L11: Chromosomal changes Flashcards
(9 cards)
detection of chromosomal changes
Cytogenetics: Karyotyping and FISH (Fluorescence in situ Hybridization).
Next-Generation Sequencing (NGS): Identification of mutations, copy number variations, and structural rearrangements.
PCR: Specific detection of translocations and gene fusions.
Somatic human cell has 46 chromosomes - Diploid
Ova or sperm has 23 chromosomes - Haploid
Numbered 1-22 in decreasing size order
Two sex chromosomes - XX in females, XY in male
chromosome structure and nomenclature
Short arm – ‘p’
Long arm ‘q’
Each arm divides into regions numbered outwards from centromere
Each region divides into bands
more or less than 46 chrosome: aneuploidy
more than 46- hyperploid
less- hypodiplpod
deletion of whole- ‘-‘ infront of chromosome number
duplication of whole chromosome- ‘+’ infront of number
deletion of part of chromsome- prefix ‘del’
extra matrial- ‘add’ prefix
chromosome translocation- ‘t’ with chromosomes involved placed in brackets with the lower numbered chromosome first
chromosome inversion where part has inverted to run in opposite direction- prefix ‘inv’
chromosome shattering
Chromosome shattering (chromothripsis)
Shattering of limited segment of chromosome followed by multiple rejoining events
Creates localised ‘firestorms’ of rearrangements
Present in 25% bone cancers and 2-3% cancers overall
cause of chromosome changes
Errors during cell division:
Mitotic spindle assembly checkpoint defects causing chromosomes to mis-segregate
Genomic instability:
Double strand DNA breaks unrepaired or improperly repaired.
Causes of DNA breaks at specific confined regions of genome often unknown with exception of translocations involving immunoglobulin (Ig) or T cell receptor (TCR) genes where mis-firing of machinery dedicated to rearranging Ig and TCR genes involved (details later)
Mutations in DNA repair pathways (e.g., BRCA1/2 role in homologous recombination)
Telomere dysfunction: Telomere shortening associated with risk of chromosomal fusions.
Environmental factors (radiation, chemicals, and viruses):
Therapy with cytotoxic drugs (e.g., increased incidence of monosomy 5 (5q-) or monosomy 7 (7q-) acute myeloid leukaemia (AML) after treatment with alkylating agents)
Chemicals: example benzene exposure can cause acute myeloid leukaemia
Cytotoxic drugs: example treatment with alkylating agents associated with increased incidence of chromosome loss (5q or 7q-) in acute myeloid leukaemia
Infections: example Epstein-Barr virus and malaria association with Burkitt lymphoma
consequences
changes leading to driver mutations that confer selective growth advantage
Cancer due to increased activity of oncogenes
BCL2 (suppress apoptosis) e.g., gene translocation in follicular lymphoma
ABL1 tyrosine kinase) e.g., gene translocation in chronic myeloid leukaemia
FLT3 (tyrosine kinase) e.g., gene duplication in acute myeloid leukaemia
MYC (transcription factor) e.g., gene amplification in neuroendocrine tumour and gene translocation in Burkitt lymphoma
Cancer due to loss of tumour-suppressor gene activity
TP53 e.g., gene deletion in Chronic lymphocytic leukaemia (CLL)
chromosomal translocations
Two mechanisms by which chromosomal translocations can lead to dysregulated gene expression
Formation of novel fusion protein
Aberrant expression of normal protein
Affects pluripotent haematopoietic stem cell
CML ccounts for 15% leukaemias and can occur at any age
White blood cell count >200x109/L (normal 4-11x109/L)
Characterised by presence of Philadelphia (Ph) chromosome
Formed by reciprocal translocation
treatment:
Small molecule inhibitor blocks adenosine triphosphate (ATP) binding site of the tyrosine kinase enzyme
Substrate cannot be phosphorylated
Induces disease remission
NOT curative as minimal residual disease (MRD) often still detected
TK inhibotors
tyrosine kinase inhibitors
Imatinib (also known as STI571 or Gleevec)
First generation TKI clinical trial 1998 induced CML remission in all 31 patients treated
Problem of emergence of drug-resistant variants due to acquired mutations in the Bcr-Abl gene which alter structure or increase gene expression
second gen (more potent but more side effects)
Nilotinib: Increased affinity for BCR-ABL1
Dasatinib: Broader TKI activity giving more rapid responses but side effects more common
Ponatinib: Effective against CML with T315I mutation in Bcr-Abl1
translocations and abbarent expression
Translocations that lead to aberrant expression of a normal protein almost always involve an immunoglobulin (Ig) or T-cell receptor (TCR) gene and most frequently the Ig heavy chain (IgH) loci on chromosome 14.
enzyme recombinase in B and T: Recombinases join up adjacent pieces of DNA after excision of intervening sequences
Recombinases recognise specific heptamer and nonamer sequences flanking gene segments
mistakes in recombinase drives translocations
Chromosome translocations are associated with B and T cells because recombinase is active in these cell types and mistakes by the enzyme during the gene rearrangement process can lead to chromosome translocation events
Immunoglobulins also subsequently undergo somatic hypermutation mediated by activation induced deaminase (AID). Mistargeting of AID contributes to B-cell malignancy
myc function
Myc forms heterodimers with other family members (e.g., Max)
Modulates transcription of genes possessing E-box sequences
Activates the activators
In partnership with Max, drives expression of many target genes (e.g. cyclin D2, CDK4 and other transcription factors such as E2Fs)
Represses the inhibitors
In partnership with Miz-1, shuts down expression of inhibitors (e.g. p15INK4B and p21Cip1)
Pushes cell through G1 phase of cell cycle
After Myc drives initiation of cell cycle, its expression diminishes in normal cells
As cells differentiate, Mxd levels rise and Myc is displaced by Mxd
Increased and persistent expression of Myc in malignant cells leads to continuous proliferation
examples of chromosomal changes in solid tumours
Breast Cancer:
Role of chromosome 17 (p53 mutations, HER2 amplification)
Colon Cancer:
Loss of chromosome 18q and APC gene mutations
Lung Cancer:
Chromosome 3p deletions and KRAS mutations
Ovarian Cancer:
Chromosomal instability, including TP53 mutations and BRCA1/2 deletions
