Molecular biology of lung cancer: clinical implications

Abstract

Lung cancer is a heterogeneous disease clinically, biologically, histologically, and molecularly. Understanding the molecular causes of this heterogeneity, which might reflect changes occurring in different classes of epithelial cells or different molecular changes occurring in the same target lung epithelial cells, is the focus of current research. Identifying the genes and pathways involved, determining how they relate to the biological behavior of lung cancer, and their utility as diagnostic and therapeutic targets are important basic and translational research issues. This article reviews current information on the key molecular steps in lung cancer pathogenesis, their timing, and clinical implications.

Figure 1. Oncogene addiction and synthetic lethality in targeting acquired tumor cell vulnerability

A) Oncogene addiction. A tumor cell contains many abnormalities in oncogenes and tumor suppressor genes (TSGs) however while some gene mutations may be critical for tumor cell survival (“driver” mutations) other gene mutations are not (“passenger” mutations). Inactivation of a critical “driver” gene in a tumor cell will result in cell death or differentiation into a normal phenotype. Inactivation of non-critical “passenger” mutations however, will not affect the tumor cell. B) Synthetic lethality arises when inactivation of two of more genes (A + B) leads to cell death whereas inactivation of either gene alone does not affect viability of the cell as the remaining gene acts in a compensatory manner. C) Synthetic lethality to target tumor cells. If a tumor cell has a non-drugable oncogene or inactivation of a TSG (Gene A), the cell will be vulnerable to inactivation of Gene B whereas a normal cell will not thus creating a second therapeutic target in addition to targeting the “driver” mutation. Adapted from^,,.

Figure 2. EGFR mutations found in lung cancer

Activating mutations, which are found with increased frequency in certain subsets of lung cancer patients, occur as three different types of somatic mutations – deletions, insertions, and missense point mutations – and are located in exons 19–21 which code for the tyrosine kinase domain of EGFR^,. Mutant EGFRs (either by exon 19 deletion or exon 21 L858R mutation) show an increased amount and duration of EGFR activation compared with wildtype receptors, and have preferential activation of the PI3K/AKT and STAT3/STAT5 pathways rather than the RAS/RAF/MEK/MAPK pathway. EGFR mutant tumors are initially highly sensitive to EGFR tyrosine kinase inhibitors (TKIs)^– however, despite an initial response, patients treated with EGFR TKIs eventually develop resistance to TKIs which is linked (in approximately 50% tumors) to the acquiring of a second mutation at T790M in exon 20^,,–. Interestingly, the presence of the T790M mutation in a primary lung cancer that had not been treated with EGFR-TKIs however, suggests that this resistance mutation may develop with tumor progression and not necessarily as a response to treatment. Adapted from^,.

Figure 3. The RAS/RAF/MEK/MAPK pathway

The RAS proto-oncogene family (KRAS, HRAS, NRAS and RRAS) encode four highly homologous 21kDa membrane-bound proteins involved in signal transduction. Proteins encoded by the RAS genes exist in two states: an active state, in which GTP is bound to the molecule and an inactive state, where the GTP has been cleaved to GDP. Activating point mutations can confer oncogenic potential through a loss of intrinsic GTPase activity resulting in an inability to cleave GTP to GDP. This can initiate unchecked cell proliferation through the RAS/RAF/MEK/MAPK pathway, downstream of the EGFR signaling pathway. Ras signaling also activates the PI3K/AKT pathway (leading to cell growth, proliferation, and survival), RalGDS and RASSF1. Adapted from^,.

Figure 4. The PI3K/AKT/mTOR pathway

Downstream targets of AKT are involved in cell growth, angiogenesis, cell metabolism, protein synthesis, and suppression of apoptosis directly or via the activation of mTOR. Activation of the PI3K/AKT pathway can occur through the binding of the SH2-domains of p85, the regulatory subunit of PI3K, to phosphotyrosine residues of activated RTKs such as EGFR. Alternatively, activation can occur via binding of PI3K to activated RAS. Mutation and more commonly, amplification of PIK3CA, which encodes the catalytic subunit of phosphatidylinositol 3-kinase (PI3K), occurs most commonly in squamous cell carcinomas^,,,. AKT, a serine/threonine kinase that acts downstream from PI3K can also have mutations that lead to pathway activation. One of the primary effectors of AKT is mTOR, a serine/threonine kinase involved in regulating proliferation, cell cycle progression, mRNA translation, cytoskeletal organization, and survival. The tumor suppressor PTEN, which negatively regulates the PI3K/AKT pathway via phosphatase activity on phosphatidylinositol 3,4,5-trisphosphate (PIP3), a product of PI3K is commonly suppressed in lung cancer by inactivating mutations or loss of expression^,.

Figure 5. The p53 and RB pathways

Regulation of p53 can occur through the MDM2 oncogene which reduces p53 levels through degradation by ubiquitination. MDM2 can in turn be inhibited by the tumor suppressor p14^ARF, an isoform of CDKN2A. As such, the genes that encode MDM2 and p14^ARF are commonly altered in lung cancer through amplification and loss of expression, respectively^–. The CDKN2A/RB1 pathway controls G1 to S phase cell cycle progression. RB acts as a tumor suppressor by acting with E2F proteins to repress transcription of genes necessary for the G1-S phase transition. RB is inhibited by hyperphosphorylation by CDK-CCND1 complexes (complexes between CDK4 or CDK6 and CCND1), and in turn, formation of CDK-CCND1 complexes can be inhibited by the p16 isoform of CDNK2A. Nearly all constituents of the CDKN2A/RB pathway have been shown to be altered in lung cancer through mutations (CDK4 and CDKN2A), deletions (RB1 and CDKN2A), amplifications (CDK4 and CCDN1), methylation silencing (CDKN2A and RB1), and phosphorylation (RB)^–.

Figure 6. Stem cell self-renewal pathways and therapeutic strategies to block these pathways in cancer

Notch, Wnt, and Hedgehog (Hh) are stem cell self-renewal pathways that are often deregulated and aberrantly activated in lung cancer, thus representing key therapeutic targets. The hedgehog pathway signals through Hh ligands binding to the Patched (PTCH) receptor and inhibiting its repression of Smoothened (SMO), allowing SMO activation which results in nuclear translocation of GLI transcription factors. Wnt signaling functions through Wnt ligands binding to the Frizzled (FZD) receptor and signaling through disheveled (DSH) leading to the stabilization of β-catenin. In the absence of Hh or Wnt ligands, GSK3 phosphorylates GLI1/2 and β-catenin, respectively, resulting in ubiquitination and degradation. Notch signaling functions through Notch ligands (DLL and JAG) binding to the Notch receptor which results in the cleavage of Notch intracellular binding domain (NICD) by γ-secretase enabling it to translocate to the nucleus, bind to CLS transcription factors and activate transcription. Some components of the pathways were omitted (dashed lines) for simplicity. Adapted from^,.