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Dr Gihan E-H Gawish, MSc, PhDMolecular Biology and Clinical Biochemistry

KSU

Cytogenetics Understanding the Disease Progression Process, Classical

and Molecular Cytogenetic Methods, Chromosome Segregation, the Abnormalities in Clinical Syndromes and

Cancer

6th Module

Dr M.Alanazi

What if mutations happen in the DNA repair genes?

Genomic instability=Accelerated mutations

Does genomic instability explain the high rate of mutations in cancer?

Is genomic instability the driving force for carcinogenesis?

Is it a late or an early phenomenon in carcinogenesis?

Genomic instability in cancerMicrosatellite instability-Caused by mutations in mismatch repair genes

-increased rate of point mutations

Chromosomal instability-Increased rate of loss or gain of whole or fraction of chromosomes

-cell to cell variability

Is genomic instability necessary for carcinogenesis?

- Only limited number of repair mutations found in tumors.

- Mismatch repair mutations in colon cancer (hMLH1 epigenetic silencing and mutations)does not generally occur before APC changes.

- Tumor suppressive mutations are not really recessive in tumors.

- Not all cancer have chromosomal instablity.

- Chromosomal instability is not frequently found in adenomas.

Genomic instability is not necessary to explain the number of mutations in cancer

Zygote to adulthood (15 yr)5x1013 cells ( 45 cell divisions)Mutation rate 5x109/nucleotide/generation5x109 bp/ genome= 25 mutations/generation= 1000 mutations in the stem cell in colonic crypt

Mutations in cell of origin before tumorigenesis (using colonic crypt as an example)

Between 15 and 65 yrs, colonic stem cells would have multiplied 5000 times= 125,000 mutations.

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Chromosomal Instability in Cancer: Causes and Consequences

Dr Gihan Gawish

• Many human malignant tumours exhibit abnormal chromosomal segregation at cell division.

• these anomalies play a role in tumorigenesis by increasing the rate of chromosome mutations,

including deletion and amplification of genes involved in cellular proliferation and/or survival.

• In vitro experiments have also shown that mitotic instability may be a mechanism for

developing resistance to cytotoxic drugs.

• Abnormal mitotic mechanisms may result in numerical or structural aberrations in the daughter

cells. Numerical aberrations can be caused either by the loss of chromosomes at

metaphase/anaphase or by multipolar divisions associated with abnormal number or structure

of centrosomes.

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What is chromosomal instability?

Dr Gihan Gawish

Chromosomal instability may be defined as a state of continuous formation of novel chromosome mutations, at a rate higher than in normal cells.

In practice, instability may be assessed by following the evolution of cytogenetic abnormalities in a tumour cell population over time and by comparing the rate of chromosome mutations with that in a normal cell population (Lengauer et al., 1997).

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1- Structural chromosome instability

Mechanisms of chromosomal instability

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2-Numerical instability

Dr Gihan Gawish

Many epithelial tumours exhibit an asymmetrical segregation of

chromosomes at the metaphase-anaphase transition, resulting in an

aberrant distribution of the genetic material to the daughter cells

(Steinbeck, 1998).

Also, abnormalities in the number and structure of centrosomes have

been observed in malignancies with aneuploid chromosome numbers,

including cancers of the breast (Lingle and Salisbury, 1999), colon

(Ghadimi et al., 2000), and the head and neck region (Saunders et al.,

2000).

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Numerical instability

Dr Gihan Gawish

changes in centrosomal configuration have been correlated with a number of molecular genetic abnormalities, including amplification of STK15 (Zhou et al., 1998), mutations in TP53 (Carroll et al., 1999), and inactivation of BRCA1 (Xu et al., 1999), BRCA2 (Tutt et al., 1999).

Abnormal centrosomal function may also be induced in vitro by expression of the papilloma virus genes E6 and E7, inhibiting normal TP53 and RB1 activity, respectively (Duensing et al., 2000).

It has also been suggested that inactivation of genes that control the timing of mitotic chromosome segregation may contribute to numerical instability. However, only rare examples of such aberrations have so far been identified (Cahill et al., 1998).

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A number of in vitro systems have shown that amplification of genes conferring resistance to cytotoxic drugs may occur through BFB events (Smith et al., 1992; Ma et al., 1993; Coquelle et al., 1997).

This suggests that a state of chromosomal instability may not only be crucial for tumour development, but may also play a role in resistance to chemotherapy.

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Mitotic-spindle checkpoints: a barrier to CIN

Cancers Usually Result from a Series of Mutations in a Single Cell

oncogene Tumor suppressorsoncogene

Genomic instability: Colon Cancer

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Genomic instability: Colon Cancer

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Dr Gihan Gawish Genomic instability: Colon Cancer

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Genomic instability: Colon CancerDr Gihan Gawish

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Searching for the molecular mechanism of CIN

Dr Gihan Gawish

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Three degree of sister-chromatid separation

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28 Dr Gihan GawishThe link between mitotic gene and CIN

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Dr Gihan Gawish

The link between mitotic gene and CIN

30 Dr Gihan GawishTherapeutic strategies targeting CIN