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Section: MBG 110-1

Homework Essay: Cancer as a genetic disease: What is the
molecular basis of inherited breast cancer

Date: 20 Dec 2017

Cancer
as a genetic disease: What is the molecular basis of inherited breast cancer

A
disease in which malignant cells form tumor in the tissue of breast is called
breast cancer. Signs of breast cancer include a lump or change in the breast.
There are 3 types of breast cancer, ductal carcinoma which begins in the cells
of the ducts of the breast, lobular carcinoma which begins in the lobes or
lobules of the breast, whereas inflammatory breast
cancer is an uncommon type in which breast show sign of inflammation that are
rubor, calor, dolor, and tumor. A family history of breast cancer
increases the risk factor for developing breast cancer.1 The risk of
developing breast cancer in a woman’s lifetime is about 12%. Women who have
mutated BRCA1 or BRCA2 genes have up to 80% risk of developing breast cancer.
Men who have mutated BRCA2 gene have 8% risk of developing breast cancer by the
age of 80.2

Humans
contain about 21000 genes. Genes are a distinct sequence of DNA (De-Ribonucleic
Acid) present in chromosomes. DNA contains instructions for building proteins
which control the function and structure of all the cells of the body. These
instructions are present in DNA as a combination of 4 nucleotides (Adenine,
Thymine, Cytosine, Guanine). When even one of the nucleotide is changed, it
changes the DNA instructions to a great extent. This results in the synthesis
of wrong protein. This wrong protein has a different function which can be
harmful for the body. 2

            Breast cancer may be caused by inherited oncogenes. This is known as hereditary breast
cancer. Out of all types of breast cancer, hereditary breast cancer comprises
5% to 10%. Oncogenes like BRCA1 and BRCA2 increase the risk factor of
developing breast cancer in a woman. 1  

            BRCA1 and BRCA2 genes are tumor suppressor genes. BRCA1
gene’s protein is called breast cancer type 1 susceptibility protein and BRCA2
gene’s protein is called breast cancer type 2 susceptibility protein. The
function of both of these proteins is to repair DNA. Both BRCA1 and BRCA2
proteins interact with RAD51 (another eukaryotic gene) during repair of DNA
double-stranded breaks (which can be caused by radiations or during crossing
over in meiosis) by homologous recombination. 3 Their proteins also make sure
that if DNA strand breaks are not repairable, then defected cells are killed.
This can be noted here that having an abnormal copy of RAD51 gene can also
increase the risk of developing breast cancer. 2

            When mutations in BRCA1 and BRCA2 occur, DNA
double-stranded breaks can no longer be repaired. These mutations also prevent
these genes to destroy defected cells. This causes a permanent faulty DNA which
results in a tumor due to uncontrolled cell division.

            BRCA1 and BRCA2 are not only known genes associated with
inherited breast cancer, there are some other genes which are also associated
with breast cancer. 2 These genes include ATM, BRIP1, CDH1, CHEK2,
MRE11A, NBN, PALB2, PTEN, RAD50, RAD51C, STK11, and TP53. The abnormalities in
these genes are not common. Mutations in these genes do not increase the risk
of breast cancer as much as mutated BRCA1 and BRCA2 genes. 2

            ATM and BRIP1 gene is responsible for the repair of
damaged DNA. Inheriting one copy of the mutated version of these genes is
linked with the development of breast cancer as the damaged genes stop the
cells to repair DNA. 2

            CDH1 gene makes protein which is helpful in binding cells
with tissues. Inheriting one copy of the abnormal CDH1 gene causes a woman to
have 39% to 52% lifetime risk of lobular cancer. 2

            CHEK2 gene makes protein which stops tumor growth.
Abnormal CHEK2 gene increases the risk of developing breast cancer by 100%.2

            MRE11A, RAD50, and NBN genes together form MRN complex
which is helpful in the repair of DNA material in the cells. Any abnormality in
these genes can increase the risk of developing breast cancer. 2

            PALB2 gene is known as partner and localizer of BRCA2
gene. It codes for a protein that is responsible for working with BRCA2 gene to
repair damaged DNA and stop tumor growth. Abnormality in this gene can increase
the risk of developing breast cancer by 33% to 58%.2

            PTEN gene is responsible for regulating cell growth.
Abnormal PTEN gene causes Cowden Syndrome. This disorder increases the risk of
developing both benign and malignant breast tumors up to 85%.2

            STK11 gene is also responsible for regulating cell
growth. The abnormal STK11 gene causes Peutz Jegher Syndrome. This disorder
increases the risk of breast cancer. 2

            TP53 gene is responsible for making a protein which stops
tumor growth. One mutated copy of TP53 gene causes Li-Fraumeni syndrome. People
with this syndrome have a higher risk of developing breast cancer than others
(100% in women and 73% in men). 2

            Apart from all these genes, there is SEC23B gene, which
is also linked by Cowden Syndrome. Clinical tests for this gene are not still
available as this is a new discovery. 2

            Summarizing it all up, breast cancer can be caused by a
mutation in BRCA1, BRCA2, ATM, BRIP1, CDH1, CHEK2, MRE11A, NBN, PALB2, PTEN,
RAD50, RAD51C, STK11 and TP53 genes. These genes mainly have tasks of repair
DNA damage, stop tumor growth or/and regulating cell growth. When these
functions are destroyed (by a mutation in the gene), cells start reproducing
without check and become tumors. Among these genes, only BRCA1 and BRCA2 gene
increase the risk of development of breast cancer noticeably, whereas other
genes do not.

References:

1.         PDQ® Adult Treatment Editorial Board. PDQ Breast Cancer
Treatment. Bethesda, MD: National Cancer Institute. Updated <11/02/2017>.
Retrieved from https://www.cancer.gov/types/breast/patient/breast-treatment-pdq.
Accessed <12/20/2017>. PMID: 26389406

2.         Breast Cancer Risk Factor: Genetics. Breastcancer.org.
Retrieved from http://www.breastcancer.org/risk/factors/genetics.
Accessed <12/21/2017>

3.         Boulton, S. (2006). Cellular Functions of the BRCA Tumour-Suppressor
Proteins. Biochemical Society Transactions, 34(5), 633-645. Retrieved from https://doi.org/10.1042/bst0340633

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