Clinical Implication Of Leucine Zipper/EF Hand-containing Transmembrane-1 Overexpression In The Prognosis Of Triple-negative Breast Cancer

Background: Breast cancer(BC) is the most common malignant tumor among females in China and the leading cause of cancer-related death worldwide. Triplenegative breast cancer(TNBC) is defined by the absence of detectable estrogen receptor(ER) and progesterone receptor(PR) expression and the lack of human epidermal growth factor 2(HER2) gene amplification, using immunohistochemical(IHC) analysis. Representing 12-17% of all BC, TNBC is generally more aggressive, with higher rates of relapse and metastasis, lower survival rates and worse survival in metastatic BC. Besides, due to TNBC is found to respond poorly or not at all to endocrine therapy, at present, treatment is mainly based on operation and supplemented by systematic chemotherapy. However, because of the significant heterogeneity of TNBC and poor prognosis, it is urgent to find effective biomarker and therapeutic target.LETM1 is one of the mitochondrial inner membrane proteins that is conserved between yeast and humans, which can reduce the mitochondrial biogenesis and mitochondrial ATP production. The disorder of LETM1 expression levels could trigger dysfunction of mitochondria, which has been associated with various human diseases including cancers. The expression levels of LETM1 were markedly increased in various human malignant tumors, however, the clinicopathological characteristics and prognostic value of LETM1 overexpression in TNBC remains unclear. The present study aims to investigate the significance of LETM1 overexpression on prognostic evaluation of TNBC.Methods: A total of 214 breast tissue samples were used for this study. One hundred and seven(107) paraffin embedded TNBC samples, 42 ductal cancer in situ(DCIS) and 65 adjacent non-tumor tissues were included. Firstly, immunofluorescence(IF) staining analysis was used to detect the cellular localization of LETM1 protein in MCF-7 BC cells. Then we measured the relative quantitation of LETM1 in TNBC using western blot method, for further validating whether LETM1 protein expression level increases in TNBC. Lastly, immunohistochemical staining(IHC) was performed to detect the expression of LETM1 in TNBC, DCIS and normal tissues, and analyze its clinicopathological implication.Results: 1. LETM1 protein located in the cytoplasm in MCF-7 BC cells by IF staining, which was consistent with our IHC staining results in TNBC tissues. 2. Western blot data demonstrated that LETM1 protein was highly expressed in TNBC tissues compared with adjacent non-tumor tissues. 3. IHC staining showed that the positive rate of LETM1 expression in TNBC was 85.0%(91/107), which was significantly higher than that in DCIS(64.3%, 27/42) and adjacent non-tumor tissues(29.2%, 19/65)(P < 0.001). Similarly, the strongly positive rate of LETM1 expression in TNBC was 69.2%(74/107), which was also significantly higher than that in DCIS(35.7%, 15/42) and adjacent non-tumor tissues(12.3%, 8/65)(P < 0.001). More importantly, the positive rate of LETM1 in DCIS was also significantly higher(64.3%, 27/42) than adjacent non-tumor tissues(29.2%, 19/65), and the strongly positive rate was also higher in DCIS(P < 0.001). Besides, the strongly positive rate of LETM1 was significantly higher in Grade 2(27/37, 73.0%) and Grade 3(34/43, 79.1%) TNBC tissues than in Grade 1 cases(13/27, 48.1%)(P both =0.020), and it was also higher in TNBC with metastasis(82.3%) than in cases without metastasis(51.1%)(P=0.001). Similarly, the strongly positive rate of LETM1 in advanced stage(III–IV) TNBC was 88.4%(38/43), but only 56.3%(36/64) in early stage(0–II) cases(P = 0.000). However, there were no significant correlations between the expression level of LETM1 protein and patient age, menopausal status, and tumor size in patients with TNBC(P 均>0.5). In addition, survival rate revealed that patients with high LETM1 expression had lower DFS and 10-year OS than those with low LETM1 expression(P = 0.000).Conclusions: High level of LETM1 demonstrates a strong association with progression and prognosis of TNBC patients. LETM1 may be of value as a potential biomarker and therapeutic target for patients with TNBC.