多糖特异性抗体介导的杀菌活性均表现出更强的抗性。脑膜炎奈瑟菌已演化出多种调控荚膜表达与结构改变的策略，该策略与其定植（colonization）及致病过程均密切相关。本研究证实，荚膜合成基因的点突变可作为自然种群中一类重要的遗传机制，显著促成荚膜表达的多样性。