Background: The calpains are intracellular cysteine proteases that function in a variety of important cellular functions, including signalling, motility, apoptosis and survival. In breast cancer high calpain-1 and calpain-2 expression has been associated with adverse clinical outcome. Calpain-9 was thought to be exclusively expressed in the digestive tract; however recent studies have shown that this protein is also expressed in breast tissue.
Methods: We investigated the expression of calpain-9 in a large cohort of early stage breast cancer patients (n = 783) using immunohistochemistry on a tissue microarray. Patients had long-term follow-up information available for analysis.
Results: Low expression of calpain-9 was associated with patients over 40 years of age (P = 0.025), smaller tumour size (P = 0.001), lower tumour stage (P = 0.009), a more favourable Nottingham Prognostic Index value (P = 0.002) and positive oestrogen receptor status (P = 0.014). Calpain-9 expression was not associated with survival in the total patient cohort, however low calpain-9 expression was associated with adverse survival in patients who received endocrine therapy (P = 0.033), which remained significant in multivariate Cox regression analysis accounting for potential confounding factors (hazard ratio (HR) = 0.56, 95% confidence interval (95% CI) = 0.36-0.89, P = 0.013). Low calpain-9 expression was also associated with adverse survival in patients with an intermediate Nottingham Prognostic Index value (P = 0.009), and remained so in multivariate analysis (HR = 0.54, 95% CI = 0.36-0.82, P = 0.003).
Conclusions: This study suggests that calpain-9 may play a role in breast cancer and that low expression is associated with poorer patient clinical outcome following endocrine therapy. Validation studies are warranted as determining expression of calpain-9 may provide important prognostic information.