High nerve density in breast cancer is associated with poor patient outcome

Active crosstalk between the nervous system and breast cancer cells has been experimentally demonstrated in vitro and in animal models. However, low frequencies of peripheral nerve presence in human breast cancers reported in previous studies (~30% of cases) potentially negate a major role of the nervous system in breast cancer development and progression. This study aimed to clarify the incidence of nerves within human breast cancers and to delineate associations with clinicopathological features. Immunohistochemical staining was conducted in formalin‐fixed paraffin‐embedded breast cancer tissue sections using antibodies against the pan‐neuronal markers protein gene product 9.5 and growth‐associated protein 43, and the sympathetic nerve‐specific marker tyrosine hydroxylase. Nerve trunks and isolated nerve fibers were quantitated. The chi‐squared test was used to determine the associations between nerve counts and clinicopathological parameters. The log‐rank test was used to compare differences in patient progression‐free survival (PFS) and overall survival (OS). The overall frequency of peripheral nerves in breast cancers was 85%, a markedly higher proportion than reported previously. Of note, most nerves present in breast cancers were of the sympathetic origin. While high density of nerve trunks or isolated nerve fibers was associated with poor PFS and OS of patients, high nerve trunk density appeared also to predict poor patient PFS independently of lymph node metastasis. Innervation of breast cancers is a common event correlated with poor patient outcomes. These findings support the notion that the nervous system plays an active role in breast cancer pathogenesis.