Assignment #2 - Paper Review

"Reduced expression of p16 and p27 is correlated with tumour progression in cutaneous melanoma"
by Amira Sanki, Wei Li, Marjorie Cloman, Rooshdiya Z. Karim, John F. Thompson, and Richard A. Scolyer. Pathology (2007). 39(6), pp. 551-557.

Summary

Tumor suppressor genes encode proteins that act to prevent or suppress the formation of cancerous tumors. These proteins affect the cell cycle, either by preventing unchecked proliferation (negative regulation) and/or by inducing apoptosis, or programmed cell death (positive regulation) [1]. When mutations in these genes occur, tumor formation can result and the cells may become malignant*. For example, an estimated 50% of all cancers are associated with a mutation in the p53 tumor suppressor gene [2]. Sanki et al. studied the relationship between a reduction in the p16 and p27 tumor suppressor gene products and melanoma, a cancer of the melanocytes and the most lethal form of skin cancer (see below; Skin Pathology).

Most of our body's normal cells progress through four stages: G1, S, G2, and M, during which the cells grow, synthesize DNA, and eventually divide. The cycle is controlled by cyclin dependent kinases (CDKs) and cyclin dependent kinase inhibitors (CDKIs). CDKs form complexes with cyclins by phosphorylating them, which together phosphorylate other proteins. The result is the release of transcription factors, which allow for the progression of the cell cycle. CDKIs block this progress by preventing the initial phosphorylation of cyclin by CDK. The CDKIs, which include p16 and p27, act in response to DNA damage which would prevent proper functioning of the cell. By stopping the cell cycle before division, the DNA can be repaired, ensuring survival of the cell. A reduction in the expression of these proteins therefore leads to the uncontrolled progression of the cell cycle, and is associated with a number of conditions including cancer of the pancreas, esophagus (p16), bowel, breast, and prostate (p27).

Sanki et al. discussed research in the areas of tumor suppressor genes and melanoma conducted prior to their findings. A genetic link has been established between a family history of melanoma and risk to the individual. For a patient with three or more first degree relatives diagnosed with melanoma, the risk of developing the condition increases by 35-70%. 30-40% of families with three or more members with melanoma have mutations in the CDKN2a gene, which encodes p16. Earlier research has also suggested that a high number of atypical naevi* increases the risk of developing melanoma.

A total of 92 melanocytic lesions, both benign* and malignant, were tested using immunohistochemistry* for the presence of p16 and p27. Antibodies to the proteins were made and allowed to bind to the p16 and p27 antigens. The amounts of these proteins present in the samples were determined by observing staining intensity and applying a staining score. The number of stained cells in each sample was also recorded.

Fig. 1: The right side is of a compound naevus stained for p27. Note the strong staining intensity (staining index = 9) . The left side is of a metastatic melanoma, negative for p27 (staining index = 0).

Sanki et. al (2007)

Sanki et al. found some very interesting results. They observed that as the tumors studied progressed from benign to malignant, the expression of p16 and p27 declined significantly. 100% of the benign samples expressed p27, whereas only 43.6% of primary melanomas expressed p27. A similar pattern was found in p16 expression: 73.7% of benign naevi, 28.8% of primary melanomas, and 14.7% of metastatic* melanomas. No significant correlation was found between p16 or p27 expression and patient survival rate or cancer recurrence.

Critique

I chose this paper based on its discussion of tumor suppressor genes and their role in cancer, a topic I find interesting and important to genetics and cancer research. With more than 4,600 new cases of melanoma reported in Canada in 2007 alone [3], research into the cause of this disease may provide insight into treatment and possibly, in the future, a practical cure.

The purpose of the researchers in writing this paper was clear and thoughtfully planned out. Their method of using immunohistochemistry to observe the presence of p16 and p27 in the samples was clever and effective. However, their method of assigning a staining value to the cells was objective and a matter of personal perception. They circumvented this by having the values assigned by a single pathologist. The paper contained a number of figures that demonstrated their findings (see Figure 1 above), which clearly differentiated between strong positive (protein present) and strong negative (protein absent) results. To someone unfamiliar with research in the area of tumor suppressor genes and cancer, the foreign terminology used could be confusing, if not completely incomprehensible. However, they do attempt to explain some background information with a nice description of the cell cycle and its components in the paper's introduction.

