Features that describe the molecular pathogenesis in a cancer include large scale genomic changes as well as mutations or alterations in specific genes or pathways. The main mechanisms responsible for large scale genomic changes in tumor cells are microsatellite instability (MSI) or chromosome instability (changes in DNA copy number) [55
]. MSI is often a direct result of defective mismatch repair mechanisms and can be identified by replication errors in repeated units of 1-4 DNA base pairs (microsatellites) that are distributed throughout the genome [56
]. In endometrial cancer, MSI most often occurs from epigenetic silencing and inactivation of the MutL Homolog 1 (MLH1)
gene through hypermethylation of CpG islands in its promoter region [57
]. This form of genetic instability increases the mutation rate and can accelerate the acquisition of further generic damage that may lead to carcinogenic transformation.
Chromosome instability reters to chromosomal modifications such as gains, losses or rearrangements that may lead to oncogene activation or rumor suppressor inactivation. Chromosome instability can be detected cytogenetically using techniques like conventional karyotyping, in which metaphase spreads of human chromosomes are analyzed, or fluorescence in situ hybridization (FISH) in which specific chromosomes or loci are marked by fluorescent probes. Comparative genomic hybridization (CGH) assesses genomic imbalance and provides a measure of gene amplication and deletion.
Allelic imbalance (AI) is another type of chromosome instability where one allele of a gene is lost or amplified. Loss of heterozygosity (LOH) is a common form of AI and refers to the situation where one chromosome has a normal allele of a gene and the other has a mutant or deleted allele. If one allele is already inactivated then only a second inactivating hit may be required, which has particular relevance for tumor suppressor genes [55
]. A method commonly applied to estimate genome-wide AI is the Single Nucleotide Polymorphism (SNP) array which measures the number of SNP markers with allelic imbalance divided by the total number SNP markers.
Inactivating mutations of specific genes generally involve base substitutions, deletions or insertions of only a few nudeotides and consequently these are often detected by direct sequencing of the gene of interest in the genomic DNA. As DNA sequencing can be labor intensive, immunohistochemical stains have been developed that allow the detection of well-studied genes, such as somatic PTEN mutation in endometrial cancer, so that gene mutation can be inferred based on positive or negative staining in formalin-fixed paraffin-embedded tissues.
The spectrum of genes affected in cancer can be wide and varied and some genes and pathways are involved in a variety of cancers. This is true for endometrial and ovarian cancer where several common genes and pathways have been described. A large amount of research has focused on p53 after it was discovered that the tumor suppressor gene TP53
is frequently mutated in a high proportion of human cancers [58
]. Activation of p53 normally occurs in response to DNA damage, aberrant proliferative growth signals, and carcinogenic factors such as exposure to UV radiation [59
]. Activated p53 carries out several functions, of which the most comprehensively understood are its ability to cause cell cycle arrest at the G2/M DNA damage checkpoint and to induce apoptosis [60
Mutations in KRAS
that cause aberrant activation have been identified in both endometrial and ovarian cancer and appear to play a central role in carcinogenesis by conducting signals that enhance cell proliferation during tumor development [62
). RAS, a small GTP binding protein, activates the core unit of a cascade composed of RAF, mitogen/extracellular signal-regulated kinase (MEK1/2) and MAP Kinase (MAPK or ERK) as well as the PI3K/ AKT pathway [64
Inactivating mutations of the tumor suppressor gene, PTEN,
are detected in both endometrial and ovarian cancer, PTEN
is an inhibitor of PI3K/ AKT signaling and acts to control the rate of cell division and promote apoptosis [67
]. Loss of PTEN
may occur through a variery of mechanisms, however the most common is inactivation of both alleles through mutation or deletion in combination with LOH at chromosome 10q23 to generate a protein deficient state with a complete loss of function phenotype [68
Gain of function mutations of the CTNNB1
gene (β-catenin) are identified in endometrial and ovarian cancer [70
], especially those with squamous differentiation. These mutations stabilize the β-catenin protein in the cell cytoplasm and nucleus which leads to activation of the lymphoid enhancing binding factor (LEF) and T cell-specific transcription factor (TCF) pathways that promote transcription of target genes involved in tumorigenesis such as C-MYC