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Genetic predisposition to cancer The basics

nicolas.janin@uclouvain.be

Cliniques universitaires Saint-Luc

Knowledge of blue pale slides is mandatory for all the students attending the BeSHG course

Plan

I. A very brief overview of carcinogenesis :I. Cancer is a multi-hit stochastic somatic genetic

disease

II. The mutator phenotype hypothesis

II. Knudson’s 1971 original two-hit hypothesis

III. Coming’s hypothesis

IV. The rules to be followed by every oncologist

V. Genetic predisposition to cancer: basic facts and figures

Molecular biology success story The unifying theory of carcinogenesis

Eighties

Same targets:Benzopyrene, aflatoxine,UVVirus…

Oncogenes Tumor suppressor genes

Multiple hits transform a normal cell into a tumor

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+ Acquisition

of a mutatorphenotype

from Hanahanand Weinberg 2000

Multiple malfunctions occur in the course of carcinogenesis

Figure 3

Douglas Hanahan and Robert A. Weinberg Cell , Volume 144, Issue 5, Pages 646-674

2011 version of this picture

Lawrence Loeb’s original hypothesis (seventies)

1. Very low mutation rate in humans,≈ 1,4 . 10-10 / nucleotide . generation

2. Cancer = acquisition of at least three independent mutations

3. With a normal mutation rate, cancer should be an exceptional disease

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Original mutator phenotype hypothesis

No mutator phenotype ⇒ No cancer

which is equivalent to

Cancer ⇒ Mutator phenotype

i.e., acquisition of a mutator phenotype is conditio sine qua non of cancer

Lawrence Loeb, Cancer Res. 1991

This hypothesis remained almost confidential for decades in the absence of a clearly detectable mutator phenotype in the majority of cancers

Discovery of MSI-H cancers (1993)New version of the hypothesis

“… mutations in one or more of these stability genes is an early event in carcinogenesis. The hypothesis proposes that mutations in stability genes would increase the overall level of mutations throughout the genome […] resulting in yet greater instability and in heterogeneity among cancer cells. […]”

Chemicals Early MutationsX-rays mutations in cancer-Endogenous in stability associatedProcesses genes genes

Lawrence Loeb, Cancer Res 1994

Mutation accumulation during tumour progression.

Loeb L A et al. Cancer Res 2008;68:3551-3557

©2008 by American Association for Cancer Research

TISSUE

NUCLEUS

Mutator phenotype (caused by loss of a caretaker)

Random mutations

Mutations inducing a loss of cell-cell adhesion

Mutations in an other caretaker gene

Mutations that block apoptosis…

LEGEND

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Mutator phenotype hypothesis: so what?

Cancer ⇒ Mutator phenotype

or

Mutator phenotype ⇒ Cancer?

Retinoblastoma : very rare disease, very simple genetics

Retinoblast

� Rétinocyte

Sporadic cases

Late, unilateral and unique

Hereditary cases

A.D. transmission with high (but incomplete) penetrance

Early bilateral and/or multifocal

“sporadic” early bilateral cases are hereditary cases caused by neomutations

� Rétinoblastoma

Comparison of the presentation of hereditary versus non hereditary retinoblastom

Non hereditary cases1. Always unique and unilateral

2. age (80% of cases) ≥ 2 years

3. No risk of other cancer

4. No risk for the children

Hereditary cases1. Often bilateral and/or multifocal

2. age (90% of cases) ≤ 2 years

3. Increased risk of other cancers

4. Autosomal dominant transmission with very high penetrance

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Alfred Knudson’s two-hit hypothesis (1971)

Two concepts enclosed in Knudson’s hypothesis

1. Retinoblastoma is a genetic somatic disease caused by the purely stochastic accumulation of 2 mutations within one retinoblast

2. Cancer prone children carry a germline mutation that is normally acquired during carcinogenesis of sporadic cases

Basic idea of Knudson’s theory« Risk = one mutation less »

Each retinoblastoma is a two-hit genetic somatic disease

One hit occurs normally in a few retinoblasts [0-3] of each retina during development

Genetically predisposed children have a constitutional first hit

PREDISPOSITION = ONE STEP SHORTER CARCINOGENESIS

Proc. Natl. Acac. Sci. USA, 1971

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The two-hit hypothesis (1971)Sporadic cases

Exceptionally clear and correct illustration of Knudson’s hypothesis found on the internet (author unknown)

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A very exceptional accident

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Non hereditary cases1. Always unique and unilateral2. age (80% of cases) ≥ 2 years3. No risk of other cancer4. No risk for the children

The two-hit hypothesis (1971)Hereditary cases

The first hit is present in each and every cell of the individual

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Thank you unknown author for this brilliant illustration!