Overall, I found this paper to be a well-written and concise illustration of the association between p16 and p27 and melanoma. The results are promising, and may some day lead to a successful treatment or preventative cure. I would very much like to see further research into the role of tumor suppressor genes in skin cancer and other malignancies.

Definitions

Benign: not cancerous; not capable of invading nearby tissue or spreading to other parts of the body.

Immunohistochemistry: a method that uses antibodies to identify, locate, and stain specific protein molecules in tissue sections

Malignant: cancerous; capable of spreading to other parts of the body

Metastatic: the spread of cancer from the primary site or origin to distant sites in the body

Naevi: pigmented lesions of the skin; moles

References

[1] Griffiths, A. J F., Gelbart, W. M., Lewontin, R. C., and Miller, J. H. (2002). Modern Genetic Analysis. 2nd ed. W. H. Freeman and Company.

[2] Benjamin, C. L., Melnikova, V. O., and Ananthaswamy, H. N. p53 protein and pathogenesis of melanoma and nonmelanoma skin cancer. Adv. Exp. Med. Biol. 2008; 624:265-82.

[3] Canadian Cancer Society, National Cancer Institute of Canada, Statistics Canada, and Provincial/Territorial Cancer Registries. (2007). Canadian Cancer Statistics 2007.

Assignment #1 - My Favorite Tissue: Skin - Introduction

This blog will focus on the function, structure, and pathology of the skin, the largest single organ in the mammalian body. The skin is composed of layers of epidermis and dermis, which are derived from different origins and divided into sublayers. Many cell types make up these layers, from the complex epithelium closest to the external environment, to the connective tissue that separates the skin from the underlying muscles and organs. The skin has a number of functions, the most obvious of which is a barrier providing protection; however, the skin is also involved in heat regulation and sensation, and plays numerous other roles essential to the maintenance of the body's homeostasis. Skin pathologies will affect nearly everyone at some point in their lifetime, and in fact, one-third of all tumors in adults are of the skin [1].

The Structure of Skin

The skin is composed of two main layers: the epidermis and dermis. Below the dermis is the hypodermis, which is composed of loose connective tissue and adipose cells, but is not considered to be part of the skin.

Epidermis:

The epidermal layer is of ectodermal origin. Skin is characterized as either being thick or thin, based on the thickness of the epidermis. Thick skin is present on the palms of the hands and soles of the feet, whereas thin skin is found elsewhere. The epidermis is made up of five sublayers of keratinocytes (keratin-producing cells) , as shown in Figure 2 below [1]:

1) Stratum Corneum:

• The sublayer closest to the external environment. The stratum corneum is composed of stratified squamous epithelial cells, which lack nuclei and are filled with keratin. These cells are metabolically inactive, and are continuously sloughed off. This sublayer forms the main protective barrier of the skin [2].

2) Stratum Lucidum:

• A thin, translucent sublayer, made up of flattened eosinophilic cells. Again, the organelles and nuclei are not visible, as the cells are dead. Thicker in thick skin than in thin skin [1].

3) Stratum Granulosum:

• Flat, hard cells that eventually move up into the stratum corneum sublayer [3]. The cells are polygonal, and filled with basophilic granules [1].

4) Stratum Spinosum:

• A sublayer made up of cuboidal cells, which are known as "prickle cells" due to the presence of thorny cell processes and desmosomes that span the intracellular spaces [4].

5) Stratum Basale:

• The sublayer separating the dermis from the epidermis. These cells may be cubiodal or columnar, and rest on the basement membrane. Most of the mitotic activity of the skin occurs in the stratum basale [1].

Fig. 2: A section of epidermis.

(from http://www.answers.com/topic/stratum-germinativum-1?cat=technology)

Dermis:

The dermal layer is of mesodermal origin. It is much thicker than the epidermis, and is composed mainly of connective tissues. Sweat and sebaceous glands, blood and lymphatic vessels, and hair follicles and nerves are also present here. The dermis layer is divided into two sublayers [1]:

1) Papillary Layer:

• The papillary layer binds the dermis to the epidermis via anchoring fibrils made of collagen. Macrophages, leukocytes, and mast cells, part of the body's immune system, are found within the loose connective tissue composing this layer [1].

