Breast Cancer Lecture 5: DNA Repair and Genomic Integrity

Breast Cancer and Genomic Integrity

• Family history influences breast cancer risk, with more relatives having breast cancer increasing the individual's risk.
• Breast cancer in young close relatives is also a risk factor.

BRCA1

• Linked to a DNA marker on chromosome 17.
• Truncation mutations are common in families with multiple cases.
• Relatively large gene, making it a big target for mutation.
• Over 1500 distinct germline mutations reported.
• BRCA1 families do not show cases of male breast cancer.
• BRCA1 breast tumors tend to be high grade and look unusual.
• BRCA1 ovarian tumors are invasive epithelial tumors.

BRCA1 Structure and Function

• The ring domain interacts with other proteins via the zinc finger.
• The NLS (nuclear localisation signal) indicates that the protein functions in the nucleus.
• The BRCT domain is seen in proteins involved in the cell cycle and checkpoints responding to DNA damage.
• BRCA1 localizes to DNA damage regions and is integral to repair.
• It co-localizes with other DNA repair proteins, including Rad51.
• BRCA1 promotes homologous recombination, a less error-prone repair process.

BRCA1 and Homology Directed DNA Repair

• BRCA1 acts within a complex crucial for homologous recombination.
• Resection of one DNA strand is necessary for homologous recombination.
• BRCA1's exonuclease activity is required for resection.
• If BRCA1 is damaged, non-homologous recombination may occur, leading to mutations.

BRCA1 and BARD1

• BRCA1 and BARD1 form a heterodimer with E3 ubiquitin ligase activity.
• The complex activates and recruits proteins to repair DNA lesions.
• BRCA1 regulates the G2/M checkpoint by activating Chk1 kinase and excluding CDC25 from the nucleus.

BRCA2

• Located on chromosome 13q12-13.
• Relatively large gene, widely expressed.
• Mutations in BRCA2 increase breast cancer risk in males to 5% (x200 that of the general population).
• 14% of male breast cancers are derived from BRCA2 mutations.
• Analysis of large families indicates a 70% risk by age 70.
• Other tumors in BRCA2 families include prostate, pancreas, and lung.

BRCA2 Structure and Function

• Less diverse than BRCA1.
• Transcriptional activation region.
• Multiple sites bind to Rad51.
• Nuclear localisation signal.
• BRCA2 helps BRCA1 carry Rad51.

Roles for BRCA2

• Disruption of BRCA1 and 2 leads to similar phenotypes.
• BRCA2 is necessary for the repair of DSBs by gene conversion.
• BRCA2 binds tightly to Rad51 via the BRC region, indicating a direct role in recombination repair.

BRCA2 and Rad52 in Homologous Recombination

• DNA damage/replication arrest causes DSBs.
• DSBs activate DNA damage signaling kinases (ATM or ATR).
• Phosphorylated BRCA2 releases Rad52, allowing it to bind to DNA and repair the damage.

Cell Cycle and Checkpoint

• Checkpoint arrest following damage is less affected by the disruption of BRCA2 than 1.
• BRCA1 may link BRCA2 DNA repair functions to the pathways that signal incomplete replication or damage.
• BRCA2 may play a role in the repair process by loading Rad51 onto BRCA1.

Mutations and Allele Penetrance

• Penetrance: the ability of a mutation to cause clinical disease in an individual carrying a specific allele.

Non-BRCA Derived Tumors

• Rare familial conditions that result in breast cancer include Fanconi Anemia and Ataxia Telangectasia (AT).
• BRCA type tumors are recognizable due to specific phenotypes, including basal tumors, negative EGFR expression, and lymphocytic infiltration.

Therapy and Treatment

• Surgery, radiotherapy, chemotherapy, and hormonal therapy (anti-oestrogen therapy) are treatment options.
• Herceptin targets the ERBB2 receptor, effective in 30% of early-stage breast cancer.
• Hormone resistance can be acquired or intrinsic.

Hormone Treatment

• 75% of sporadic tumors are Oestrogen receptor (ER) positive.
• Tamoxifen is an oestrogen agonist, effective in 70% of ER-positive and/or progesterone-positive tumors.
• However, 30-50% relapse, and tamoxifen can only be taken for 5 years.

Hormone Resistance

• Intrinsic resistance: tumors that do not express receptors will be resistant.
• Acquired resistance: tumors that stop expressing the receptor and grow in the absence of oestrogen.

