The spatio-temporal evolution of lymph node spread in early breast cancer

The most significant prognostic factor in early breast cancer is lymph node involvement. This stage between localised and systemic disease is key to understanding breast cancer progression, however our knowledge of the evolution of lymph node malignant invasion remains limited, as most currently available data derives from primary tumours. In 11 treatment-naïve node positive early breast cancer patients without clinical evidence of distant metastasis, we investigated lymph node evolution using spatial multi-region sequencing of primary and lymph node deposits and genomic profiling of matched longitudinal circulating tumour DNA (ctDNA). Linear evolution from primary to lymph node was rare (1/11) whereas the majority of cases displayed either early divergence between primary and nodes (4/11), or no detectable divergence (6/11) where both primary and nodal cells belonged to a single recent expansion of a metastatic clone. Divergence of metastatic clones was driven in part by APOBEC. Longitudinal ctDNA from 2 subjects taken peri-operatively reflected the two major evolutionary patterns and demonstrates, even in this early disease cohort, the principle of detection of private mutations from an early metastatic nodal deposit. This study sheds new light on a crucial evolutionary step in the natural history of breast cancer.

早期乳腺癌最重要的预后因素为淋巴结受累（lymph node involvement）。这一介于局限性疾病与全身性疾病之间的阶段，是解析乳腺癌进展机制的关键环节，但目前我们对淋巴结恶性侵袭演化过程的认知仍较为有限，因现有多数数据均来源于原发肿瘤。本研究纳入11例无远处转移临床证据的初治淋巴结阳性早期乳腺癌患者，通过对原发肿瘤及淋巴结病灶开展空间多区域测序，并对匹配的纵向循环肿瘤DNA（circulating tumour DNA，简称ctDNA）进行基因组特征分析，以此探究淋巴结的演化过程。从原发肿瘤到淋巴结的线性演化模式极为罕见（仅1/11例）；多数病例要么表现为原发肿瘤与淋巴结之间存在早期分化（4/11例），要么未检测到明显分化（6/11例），此时原发肿瘤与淋巴结肿瘤细胞均起源于转移克隆的单次近期扩增。转移克隆的分化部分由APOBEC介导。对2例患者围手术期采集的纵向ctDNA样本的检测结果契合上述两种主要演化模式，且即便在该早期疾病队列中，也证实了可从早期转移性淋巴结病灶中检出私有突变的可行性。本研究为乳腺癌自然病程中这一关键演化环节提供了全新的认知视角。