Rocio Sotillo M.D.

This grant is being fully funded by the Thomas G. Labrecque Foundation, through the Uniting Against Lung Cancer Grant Program.

Lay Description

Lung cancer is the leading cause of cancer-related deaths worldwide. Lung cancer cells have been shown to carry frequent structural chromosomal abnormalities such as deletions of specific regions or numerical changes in their chromosome numbers, a phenomenon known as aneuploidy. Moreover, chromosomal instability (CIN) has been shown to be a risk factor for poor prognosis of lung cancer.

Aneuploidy is generally assumed to be a consequence of a deregulated Spindle Assembly Checkpoint (SAC), a surveillance mechanism that ensures that all the chromosomes are correctly distributed between the two daughter cells. Several proteins have been shown to play a role in this checkpoint and our laboratory has focused on two of these, Mad2 and Hec1, since both proteins are frequently found upregulated in human tumors. Previous work from our lab has shown that Hec1 or Mad2 overexpression leads to tumor formation in animal models.

To further examine if the chromosomal instability generated by high levels of Mad2 plays a role in lung tumorigenesis, we decided to study if Mad2 cooperates with a mutant form of the KRAS oncogene, a gene frequently mutated and participant in human tumor pathogenesis. We crossed mice that overexpress Mad2 and mutant Kras specifically in lung epithelial cells. Preliminary data suggest that chromosomal instability generated by high levels of Mad2 is involved in lung cancer development, since 1) in a murine mouse model Mad2 overexpression is synergistic with a Kras activating mutation in accelerating lung cancer development and 2) 45% of these mice develop lung recurrences after inhibition of the oncogene.

Lung tumors derived from these transgenic mice provide an ideal opportunity to identify additional genes that participate in the initial steps of lung tumorigenesis. Furthermore, comparison with “control” lung tumors derived from Kras transgenic mice will help to identify which pathways and genes are important for CIN dependent tumor relapse.

The overall goal of this proposal is to generate new mouse models that combine chromosomal instability with other genes that are known to be important in lung tumorigenesis. These models will serve to further our understanding of the role of CIN in tumor progression and relapse and identify new mechanisms of resistance to therapies. To that end, we are using transgenic mice that overexpress Hec1 or Mad2 in combination with mice that express mutant KRAS or EGFR. Mutations in the EGF-receptor are commonly found in lung tumor patients who are successfully treated with Gefitinib. However, most patients eventually relapse on treatment, often with a more aggressive disease.

Growing evidence suggests that CIN plays a major part in the pathogenesis of lung cancer. Nevertheless, what particular roles CIN plays in the initiation and progression of this disease remains unknown. Perhaps more importantly, the design of targeted therapies for specific types of lung cancer is likely to be faced with the challenge of therapeutic resistance. How CIN influences the response to these therapies will be crucial to determine the most efficient way of combating lung cancer.

Scientific Abstract

Lung cancer is the leading cause of cancer-related deaths worldwide with non small cell lung cancer (NSCLC) accounting for 80% of lung cancer mortality. Abnormal chromosome numbers are frequently observed in lung cancer and this often correlates with tumor grade and prognosis. Moreover, chromosomal instability (CIN) in surgical specimens of NSCLC is associated with a decreased survival. Though the exact contribution of CIN to tumor progression remains difficult to prove it is becoming clear that CIN can contribute to tumor development and progression by facilitating the loss of tumor suppressor genes as well as favoring polysomy of chromosomes that harbor growth-promoting genes.

Aneuploid cells can result from a deregulated Spindle Assembly Checkpoint (SAC), a surveillance mechanism that delays anaphase until all kinetochores are correctly attached to the mitotic spindle. Work from our lab and others has shown that both down regulation as well as overactivation of the SAC leads to lung tumorigenesis in several mouse models, implying that aneuploidy is particularly effective in driving tumor progression in lung epithelial cells. Mad2, an integral component of the spindle assembly checkpoint, and Hec1, a key component of the Ndc80 complex thought to mediate microtubule to kinetochore attachment, are frequently upregulated in human lung tumors. Moreover, Mad2 is an E2F target and its upregulated upon deregulation of the Retinoblastoma pathway. Overexpression of these genes in doxycycline inducible transgenic mice leads to a wide spectrum of tumors, with lung adenocarcinomas among the most prevalent. In addition, we have shown that these tumors are highly aneuploid, supporting the causal role at aneuploidy driving the pathologic process.

Our preliminary data suggest that chromosomal instability generated by high levels of Mad2 is involved in lung cancer development, since 1) in a murine mouse model Mad2 overexpression is synergistic with a Kras activating mutation in accelerating lung cancer development and 2) 45% of these mice develop lung recurrences after oncogene withdrawal.

Lung tumors derived from these transgenic mice provide an ideal opportunity to identify additional genes that participate in the initial steps of lung tumorigenesis. In addiction they will help to identify which pathways and genes are important for CIN dependent tumor relapse.

We aim to extend our preliminary findings that CIN plays a role in tumor progression and relapse by generating new mouse models that combine CIN with other mutations that are known to be important in lung tumorigenesis. To that end we are using transgenic mice that overexpress Hec1 or Mad2 in combination with mice that express mutant KRAS or EGFR. Mutations in the EGF-receptor are commonly found in lung tumor patients although they can be successfully treated with Gefitinib. However most patients eventually relapse with a more aggressive disease. The fact that CIN present in the primary tumor predisposes mice to relapse represents a difficult challenge to the emerging field of targeted therapies. It is now clear that combined approaches may be more advantageous than single agent ones but in vivo models of ‘oncogene escape’ in the face of CIN are lacking. For that reason, I aim to study the behavior of escape from oncogene addiction in an EGFR transgenic model where the oncogene can be inhibited pharmacologically.

It is important to elucidate how the presence of CIN in lung cancer cells contributes to tumor relapse and resistance to therapy. Clarification of the molecular bases and biological significance of CIN provide important clues for the development of new therapeutic approaches for this fatal disease.