Genetics of Cancer

The Oncogene Hypothesis

Cancer is in the DNA

DNA from one tumor can cause other cells to become tumorous (called transformation)
animal tumor viruses can transduce new DNA into cells to make them tumerous (may occur in humans too)
an oncoprotein is a protein that, when mutated or inappropriately or excessively expressed, results in cancer
a proto-oncogene can be mutated or overexpressed to form an oncogene, which encodes an oncoprotein

most malignant cancers involve 5-6 oncogene mutations, and more severe cancers involve more mutations

Cancer can result from:

(1) gain of function of a growth factor or other protein that promotes cell growth (dominant oncogene)

(2) loss of function of a tumor suppressor or other protein that inhibits cell growth (a recessive oncogene)

Dominant Oncogenes – Two Types:

(1) Type 1 Dominant Oncogenes – Growth Factor Signalling Pathway Components

There is a signal transduction cascade by which cells respond to growth factors secreted by other cells

growth factor Þ growth factor receptor Þ signal transducer Þ signal carrier Þ effector (affect transcription)

mutation in any of these could theoretically lead to cancer – most human cancers from mutations early in cascade

(a) Growth Factors – secreted by a cell to tell other cells in the vicinity to proliferate.

oncogenes encode growth factors in wrong place or at wrong time
e.g., sis proto-oncogene encodes beta chain of PDGF; mutation causes proliferation and tumor

(b) Growth Factor Receptors – bind growth factor on surface of responding cell – usually receptor tyrosine kinases

oncogenes encode growth factor receptors that act like they are bound to a growth factor even when they are not
e.g., Erb-B, neu, trk

Clinical Examples:

breast carcinoma – overexpression of neu (c-erb-2), a receptor tyrosine kinase on surface of breast epithelium
diagnose by excess of DNA (Southern blot), RNA (Northern blot), or protein (immunocytochemistry)
medullary thyroid carcinoma (MTC) – mutation in tyrosine kinase, so constitutively active
hereditary and sporadic forms
CML (Chronic Myelogenous Leukemia) – fusion of abl tyrosine kinase to another gene (BCR, unknown function)
result is constitutively active tyrosine kinase
caused by translocation of chromosomes 9 and 22, producing the "Philadelphia chromosome"
diagnosed by visualization of altered chromosome, Southern blot, or rtPCR (PCR of mRNA)

oncogenes cause transducers to be too active

e.g., ras proto-oncogene is a G-protein that transmits growth factor signals (active when bound to GTP)

ras-related tumors occur in several ways:

(1) hydrolysis (GTPÞ GDP) can be slowed down (mutation in GAP, G-protein affector protein)

(2) exchange (GDPÞ GTP) can be accelerated (recessive oncogene)

(3) point mutation in ras itself (at codon 12, 13, or 61) is often found in late-stage cancers

causes decreased rate of GTP hydrolysis

diagnosed by probing amplified DNA with oligonucleotide

(d) Signal Carriers – transmit signal to nucleus – often serine/threonine kinases (e.g. ras Þ raf Þ MEK Þ MAPK)

oncogenes cause constitutive activity

could include raf and mos, although these have not been shown to cause human cancers

ras activates raf Þ MEK Þ MAPK (mitogen activator protein kinase), neg. feedback by MAPK phosphatase

raf mutations cause some animal tumors

(e) Effectors – affect transcription in nucleus

oncogenes cause inappropriate activity
could include fos/jun (AP-1 transcription regulators), myc immortalizing genes, Elk1 (end of ras pathway)

(2) Type 2 Dominant Oncogenes – Enablers of Immortal Growth and Avoidance of Apoptosis

Many proteins that ordinarily cause apoptosis (programmed cell death) can be mutated to cause immortalized cell growth
(a) p53 Mutation – normal p53 rapidly degraded unless genomic integrity compromised – mutant is stable

causes immortalization of cells (unknown process)

Clinical Example:

Li-Fraumeni syndrome – rare p53 dominant germline mutation – may be involved in other tumor syndromes
causes bone and soft tissue sarcomas, breast cancer, brain cancer, leukemia, adrenocortical carcinomas
multiple primary tumors
TP53 germline mutations assoc with most cases – codon 248 middle of p53 – recessive tumor suppressor

(b) Other mutations that cause immortalized cell growth - include myc and bcl2

Clinical Example:

Burkitt’s lymphoma – excess of normal c-Myc caused by translocation on chromosome 8

several different translocations have been observed, each to an Ig enhancer (so lymphocytes turn it on)
can also be tandem repeat or back to back telomere that lacks a centrimere so it is excessively replicated

Recessive Oncogenes

These oncogenes involve loss of a protein that inhibits proliferation (tumor suppressor protein)

Although recessive at a cellular level, they are usually inherited as autosomal dominant – Knudson’s two hit hypothesis

loss of heterozygosity – start out with one mutated allele in all cells, the other is mutated in tumorous cells

Examples of tumor suppressor proteins:

Rb – controls passage through the cell cycle – inhibits replication by binding E2F
p53 – monitors genomic integrity – if DNA is damaged, causes G1 arrest if repair is possible, apoptosis if it is not

concentrations increase hundredfold within 5 minutes of UV irradiation – activates its own transcription
activates transcription of G1 arrest proteins p21-CIP and p16-INK4 (inhibit CDK2 and CDK4, respectively)

Many DNA tumor viruses have proteins that inactivate these two tumor suppressors:

Virus	Inactivates Rb	Inactivates p53
HPV (Human Papilloma Virus)	E7	E6
Adenovirus	E1A	E2A
SV40	T-antigen	T-antigen

Clinical Examples:

Retinoblastoma – defect of Rb (chromosome 13) – 27 exons, mutated at rate of 10^-6 (most mutations affect spicing)

most common cause of eye cancer in children (occurs 1/20,000)
familial retinoblastoma accounts for 40% of cases – loss of heterozygosity at Rb locus

increased risk of osteosarcoma in face exposed to normal radiation
radiation therapy increases mortality

also a sporadic form – never bilateral

breast cancer – 5-10% is familial, some involve one of two tumor suppressors (which ordinarily stabilize genome)

BRCA1 (chromosome 17, 20-40%) and BRCA2 (chromosome 13,10-30%)
more than 500 mutations (mostly truncation) discovered for each – mechanism is loss of heterozygosity

familial adenomatous polyposis – defect of APC (on chromosome 5q, involved in apoptosis) – 10% of colon cancer

most mutations are truncations near the N-terminus – 30% have de novo germline mutation
mostly colon cancer, but also risk of extracolonic tumors – upper GI, osteoma, thyroid, brain, etc.
greater than 90% penetrance, and 100% cancer risk if untreated
most families have unique mutations (unlike Li-Fraumeni) – mutation relatable to severity of phenotype
loss of APC is early event – malignancy caused by defect in c-ras, deletion of 18q, loss of p53, etc.

Lynch syndrome (HNPCC – hereditary non-polyposis colon cancer) - left colon and extracolonic – 5% of colon cancer

involves defect in DNA mismatch repair (correct wrong strand after missense mutation)

leads to microsatellite instability – no pathological significance but useful for diagnosis

recently discovered: specific mutation in TGF-b receptor (inhibitory cytokine)

Neurofibromatosis Type 1 – involves loss of NF-1 (ordinarily accelerates GTPase of RAS G-proteins)

Wilms’ tumor – kidney cancer – involves loss of WT1 (a transcription factor)