31_01_01

Chapter 31: Cancer of the Lung
31.1: Molecular Biology of Lung Cancer

Yoshitaka Sekido
Kwun M. Fong
John D. Minna

Cancer: Principles and Practice of Oncology, 6th Edition
Published by Lippincott Williams & Wilkins, Copyright 2001

Lung cancer cells have accumulated a number of molecular genetic and
epigenetic lesions, which appear necessary to transform normal
bronchial epithelium to an overt lung cancer. There is complex
interaction between the various molecular changes that ultimately
result in the abrogation of key cellular regulatory and growth control
pathways. Of the three major classes of human "cancer" genes, the
protooncogenes and tumor suppressor genes (TSGs) are involved in lung
carcinogenesis, whereas evidence implicating DNA repair genes is not
yet conclusive. Many of the protooncogene and TSG changes are present
in both major lung cancer subtypes: small cell lung cancer (SCLC) and
non-small cell lung cancer (NSCLC), although certain mutations have
subtype specificity (Table 31.1_1). Protooncogenes generally encode
proteins that are positive effectors of the transformed phenotype and
can simplistically be considered positive growth regulators. Their
"activation" results in their functional deregulation, leading to a
gain in function or "dominant" effect. Conversely, TSG products are
negative growth regulators and their "inactivation" results in a loss
of function that contributes to malignancy. Interacting with yet other
biologic changes, these fundamental molecular events appear to underlie
the characteristics of dysregulated growth, clonal expansion, and
immortality, which are typical of overt lung cancers. In addition,
these, and yet other to be discovered molecular changes, may affect the
processes of invasion, metastasis, and resistance against cancer
therapy. In translating these laboratory discoveries into the clinic,
it is important to identify these various changes, determine the
frequency of occurrence, and test whether they have clinically
important associations (e.g., with histologic type, stage, survival,
response to therapy), as well as to determine if they could be used for
early diagnosis, to monitor prevention and treatment efforts, and as
targets for the development of new treatments. In addition, these
abnormalities will probably also give us important understanding about
lung development and differentiation.

Genetic and Epigenetic Alterations in Lung Cancers

Chromosomal Abnormalities

Lung cancer cells display numerical abnormalities (aneuploidy) of
chromosomes, which are suggestive of allele loss or gain, as well as
structural cytogenetic abnormalities. The latter include nonreciprocal
translocations and deletions, whereas the presence of double minutes
and homogeneously staining regions indicate gene amplification, such as
for the MYC gene family. [ref: 1] In SCLCs, losses from chromosomes 3p,
5q, 13q, and 17p predominate, but double minutes may be common late in
disease. In NSCLCs, deletions of 3p, 9p, and 17p, together with +7,
i(5)(p10), and i(8)(q10) are often seen. Molecular cytogenetic analysis
with comparative genomic hybridization has identified hitherto
unrecognized abnormalities, including deletions at 10q26, 16p11.2, and
22q12.1-13.1 and amplification at 1q24, 3q, 5p, 17q, and Xq26.
It has been proposed that human tumors may be genetically unstable at
two levels: at the chromosomal level, including losses and gains
(amplification), and at the DNA nucleotide level, including single or
several base changes. [ref: 2] It will thus be important to determine
if aneuploidy and structural cytogenetic abnormalities, apart from
targeting key genes, actually represent the phenomenon of chromosomal
instability in lung cancers.

Microsatellite Instability

A genetic change that manifests itself as a mutator phenotype (often
called the replication error repair phenotype) in human cancers results
in widespread microsatellite instability. The result of microsatellite
instability is a "laddering" of short-tandem DNA repeat sequences at
multiple loci seen on high-resolution polyacrylamide electrophoretic
gels. This phenotype is usually due to mutational inactivation of DNA
mismatch repair enzymes, resulting in marked instability of these
polymorphic DNA repeat sequences. This phenotype was initially reported
in hereditary nonpolyposis colon cancers. Lung cancer frequently
exhibits microsatellite instability; however, this occurs at only a few
loci and results only in "shifts" of individual allelic bands
("microsatellite alterations") compared to normal DNA in the same
patient. The abnormal mechanism underlying this phenotype is currently
unknown, and apparently mutations in DNA mismatch repair enzymes are
very uncommon in lung cancer. The human 8-oxoguanine DNA glycosylase
(hOGG1) gene, involved in the repair of oxidative DNA damage, is
another candidate for involvement in generating multiple lung cancer
mutations. However, abnormalities in this gene only rarely occur in
lung cancer, and mutations in other DNA repair genes have not yet been
reported in lung cancer. Overall, approximately 35% (37 of 106) of
SCLCs and 22% (160 of 727) of NSCLCs showed some examples of
microsatellite alterations at individual loci. [ref: 3] Microsatellite
alterations in lung cancer have been reported to be associated with
younger age, reduced survival, and advanced tumor stage. Regardless of
the underlying mechanism, many groups are testing the possibility of
using this microsatellite alteration phenotype for the early diagnosis
of lung cancer by detecting these shifted DNA bands in sputum,
bronchial washings, or blood.

Aberrant DNA Methylation

DNA methylation involves covalent modification at the fifth carbon
position of cytosine residues within CpG nucleotides of DNA, which tend
to be clustered around the 5' ends of many housekeeping genes (CpG
islands). Hypermethylation in the 5' promoter region of genes is
associated with transcriptional silencing and is an alternative
mechanism for down-regulating TSG expression rather than gene deletion
or mutation.
Hypermethylation of the promoter region of the p16**INK4A gene in a
subset of NSCLCs results in its down-regulation and may be an early
event in lung cancer pathogenesis. [ref: 4] Other genes also have been
found to undergo aberrant promoter methylation in lung cancer but are
not found in the normal lung associated with these tumors, including
DAP (death associated protein) kinase, GSTP1 (glutathione S-
transferase), and MGMT (O**6-methylguanine-DNA-methyltransferase).
[ref: 5] In addition, DNA containing these methylated sequences could
be detected in the corresponding blood samples from the same patients,
indicating that the tumor cells had shed DNA into the peripheral blood.
Thus, aberrantly methylated DNA sequences, which can be sensitively
detected among a background of normal DNA, represent an attractive
strategy for early molecular detection. Other sites of
hypermethylation, including 3p, 4q34, 10q26, and 17p13, have been
implicated in lung cancer pathogenesis, although the precise gene
targets are uncertain. In addition to use as an early detection target,
it may be possible to reverse methylation pharmacologically. In tissue
culture systems, this is routinely done with the demethylating agent 5-
aza-2'-deoxycytidine. Clinical trials with such agents have been
attempted in other diseases, and agents with less toxicity need to be
developed and tested in lung cancer.
Another acquired tumor abnormality is loss of imprinting (loss of
methylation) to allow the expression of genes in lung cancer.
Methylation plays a role in mediating genomic imprinting, which is a
gamete-specific modification causing differential expression of the two
alleles of a gene in somatic cells. Loss of genomic imprinting of the
insulin-like growth factor-2 (IGF-2) gene and the H19 gene (associated
with hypomethylation of its promoter region) also occurs in lung
cancer.