Breast cancer pathophysiology

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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Overview

Genes involved in the pathogenesis of breast cancer include BRCA1, BRCA2 and p53. On microscopic histopathological analysis, minimal tubule formation, marked pleomorphism, and numerous mitotic figures are characteristic findings of breast cancer.

Pathogenesis

Majority of breast malignancies arise from epithelial cells and hence are carcinomas. Microscopic appearance and biologic behavior divide breast carcinomas to several diverse groups. based upon the growth pattern and cytologic features of the lesions,The in situ carcinomas of the breast divide into ductal (also known as intraductal carcinoma) or lobular. Surprisingly their anatomic location within the mammary ductal-lobular system. is not the determinant factor in this classification.

Table below shows the invasive breast carcinomas several histologic subtypes; based on a series of 135,157 women with breast cancer reported to the SEER database o between 1992 and 2001.

Histologic subtypes of invasive breast carcinomas
Infiltrating ductal	76 %
Invasive lobular	8 %
Ductal/lobular	7 %
Mucinous (colloid)	2.4 %
Tubular	1.5 %
Medullary	1.2 %
Papillary	1 %
Other subtypes, including: ● Metaplastic breast cancer ● Invasive micropapillary breast cancer	less than 5 %

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Bone Metastasis

• Bone is the most common site of breast cancer distant spread. Bone metastases due to breast cancer cause major morbidity, decrease survival and reduce quality of life of many patients.

Cancer influence on the skeleton results in two main negative consequences: pain and Skeletal-Related events (SREs), defined as any of the following:

• pathologic fracture,
• a requirement for surgical intervention and palliative radiotherapy to bone lesions,
• spinal cord compression,
• hypercalcemia of malignancy .

In fact, SREs constitute readily measured clinical parameters that are employed in clinics and clinical trials.

• Many disciplines should be involved in the management of breast cancer bone metastases, including medical oncology, pain and palliative care, radiation oncology, orthopedic surgery and neurosurgery. Systemic therapy delays the progression of bone metastases and provides palliation; it includes endocrine therapy, biologic agents, chemotherapy, bisphosphonate therapy and the new osteoclast inhibitors.
• A thorough knowledge of the molecular basis of bone metastasis caused by breast cancer is essential for the understanding of the therapeutic approach. In fact, The normal balance between bone resorption and deposition is significantly affected by cancer. Bone metastases due to breast cancer are mostly osteolytic lesions, though predominant osteoblastic disease can occur .

The breast cancer cells and the bone microenvironment interact extensively through many chemical mediators resulting in bone destruction and tumor growth. These molecular mediators (pimarily Osteopontin, CXCR4, CTGF and Interleukin-11) exert their effect on osteoclasts which in turn cause bone resorption. This osteoclast-mediated bone resorption is thought to be the product of the action of numerous molecules including:

• PTHrP (Parathyroid Hormone–related Peptide),
• Tumor Necrosis Factor α (TNF-α),
• Cytokines such as Interleukin-1, Interleukin-6, Interleukin-8, and interleukin-11

• These factors signal osteoblasts (the bone-building cells) to induce osteoclast differentiation through the RANKL (the ligand for the receptor activator of nuclearfactor-κB [RANK])- RANK signaling. When Osteoclasts lyse bone, they cause the release of growth factors such as bone morphogenetic proteins (BMPs), IGF-I and TGF-β from the bone matrix which stimulate and maintain tumor cell proliferation and induce further release of PTHrP .

Genetics

• Today, breast cancer, like other forms of cancer, is considered to be the final outcome of multiple environmental and hereditary factors. Some of the effects of environmental and hereditary factors that ultimately cause breast cancer are:

1. Lesions to DNA such as genetic mutations. Exposure to estrogen has been experimentally linked to the mutations that cause breast cancer. Beyond the contribution of estrogen, research has implicated viral oncogenesis and the contribution of ionizing radiation.
2. Failure of immune surveillance, which usually removes malignancies at early phases of their natural history.
3. Abnormal growth factor signaling in the interaction between stromal cells and epithelial cells. For example: in the angiogenesis, it is necessary to promote new blood vessel growth near new cancers.
4. Inherited defects in DNA repair genes, such as BRCA1, BRCA2 and p53.

• Breast cancer, like other cancers, occurs because of an interaction between the environment and a defective gene. Normal cells divide as many times as needed and stop; they attach to other cells and stay in place in tissues. Cells become cancerous when mutations destroy their ability to stop dividing, to attach to other cells, and to stay where they belong.

• Normal cells will commit cell suicide (apoptosis) when they are no longer needed. Until then, they are protected from cell suicide by several protein clusters and pathways. One of the protective pathways is the PI3K/AKT pathway; another one is the RAS/MEK/ERK pathway. Sometimes the genes along these protective pathways are mutated in a way that turns them permanently "on", rendering the cell incapable of committing suicide when it is no longer needed. This is one of the steps that causes cancer in combination with other mutations. Normally, the PTEN protein turns off the PI3K/AKT pathway when the cell is ready for cell suicide. In some breast cancers, the gene for the PTEN protein is mutated, so the PI3K/AKT pathway is stuck in the "on" position, and the cancer cell does not commit suicide.
• Mutations that can lead to breast cancer have been experimentally linked to estrogen exposure and failure of immune surveillance, the removal of malignant cells throughout one's life by the immune system.
• Abnormal growth factor signaling in the interaction between stromal cells and epithelial cells can facilitate malignant cell growth. In breast adipose tissue, overexpression of leptin leads to increased cell proliferation and cancer.
• In the United States, 10 to 20 % of patients with breast cancer or ovarian cancer have a first- or second-degree relative with one of these diseases. The familial tendency to develop these cancers is called hereditary breast—ovarian cancer syndrome. The best known of these, the BRCA mutations, confer a lifetime risk of breast cancer of between 60 and 85 % and a lifetime risk of ovarian cancer of between 15 and 40 %. Some mutations associated with cancer, such as p53, BRCA1 and BRCA2, occur in mechanisms to correct errors in DNA. These mutations are either inherited or acquired after birth. Presumably, they allow further mutations, which lead to uncontrolled division, lack of attachment, and metastasis to distant organs.However, there is strong evidence of residual risk variation that goes well beyond hereditary BRCA gene mutations between carrier families; this is caused by unobserved risk factors. It implicates that environmental factors and other causes are triggers for breast cancer. The inherited mutation in BRCA1 or BRCA2 genes can interfere with repair of DNA cross links and DNA double strand breaks (known functions of the encoded protein). Because of this repair deficit, risks from carcinogenic chemicals and ionizing radiation can increase.These carcinogens cause DNA damage, such as to DNA cross links and double strand breaks that often require repairs by pathways containing BRCA1 and BRCA2. But it is these repair pathways that can be crippled by inherited mutation. There is evidence that cancer risks increase in mutation carriers exposed to such opportunistic carcinogens. Thus, risks for cancers may be reduced by avoiding or compensating for carcinogens that exploit the inherited BRCA gene deficiency However, mutations in BRCA genes account for only 2 to 3 % of all breast cancers. About half of hereditary breast–ovarian cancer syndromes involve unknown genes.

Microscopic Pathology

• Shown below is a micrograph of high grade invasive ductal carcinoma showing minimal tubule formation,marked pleomorphism, and numerous mitotic figures.