Snyder et al. recently published an intriguing report in Molecular Cell (21 March 2013) (featured free article) in which they convincingly demonstrate how tumors in which Nkx2-1 was deleted show striking different morphology from those in which Nkx2-1 is expressed.
Nkx2-1/TTF-1 is a highly conserved homeodomain-containing transcription factor that is expressed at the onset of lung and thyroid development and has been shown from murine knockout experiments to be required for branching morphogenesis in very early lung development. Nkx2-1 is expressed in about 80% of human lung adenocarcinomas (ADCs). Recent studies have shown that Nkx2-1-negative ADCs have a worse prognosis and have a higher histologic grade (more poorly differentiated) compared to Nkx2-1-positive tumors. These data suggest that Nkx2-1 may coordinate a lineage-specific differentiation program on lung adenocarcinomas that restrains their malignant potential. Further data have shown that the Nkx2-1 gene is amplified in 10%–15% of human lung ADCs, suggesting that it can also act as a lineage-survival oncogene in a subset of tumors. The authors' previous work using a mouse model for lung ADC showed that Nkx2-1 restrained the ability of Kras-driven lung tumors to evolve to a poorly differentiated, Hmga2-positive state.
The authors cleverly designed a mouse model in which Cre recombinase can activate a conditional allele of Kras G12D and delete both alleles of Nkx2-1 from the lungs of mice. Simultaneous activation of Kras G12D and deletion of Nkx2-1 yielded peripheral lung ADCs that showed a distinct glandular growth pattern of malignant cells with abundant MUC5A-positive mucin resembling human lung mucinous ADC. In contrast, control Nkx2-1-positive mice developed tumors showing a predominantly papillary non-mucinous pattern. They also showed in a different model that in established tumors, Nkx2-1 deletion induced a glandular pattern and mucin production similar to de novo tumors and promoted long-term tumor growth. In addition, this study demonstrated that Nkx2-1 deletion induced diffuse alveolar epithelial hyperplasia in the alveolar space. The hyperplastic cells lacked expression of canonical Nkx2-1 targets and exhibited increasing mucinous accumulation and morphology over time.
The authors compared Kras-mutant/Nkx2-1-deleted tumors and Nkx2-1-positive tumors by mRNA expression profiling and found 1,828 differentially expressed genes, including 1,137 upregulated and 691 downregulated genes. MetaCore analysis for disease-specific gene collections and networks identified genes associated with gastric diseases that were enriched in ‘‘stomach neoplasms’’ and ‘‘stomach diseases’’ as categories that were enriched in Kras-mutant/Nkx2-1-deleted tumors. Finally, they determined whether human mucinous lung adenocarcinomas also exhibit evidence of gastric differentiationby evaluating the expression of two stomach-restricted proteins (GKN1 and CTSE) in 37 human lung adenocarcinomas. In 11 NKX2-1-negative mucinous human lung adenocarcinomas (n = 11), they found that GKN1 was expressed in 6 out of 11 cases and that CTSE was strongly and diffusely expressed in all 11 tumors. In contrast, NKX2-1-positive lung adenocarcinomas (n = 26) were entirely negative for GKN1. Most NKX2-1-positive lung tumors were either negative (n = 14) or only focally positive (n = 10) for CTSE.
There is much more to unpack from this article than I can concisely summarize in this post. The fascinating point for me is that lung alveolar epithelial cells have a latent gastric differentiation program that becomes activated by default when Nkx2-1/TTF-1 is deleted and this is most dramatically evinced in pulmonary mucinous adenocarcinomas. These particular tumors are nettlesome diagnostically because of the question raised of whether the tumor is primary or metastatic.
Wonderful paper and lots to think about!