Breast Cancer and Genomic Integrity

• Family history influences breast cancer risk, with more relatives having breast cancer increasing the individual's risk.
• Breast cancer in young close relatives is also a risk factor.

BRCA1

• Linked to a DNA marker on chromosome 17.
• Truncation mutations are common in families with multiple cases.
• Relatively large gene, making it a big target for mutation.
• Over 1500 distinct germline mutations reported.
• BRCA1 families do not show cases of male breast cancer.
• BRCA1 breast tumors tend to be high grade and look unusual.
• BRCA1 ovarian tumors are invasive epithelial tumors.

BRCA1 Structure and Function

• The ring domain interacts with other proteins via the zinc finger.
• The NLS (nuclear localisation signal) indicates that the protein functions in the nucleus.
• The BRCT domain is seen in proteins involved in the cell cycle and checkpoints responding to DNA damage.
• BRCA1 localizes to DNA damage regions and is integral to repair.
• It co-localizes with other DNA repair proteins, including Rad51.
• BRCA1 promotes homologous recombination, a less error-prone repair process.

BRCA1 and Homology Directed DNA Repair

• BRCA1 acts within a complex crucial for homologous recombination.
• Resection of one DNA strand is necessary for homologous recombination.
• BRCA1's exonuclease activity is required for resection.
• If BRCA1 is damaged, non-homologous recombination may occur, leading to mutations.

BRCA1 and BARD1

• BRCA1 and BARD1 form a heterodimer with E3 ubiquitin ligase activity.
• The complex activates and recruits proteins to repair DNA lesions.
• BRCA1 regulates the G2/M checkpoint by activating Chk1 kinase and excluding CDC25 from the nucleus.

BRCA2

• Located on chromosome 13q12-13.
• Relatively large gene, widely expressed.
• Mutations in BRCA2 increase breast cancer risk in males to 5% (x200 that of the general population).
• 14% of male breast cancers are derived from BRCA2 mutations.
• Analysis of large families indicates a 70% risk by age 70.
• Other tumors in BRCA2 families include prostate, pancreas, and lung.

BRCA2 Structure and Function

• Less diverse than BRCA1.
• Transcriptional activation region.
• Multiple sites bind to Rad51.
• Nuclear localisation signal.
• BRCA2 helps BRCA1 carry Rad51.

Roles for BRCA2

• Disruption of BRCA1 and 2 leads to similar phenotypes.
• BRCA2 is necessary for the repair of DSBs by gene conversion.
• BRCA2 binds tightly to Rad51 via the BRC region, indicating a direct role in recombination repair.

BRCA2 and Rad52 in Homologous Recombination

• DNA damage/replication arrest causes DSBs.
• DSBs activate DNA damage signaling kinases (ATM or ATR).
• Phosphorylated BRCA2 releases Rad52, allowing it to bind to DNA and repair the damage.

Cell Cycle and Checkpoint

• Checkpoint arrest following damage is less affected by the disruption of BRCA2 than 1.
• BRCA1 may link BRCA2 DNA repair functions to the pathways that signal incomplete replication or damage.
• BRCA2 may play a role in the repair process by loading Rad51 onto BRCA1.

Mutations and Allele Penetrance

• Penetrance: the ability of a mutation to cause clinical disease in an individual carrying a specific allele.

Non-BRCA Derived Tumors

• Rare familial conditions that result in breast cancer include Fanconi Anemia and Ataxia Telangectasia (AT).
• BRCA type tumors are recognizable due to specific phenotypes, including basal tumors, negative EGFR expression, and lymphocytic infiltration.

Therapy and Treatment

• Surgery, radiotherapy, chemotherapy, and hormonal therapy (anti-oestrogen therapy) are treatment options.
• Herceptin targets the ERBB2 receptor, effective in 30% of early-stage breast cancer.
• Hormone resistance can be acquired or intrinsic.

Hormone Treatment

• 75% of sporadic tumors are Oestrogen receptor (ER) positive.
• Tamoxifen is an oestrogen agonist, effective in 70% of ER-positive and/or progesterone-positive tumors.
• However, 30-50% relapse, and tamoxifen can only be taken for 5 years.

Hormone Resistance

• Intrinsic resistance: tumors that do not express receptors will be resistant.
• Acquired resistance: tumors that stop expressing the receptor and grow in the absence of oestrogen.