Pretreatment Chromosomal Instability Correlates With Radiation Sensitivity in Squamous Cell Cancers

Continuous chromosome missegregation over successive mitotic divisions, known as chromosomal instability (CIN), is common in cancer. Though it has been associated with treatment resistance and poor prognosis, increasing CIN above a maximally tolerated threshold leads to cell death because of loss of essential chromosomes. Because radiation causes CIN, we hypothesize that pre-existing CIN sensitizes tumor cells to radiation therapy.

We induced mitotic defects that lead to CIN in FaDu (head and neck cancer, HNC) and HeLa (cervical) cells by knocking down or overexpressing the mitotic checkpoint protein mitotic arrest deficient 1 (Mad1), which induces lagging chromosomes. Radiation sensitivity was tested with clonogenic assays in vitro and tumor regression in patient-derived xenografts in vivo. MTT assays were used to determine the sensitivity of human papillomavirus (HPV) positive and HPV-negative HNC cells to docetaxel, and mitotic defects were quantified using immunofluorescence microscopy. Docetaxel-induced mitotic errors and tumor growth delay were evaluated in vivo. Six-chromosome fluorescence in situ hybridization was used to quantify CIN in a cohort of patients with laryngeal cancer treated with definitive radiation.

Here, we show in two tissue contexts using engineered isogenic cancer cell lines that higher rates of chromosome missegregation sensitize to ionizing radiation, which itself induces mitotic errors. Consistent with this result, higher rates of anaphase defects in HPV-positive and HPV-negative HNC patient-derived xenograft tumors correlate with response to radiation. Moreover, laryngeal tumors with higher CIN before treatment tend to have an improved response to radiation therapy in the clinic. Furthermore, we show that docetaxel, a microtubule-stabilizing drug commonly used in combination with radiation, causes cell death and radiosensitizes cells by inducing abnormal multipolar spindles rather than causing mitotic arrest.

These results mechanistically implicate CIN as an inducer of radiation response and provide evidence that increasing the rate of CIN is a rational method to enhance radiation sensitivity, which has significant implications for personalized therapy.