Hereditary cases1. Often bilateral and/or multifocal2. age (90% of cases) ≤ 2 years3. Increased risk of other cancers4. Autosomal dominant transmission

with very high penetrance

Knudson’s hypothesis predictions: the semiology of genetic cancer predisposition

1. Autosomal dominant transmission of the risk

2. High but incomplete penetrance

3. Age at diagnosis < usual age in sporadic cases

4. Multifocal and/or bilateral disease1. Diagnosis made at the same moment (synchronous cases)

2. Or at different times in life (asynchronous cases)

5. Risk of another cancer in another tissue

6. Cancer developing in the context of predisposition are a priori identical to sporadic cancersNB: This is NOT TRUE for Lynch and breast-ovarian syndromes (these syndromes predispose to a minor genetic subtype of cancers with a very particular behavior: speed of evolution, drug sensitivity…)

The signs you must be looking for (and note) when taking the anamnesis

Autosomal dominant transmission means

1. Cancers in many generations in the same branch of the family

2. Risk transmission = ½ for men as well as for women

3. Beware of the incomplete penetrance

Luckily, tumor risks are very often restricted to one or to a few tissues (spectrum –> syndrome). Ex: breast-ovarian syndrome,Lynch syndrome = {colon, endometrium, ovaries, urothelium}People getting cancer in high risk families

� are often younger than people in sporadic cases

� often develop a 2nd cancer within the same tissue or in another tissue

NB: Knudson does not predict a very important fact in Lynch and breast & ovarian syndrome. When compared to sporadic cases, cancers evolve very rapidly in these syndromes…

In case you have a doubt ���� oncogenetics advice

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Caretakers and gatekeepers

Most tumor suppressor genes can be broadly divided into two groups called gatekeepers and caretakers.

1. Gatekeepers directly regulate the growth of tumors by inhibiting their growth or promoting their death.

2. Inactivation of caretakers leads to genetic instability that indirectly promotes growth by causing an increased mutation rate.

Bert Vogelstein & Kenneth Kinzler (Eds.) The genetic basis of human cancer.Copyright 2002, 1998 McGraw-Hill

Practice in case of a breast cancer

Always take the family history into account when dealing with a breast tumor

Note the age of diagnosis

� suspicious if age < 40

Note the pathological variety

� suspicious if triple negative SBR III ductal carcinoma

Search the personal and familial history

� for breast and ovarian cancer

� Mind the fact that carrier men have almost no risk

Look for Jewish origin

� prevalence of BRCA1&2 germline mutations >10x higher among Ashkenazi Jews than among standard Europeans

A rule that applies particularly well to oncogenetics

If the hypothesis of a cancer predisposition syndrome has not been put forward by the end of anamnesis (Evolution of disease + Personal and family history), you will never make the diagnosis.

Original de T. Alajouanine (1890-1980) : « Si vous n'avez pas fait votre diagnostic à la fin de l'interrogatoire, vous ne le ferez jamais. »

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1973: Comings’ hypothesis

Autosomal dominant hereditary tumors […] are the result of germ-line inheritance of one inactive regulatory gene. Subsequent somatic mutation of the other regulatory gene leads to tumor formation.

It is suggested that all cells possess multiple structural genes (Tr)capable of coding for transforming factors which can release the cell from its normal constraint of growth. In adults, they are suppressed by diploid pairs of regulatory genes (Reg)…

Early seventies, Knudson and Comings: synopsis

Knudson (1971) Comings (1973)

Problem

Basic idea

Tools that proved the idea to be correct

Application

Inheritance of cancer risk within families

Biological mechanism of carcinogenesis

Linking Knudson’s idea with the idea of tumor

suppressor gene

Statistics: distribution of cases/ages Poisson

(1 hit versus 2 hits)

Molecular and cellular biology

Understanding autosomal dominant cancer predisposition

Gene therapy???

1. RB ⇐ 2 random hits

2. Cancer risk ⇐ n-1

Eighties and nineties: Knudson became universally known for having put forward Comings’ idea

Examples of this illegitimate and illogical fusion of the two concepts

“Knudson (1971) has suggested that the mutation for a dominantly inherited cancer susceptibility may be the first step in a recessive change in tumour cells, and that the same gene may be involved in both familial and non-familial cases of a given tumor.” Nature 1987

“Molecular analysis has fully confirmed the ingenious theory of Knudson that retinoblastoma arises after the loss of both alleles at the same locus (RB-1)” George Klein, Science 1987

Fusion between the "N-1 hypothesis" and the anti-oncogene hypothesis

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Students now learn the misleading and confusing paradigm

“In 1971, Alfred Knudson… proposed on the basis of statistical analysis that both the inherited and sporadic cases of retinoblastoma occur because the two copies of a particular

gene have been inactivated”

PRINCIPLES OF GENETICS, 3rd editionJohn Wiley & Sons, 2003

Knudson & Comings ideas stick together in most diagrams

= Comings’ hypothesis

?This mistake is so common… Why is it so?

