Pathobiology

Driver mutations that confer selective growth advantage

Subdivided into

Oncogenes:

Mutational product drives tumour generation
e.g. KRAS
Missense point mutations at key locations:

These single base substitutions in the DNA code introduce a specific functional change in the amino acid sequence, resulting in initiation of oncogene activity.

Chromosomal rearrangements → fusion proteins
Requires mutation of one allelle

Tumour suppressor genes:

Loss- of- function genes
e.g. NF2
Inactivation by any mutation resulting in the incidence of a stop codon within the reading frame, including insertions, deletions, and even single base substitutions occurring throughout the coding sequence.
Requires mutation of two allelle

‘Passenger’ mutations

Mutation that does not allow growth advantage.

Varies between and within tumours, and can be used to map tumour evolution

Chromosomal alteration

Aneuploidy consistently observed in cancers from a variety of origins.

Copy number variation is prevalent in cancer cells with amplification of gene expression, through simply the increased number of gene copies per cell;

For example,

EGFR occurring alongside deletions of genomic regions, where tumour suppressor genes are lost (e.g. phosphatase and tensin homolog (PTEN), resulting in a decrease or absence of their expression

Eg Gliomas

A non- balanced translocation affecting chromosome arms 1p and 19q

Eg Oligodendrogliomas

Chromosomes can also undergo fusion events whereby breakage of DNA and subsequent mal- reconstitution lead to the generation of an oncogenic target

E.g.: C11orf95- RELA in supratentorial ependymoma

Hallmarks of cancer

Sustain signalling

Upregulation of receptor proteins,
Autocrine proliferative signaling,
Constitutive receptor activation

eg. activation even if no ligand is present, e.g. EGFRvIII amplification in GBM

Disruption of intracellular negative feedback loops

E.g.: PTEN loss

Evading growth suppressors

Normal cell cycle regulatory proteins are commonly mutated in cancer.

Uncontrolled flux through the cell cycle occurs following mutation of retinoblastoma (Rb) via the loss of intracellular transduction of extracellular inhibitory signals and through mutation of TP53, which detects intracellular stress and induces apoptosis accordingly.

Evading contact inhibition

Cell contact inhibition is often bypassed in cancer.

NF2 mutation → affects the production of merlin (schwannomin), a cytoskeletal protein that limits mitogenic signalling through intracellular tyrosine kinase binding.
Thus loss of this inhibition results in cellular proliferation under conditions in which it would usually be suppressed
>½ of meningiomas contain NF2 tumour suppressor gene mutations.
Through clustering of this and other somatic mutations in meningioma, the mutation of NF2 and deletion of chromosome 22 correlated with a hemispheric origin of the meningioma

Genomic instability, mutation, and avoiding cell death

TP53

Induces apoptosis following DNA strand break and chromosomal injury (e.g. following cell stress or during cell division).
Loss of p53 protein thus enables the cell to evade programmed cell death and continue to proliferate.
Concomitantly cells with mutations in TP53 thus have an ongoing increased rate of mutation due to the loss of this checkpoint.

Mismatch repair gene mutation, occurring following temozolomide treatment in LGG, has also been shown to lead to the development of a hypermutated phenotype (Turcan et al., 2012).

The loss of apoptosis within cells does not always lead to continued cell proliferation.

Cells often become non- viable and although unable to undergo apoptosis, they undergo necrosis.
This is potentially protumourigenic as it induces a pro- inflammatory microenvironment that stimulates cytokine secretion that induces angiogenesis and promotes tumour cell invasion and proliferation

Enabling replicative immortality

Normal cells can undertake only a limited number of cell divisions:

Hayflick limit:

The loss of telomeres during division process results in replicative senescence
After ~50 divisions

Two common mutations to affect this process that are commonly mutated within glioma are

Inducing angiogenesis

Angiogenesis is a key factor in the instigation, sustenance, and progression of a variety of neoplastic processes both local to and distant to the primary tumour,

An independent predictor of poor prognosis in GBM

Has been shown to display the highest degree of vascular proliferation and endothelial cell hyperplasia of any solid tumour
Gliomas need to develop a blood supply to promote tumour growth.
Steps

Vascular co-option where the tumour takes over normal brain vessels. → neoangiogenesis.

VEGF

Is a key driver of tumour angiogenesis,
VEGF receptor inhibitor bevacizumab has a potent but short- lived effect on tumour growth although it may also promote tumour invasion.

Deregulating cellular energetics

The development of abnormal cellular metabolism allows

The energy requirements of upregulated cellular proliferation
Generating the biosynthetic precursors required for rapid cell turnover

Tumour- promoting inflammation

A supportive microenvironment is also required to drive tumour progression.

The complex interplay between a tumour and the immune system results in the expression of both suppressive cytokines as well as chemokines

Studies within GBM have suggested a correlation between GBM subtype and immune characteristics.

The correlation between immune cell infiltration and differing underlying mutational landscapes alongside the ability of prominent cytokines (e.g. TGF- b) to regulate proliferation, migration, and tumorigenicity in mesenchymal GBM further defines the role of the immune microenvironment in GBM.

TGF-β

Is a multifunctional polypeptide implicated in the regulation of various cellular processes including growth, differentiation, apoptosis, adhesion, and motility.
Abnormalities in the TGF-β signaling pathway are implicated in the development and progression of brain tumors.

Epigenomics

DNA modifications that result in gene expression changes that do not alter the base pair sequence.

Methylation of the MGMT promoter is an epigenomic modification resulting in reduced expression of this DNA repair gene, thereby conferring increased sensitivity of GBM to alkylating chemotherapy.

Genomic mutations can also drive epigenomic progression:

Unbiased genome sequencing based approaches have repeatedly and unexpectedly identified examples of driver mutations in genes concerned with cellular regulation of gene expression including

Histone proteins responsible for DNA packaging in paediatric GBM (H3K27M and H3G34R/ V mutations)
IDH mutations driving production of the oncometabolite 2- hydroxyglutarate which inhibits DNA methylation turnover in diffuse glioma