PD-2 In vivo targeting delivery of miR-26a inhibits tumor growth and protects host from hematological toxicity of chemotherapy in breast cancer
Authors: Tanno Toshihiko , Zhang Peng , Lazarski Christopher , Liu Yang , Zheng Pan 
Published: April 2019

Breast cancer is the most common malignant disease in women. In the United States, it is estimated that 252,710 women will be diagnosed with invasive breast cancer, and 40,610 women will die of breast cancer in 2017. Recent molecular-based classification categorizes the breast cancer into HER-2+, luminal-like, normal-like and basal-like breast cancer. The 12%–37% of breast cancer patients are classified into the basal-like breast cancer. The basal-like breast cancer has an unfavorable prognosis with high risk of early relapse within the first 2–5 years treatment, resulting in lower 5-year survival rate than other types of breast cancer due to lack of effective target therapy. Thus, development of novel therapeutics is urgently needed.

The recent rapid expansion of genomic data greatly contributes to the understanding of disease development and progression. To translate these genetic discoveries into clinical applications, we need to develop therapeutic platforms that can specifically modulate the expression of disease-related genes in vivo. Small RNAs, such as microRNAs (miRNAs) and siRNAs function in messenger RNA silencing and post-transcriptional regulation of gene expression. Accumulating evidences have demonstrated the therapeutic potential of these small RNAs that modulate the gene expressions in many diseases including cancers. However, difficulties of in vivo delivery to specific cells have limited the potential utility of miRNA/siRNA in cancer therapy.

Given the challenge of in vivo delivery, we developed a small RNAs targeting delivery platform, comprising a miRNA/siRNA sequence, a cell surface receptor-targeting DNA aptamer (a “chemical antibody”) and a complimentary passenger sequence of the miRNA/siRNA conjugated with cholesterol. These oligonucleotide sequences were heavily modified to prevent the degradation by nucleases in serum. Our newly designed delivery platform enables target-specific delivery of the small RNAs into desired cells and tissues in vivo.

Our in silico gene expression analysis using data from the breast cancer and normal tissues in The Cancer Genome Atlas (TCGA) database revealed that the suppression of miR-26a expression and overexpression of a receptor tyrosine kinase, c-KIT in the basal-like breast cancer were significantly associated with disease outcome. Therefore, we hypothesized that restoration of miR-26a loss in the cancer cells using our small RNA delivery platform can inhibit the cancer growth. To test this hypothesis, we developed c-KIT targeting miR-26a delivery therapeutic using a c-KIT targeting aptamer (termed “miR-26a chimera”). This miR-26a chimera significantly inhibited the tumor growth of c-KIT+ basal-like breast cancer cells by silencing an oncogene, EZH2 in xenograft models. Interestingly, the miR-26a chimera not only enhanced the anti-tumor effect of chemotherapy (5-FU or carboplatin) in its combinational use, but also ameliorated myelosuppresive adverse effects of the chemotherapy by protecting c-Kit+ hematopoietic progenitor cells from apoptosis via silencing a pro-apoptotic gene, Bak1. By preventing the major adverse effect of chemotherapy, our new approach may allow even broader use of chemotherapeutic drugs for the advanced breast cancer.

Although our study focused on breast cancer models, miR-26a as a tumor suppressor has been observed in other cancer types, including prostate cancer, pancreatic cancer and lung cancer. Likewise, the c-KIT is also widely expressed among human cancers, including gastrointestinal stromal tumors, myeloid leukemia, small-cell lung cancer, prostate cancer, pancreatic cancer, ovarian cancer and glioblastoma. Therefore, the clinical significance of our miR-26a chimera targeting c-KIT+ cancers could extend well beyond breast cancer. Besides, our targeting delivery platforms will provide not only novel therapeutics for a wide range of diseases, but also research tools to understand the roles of disease-related genes in specific tissues in vivo.