8 Multi-step cancer progression

We have previously seen that mutations to genes that have oncogene or tumour suppressor gene function can drive cells towards the development of cancer. One of the most important question that has interested scientists since the realisation that mutations of genes could lead to the development of cancer has been to understand how many mutations it will actually take for cells to develop cancer.

Would one mutation, in the case of an oncogene (dominant), or two mutations i.e. of both allels of a tumour suppressor gene (recessive) be sufficient to trigger the development of cancer?

Various mathematical modeling studies in the ’50s suggested that the age distribution of the typical incidence of cancer, i.e. typically much higher risk for people over the age of 40-50 could be explained if one assumed that more than one mutation was required to trigger cancer development as the likelihood of these events happening more than once would increase with age.

In the 1950’s there was evidence from studies looking at the typical frequency of mutations occuring in tissues as well as mathematical considerations to suggest that more than one or two mutations would required (Figure 8.1). Mathematical modeling of cancer incidence by Nording et al. suggested in many cancer types an increase with the 6th power of age leading to the suggestion that cancer was likely to be a multi-step process and that around 6 mutations might be required (Nordling (1953)).

The cancer incience rate when plotted against age show a strong correlation (Knudson:2001ik).

Figure 8.1: The cancer incience rate when plotted against age show a strong correlation (Knudson:2001ik).

8.1 Evidence of multistep cancer development in colon cancer

As molecular biological techniques improved laboratories started to be able to address this questions much more specifically looking at the actual changes occurring in patients. In this context, colon cancer is a particularly interesting disease as in many patients (50-85%) the disease evolves along a number of defined stages, i.e.,

  • starting with local tissue hyper-proliferation,

  • adenomatous polyps,

  • severe pre-cancerous dysplasia,

  • adenocarcinoma

    • local,
    • invasive
    • metastatic
Colon cancer development follows a common pattern of mutation of specific genes over a number of decades.

Figure 8.2: Colon cancer development follows a common pattern of mutation of specific genes over a number of decades.

Vogelstein and colleagues {Fearon and Vogelstein (1990)} showed that these clinically observable stages precisely map onto specific genetic changes in the associated colon tissue and occurred very gradually over many years (Figure 8.2). They observed that the initial development of small adenomas was typically observed in patients aged 30-50 and involved mutations of the APC{#apc} gene. A frequently observed next step involves the mutations in the (RAS gene){#ras} and is linked to the development of large adenomas at 40-60 years of age. Finally, at the age of 50 -70 years malignant tumours in the form of carcinomas would emerge with associated changes to pathways such as PDK, cell cycle and/or apoptosis, and TGF-β. Cells in those tumours thus have accumulated genetic changes over many years before the cancer develops.

This gradual development of cancer over many years and by accumulation of a number of genetic changes has since been shown to be typical of many common types of cancer although not all will follow quite such a predictable development.

8.1.1 Mutations are common in many cancer types

In the following decades the laboratory techniques for the analysis of DNA sequence have increased by orders of magnitude in terms of sensitivity and throughput making a mch more detailed mapping of genetic changes in various cancer types possible.

For example, Vogelstein and colleagues published a detailed analysis of the number of mutations observable on average in different tumour types (Figure 8.3). The number of mutations observed differ over orders of magnitude, ranging from over 500 in some forms of colorectal cancer to below 10-20 for many forms of childhood cancers.

The number of different (non-synonymous) muations in different types of cancer. From Vogelstein, B. et al., 2013. Cancer Genome Landscapes. Science, 339(6127), pp.1546–1558.

Figure 8.3: The number of different (non-synonymous) muations in different types of cancer. From Vogelstein, B. et al., 2013. Cancer Genome Landscapes. Science, 339(6127), pp.1546–1558.

Interestingly, it appears that the number of mutations can also be understood in relation to various risk factors: While on the one hand cancers associated with tissues with a high risk of mutagen exposure will show high numbers of mutations (lung, skin melanoma etc.) it is also obvious that typically cancers that develop in childhood and thus have little time to accumulate muations in deed show low numbers of mutations on average. Many common forms of cancer in adults appear to sit between these extremes.

One possible conclusion from this is that cancer development does indeed reuqire the accumulation of multiple mutations but it also suggests that not all mutations are equally effective or relevant as drivers of cancer progression.

8.2 Passengers and drivers

As methods for rapid and high resolution gene sequencing have become available various studies have in a number of different cancers have tried to answer this quesion. Importantly, this now also allows analysis of the types of genes involved.

The first discovery of the involvement of specific genes in cancer development had highlighted the potential role of oncogenes and [tumour suppressor][Tumour Suppressor Genes] genes as drivers of cancer progression. It thus makes sense to no only look at the overall number of mutations but to distinguish between driver genes that have a potential role in cancer development and those passenger genes whose mutation have no role in cancer.

Not all mutations affect genes that are involved in cancer development. Mutations that drive cancer development and progression are known as _driver_ mutations while those that have no impact are called _passenger_ mutations.

Figure 8.4: Not all mutations affect genes that are involved in cancer development. Mutations that drive cancer development and progression are known as driver mutations while those that have no impact are called passenger mutations.

Making the distinction between passenger vs. driver gene mutations leads to estimates that there are around 120 genes that when mutated could become drivers of cancer progression (driver genes); and estimated around 70 of them involve genes that appear to act as tumour suppressors and around 50 that have been classified as oncogenes.

When looking at the average number of these more relevant mutations suggests that much fewer mutations are required, e.g. for some lung cancers around 450 genes have been implicated but only estimated 11 of these are driver mutations.

8.2.1 A model of multi-step and multi-stage carcinogenesis

Recent estimates suggest that for some common forms of cancer three (or more) driver mutations may be required for the development of malignant tumours if specific important genes are affected (Figure 8.5){Vogelstein and Kinzler (2015)}. This model combines the observations of the Muti-step process of cancer development in colon cancer with the obeservation or driver vs. passenger mutations. This is supported by the fact that in many adult cancers at least three driver mutations may be observed to occur. These mutations can be broadly linked to three phases in the development of cancer these emerge slowly over a long period of time (30 years).

1. Break through phase. The initial driver mutation is thought to typically affect a growth-related pathway relevant in this cell type. Daughter cells derived from this original mutated cell will carry the same mutation which will support abnormal but slow proliferation. In many cancer the initial driver mutation commonly affects a specific cell type specific pathways, e.g. APC in colon cancer. This could suggest that these pathways are more active or relevant to the affected cells.
(more variability for later driver mutations)

2. Expansion phase. Any of the cells in this local clone of cells with a benign hyper-proliferation could receive a second mutation. If this is a driver mutation it is thought to affect genes that facilitate further ‘thriving’ or the cells in the tissue environment without triggering invasion. This would map on to the stage of large adenomas in colon cancer. These still are benign tumours.

3. Invasive phase. A 3rd driver mutation initiates invasion and thus the transformation to a malignant tumour; this may already be enough to also allow eventual metastasis to occur.

For a number of different cancer an estimated three mutations ("strikes") to specific driver genes are likley to be sufficient for cancer development

Figure 8.5: For a number of different cancer an estimated three mutations (“strikes”) to specific driver genes are likley to be sufficient for cancer development

8.3 Summary

  • TO BE EDITED
  • The number of mutations varies widely between different types of cancers but not all
  • Typically mutations and epimutations affect DNA regions in a random fashion.