3 categories of heterozygous germline mutations in AD cancer predisposition syndromes

1. Activated oncogenes (MEN2)very rare because the oncogenic gain of function must be

tolerated during embryogenesis

2. Inactivated anti-oncogenes or gatekeepers (NF1 and dominant FAP)very precisely matching the retinoblastoma paradigm

3. Inactivated caretakersformally fitting the retinoblastoma paradigm

NB: 1971 Knudson’s hypothesis is valid in all 3 casesComings’ hypothesis in valid only in syndromes of category 2

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Features shared by all autosomal dominant cancer predisposition syndromes

Tumor spectrum largely unexplained at present time� Whereas the responsible genes are ubiquitously expressed,

risk is restricted to one or a few organs

� Whereas acquired mutations in one gene may be important for the genesis of one particular cancer (i.e. somatic retmutations in papillary carcinomas of the thyroid), people carrying a germline mutation in this gene may have no risk in this particular organ (no papillary cancers in MEN2)

Anyone knowing Knudson’s 1971 hypothesis should be able to recognize such a syndrome � using the clinical check-list drawn from this hypothesis

� precise diagnosis may be a matter of specialist

Broad clinical features common to tumor suppressor gene-associated syndrome

I. Affected subjects often have many benign tumors and/or malformations� … benign tumors linked or not to the cancer risk

� Hundreds to thousands of adenomas in FAP (precancerous)

� Mandibular osteomas in FAP (not precancerous)

� Neurofibromas in neurofibromatosis type 1

� Renal and pancreatic cysts in Von Hippel-Lindau

� � Obvious phenotype – Easy diagnosis

II. Precancerous tumors progress slowly to cancer

Broad clinical features common to caretaker-associated syndromes

I. Affected subjects have a normal phenotype

� Normal phenotype – Difficult diagnosis

� They are the most common syndromes � Systematically use Knudson’s check-list each and every time you see a cancer patient

II. Affected subjects develop a particular subtype of cancers that behave very differently from the most common cancers

1) may sometimes be recognized as suspect on pathological reports � 3xNEG SBR III BREAST CANCER

� MISMATCH REPAIR DEFICIENT CANCER (IF SEARCH FOR MMRD IS DONE)

2) evolve very rapidly once carcinogenesis has started (NB: this is the only clinical feature not explained by Knudson’s hypothesis)

3) respond differently to chemotherapy� BRCA-associated cancers are sensitive to doxorubicin and cisplatin, not to taxanes

� MMRD cancers are sensitive to irinotecan, but intrinsically resistant to 5-FU

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Very important fact about caretaker-syndromes

Catégories Gène Fonction Si mutation germinale

Commentaire

Gate-keepers

p53 Transcription factor

Syndrome de Li-Fraumeni

Mutations acquises dans ≈50% de tous les cancers

Rb1 Transcriptional regulator

Prédisposition au rétinoblastome

Souvent muté dans d’autres cancers

APC Regulates β-catenin function

Polypose recto-colique familiale

Perte de fonction fréquente avant l’adénome

Caretakers

BRCA1 DNA repair Syndrome sein-ovaire

Mutations très rares dans les cancers dusein sporadiques

BRCA2 DNA repair Syndrome sein-ovaire

MLH1 MSH2MSH6

Réparation des mismatch lors de la réplication de l’ADN

Syndrome de Lynch ou HNPCC

Evolution of caretaker-linked cancers typically

is very fast

2012 basic epidemiology about cancer predisposition syndromes

≈3% of the cancers are linked to high penetrance hereditary cancer predisposition syndromes

Transmission almost always autosomal dominant

Almost every organ may be affected� Breast + ovaries (>2 genes) ; colon (many syndromes) ;

� Kidneys (many syndromes) ; skin (BCC ; malignant melanoma)

� Adrenal gland… A few dozens of clearly delineated cancer predisposition syndromes

You do not need to know all these syndromes. You must know that they exist. You must be able to identify them.� Almost all you need to know is Knudson’s hypothesis

THE END