Chromosomally unstable mouse tumors have genomic alterations similar to diverse human cancers

Journal Club

14.09.2007

Introduction

• A hallmark of human cancer is highly disorganised, unstable genomes driven by alterations in multiple pathways. Single gene mouse models of cancer development have, in general, not faithfully recapitulated this facet of human disease leading to restrictions on their utility in comparative cancer genomics.

Generation of murine lymphoma model

• Terc, Atm and Trp53 mutant mice were cross-bread. Designated TKO– Terc: stabilize chromosome– Atm: maintain DNA integrity – Trp53: tumor suppressor gene

• Trp53+/- and Trp53-/- mice were more susceptible to develop lymphomas and at a faster rate than Trp53+/+

• Trp53+/- derived tumors showed loss of heterozygosis

Spectral karytype analyses

• Each chromosome is visualised with chromosome specific DNA labelled with a fluophore

• Used to identify structural chromosome aberrations

Generation of murine lymphoma model

• Spectral karytype analyses– G0 telomere intact (Terc+/+ and Terc+/-)– G1 – G4 telomere deficient (Terc-/-)

• G1 – G4 are more prone to chromosomal aberration compared to G0

Translocation

• Non-reciprocal– Robertsonian translocation

• acrocentric chromosomes in the human genome: 13, 14, 15, 21 and 22.

• Long arm fusion and loss of short arm

– Dicentric chromosome • Formation of a chromosome with two centromeres

• Reciprocal– Exchange of material between two non-homologous

chromosomes

Comparative Genomic Hybridization (CGH)

Copy Number Alteration

• Copy Number Alteration (CNA) detected by Comparative Genomic Hybridization (CGH) in TKO lymphomas

• Identification of relevant CNA’s. Notch1 and T-Cell receptors (Tcr)

Notch1

Notch1

• Re-sequencing of TKO tumors missing Notch1 genomic rearrangement– Deletion/insertion – Activation of Notch1

and an up regulation of its transcriptional targets

Correlation of TKO lymphoma to human T-Cell acute lymphoblastic leukemia/lymphoma (T-ALL)

• Minimal Common Region (MCR)• Comparison of TKO and T-All MCR by synteny mapping

Synteny

• Preserved order of genes on chromosomes of related species, as a result of descent from a common ancestor.

FBXW7

• FBXW7 is deleted in TKO lymphomas and T-ALL cells

• Fbxw7 is under expressed in TKO lymphomas

• FBXW7 was mutated or deleted in – 48% cell lines – 29% clinical samples

FBXW7

• Fbxw7 binds to the PEST domain in Notch1 and activates degradation of Notch1

• When the PEST domain is mutated there is a less frequent FBXW7 mutation/deletion compared to heterodimerization domains mutation

• Fbxw7 and PEST is part of the same degradation pathway

PTEN

• PTEN is deleted in TKO lymphomas and T-ALL cells

• 30.4% cell lines• 5.2% clinical samples

• Act as a tumor suppressor by inhibiting the phospho-AKT pathway

PTEN

Correlation to other human tumors

• 62% of amplification in 2 or more tumor types

• 53% of deletions in 2 or more tumor types

• 24 genes from the Cancer Gene Consus could be found within the 104 MCR from TKO tumors (17 oncogenes and 7 tumor suppressors)

Conclusion

• TKO mouse lymphomas resemble the genomic alterations observed in human tumors

• Syntenic mapping of TKO lymphomas with human cell lines or patient samples identified known genes relevant in cancer – Fbxw7, Notch1, Tcr and Pten

• TKO mouse can be used as a screening model for identifying genomic alterations relevant in human cancer

Weakest point

• Have they shown that it is the same mechanisms that drive human and murine tumors?

Strongest point

Murine TKO tumor

Identify CNA’s

Compare murine MCRto human MCR in T-ALL

Notch1 and Tcr

Compare murine MCRto human MCR in 6 different tumor types

In 102 syntenic MCR’s 24 known genes involved in

cancer could be found

FBXW7 and PTEN

Breakage Fusion Bridge

• G0 Terc+/- Terc+/+ Atm-/- Atm+/+

• G1 Terc-/- Atm-/- Atm+/+

• G2 Terc-/- Atm-/- Atm+/+

• G3 Terc-/- Atm-/- Atm+/+

• G4 Terc-/- Atm-/- Atm+/+