2) Reticular Layer:

• Irregular dense connective tissue formed from type I collagen is found in the reticular layer. As well, a network of elastic fibers provide the skin with its elasticity [1]. See Figure 3 below.

Fig. 3: A section of the skin stained to show the elastic fibers of the reticular layer.

(from http://missinglink.ucsf.edu/lm/DermatologyGlossary/dermis.html)

The Function of Skin

The most obvious function of the skin is as a protective barrier against harmful agents in the outside environment. The dense, thick layers of cells help to prevent physical and chemical damage, while preventing microscopic pathogens from entering the body. The cells of the immune system present in the skin, as well as acidic and microbicidal secretions (sweat, oil, etc.) from subcutaneous glands also prevent the entry of potentially harmful invaders. Microbes are unable to colonize the skin due to the constant shedding of the external layer. The damaging effects of the sun's UV radiation on DNA are reduced due to the pigment melanin, present in the cells of the epidermis [1].

The skin also provides a barrier that prevents the loss of components out of the body. The stratum corneum forms an impermeable layer, preventing the evaporation of water and therefore dehydration [1].

The skin also plays a role in heat regulation. An increase in heat production will stimulate the excretion of sweat, which cools the body surface, from merocrine sweat glands which span the layers of the skin (see Figure 4). Heat leaves the body via conduction, passing from dilated blood vessels, through the skin, and into the external environment. A decrease in circulation under the skin will conserve heat [5].

Fig. 4: A section of sweat glands in the skin.

(from Junqueira and Carneiro, 2005)

The epidermal layer is rich in free nerve endings, as are the dermal papillae (extensions of the dermis into the epidermis). These nerve endings (Figure 5) provide the body with critical information from the environment. Pain, temperature, pressure, and tactile stimuli are received at the skin's surface, sending information to the nervous systems so as to elicit an appropriate response [1].

Fig. 5: Four types of nerve endings found in the skin.

(from Junquiera and Carneiro, 2005)

Skin Pathology

A number of conditions affect the skin, including rashes and acne, which are relatively harmless. The most serious disease affecting the skin is skin cancer, which is the most common form of cancer [6]. There are two types of skin cancer: non-melanoma and malignant melanoma.

Non-melanoma Skin Cancer:

• The most common type of skin cancer, non-melanoma is cancer of the basal or squamous cells of the skin. Basal cell cancers are the result of UV damage to basal cells [7]. They have shiny or pearly surfaces [6]. Squamous cell cancers have numerous causes and affect the skin's squamous epithelial cells. They have reddish, scaly surfaces. Non-melanoma cancers are unlikely to metastasize (i.e. spread throughout the body) [6].

Malignant Melanoma Skin Cancer:

• Malignant melanoma is cancer of the melanocytes, melanin-producing cells in the skin. This cancer is also caused by damaging UV radiation, which stimulates melanin production and the clustering of melanocytes, forming moles. Malignant melanoma is the most serious type of skin cancer [8].

The successful treatment rate of skin cancer is generally very high, due to programs aimed at prevention and early detection. The "ABCD's" of skin cancer is a guideline allowing the identification of irregular, suspicious moles (see Figure 6). Through the use of such educational techniques, it may be possible to increase the chance of a successful recovery from skin cancer [9].

Fig. 6: The ABCD's of skin cancer.

(from http://www.revolutionhealth.com/conditions/skin/skin-care/your-health/melanoma-photos

References

[1] Junqueira, L. C., and Carnerio, J. (2005). Basic Histology: Text & Atlas. 11th ed. McGraw-Hill.

[2] Barker, R. L., and Coletta G. C. (1986). Performance of Protective Clothing. ASTM International.

[3] Definitions. Retrieved March 1, 2008, from Pharmagel Official Site web site: http://www.pharmagel.net/definitions.php

[4] Kühnel, W. (2003). Color Atlas of Cytology, Histology, and Microscopic Anatomy. 4th ed. Thieme.

[5] Keipert, J. A. (1990). Essential Pediatric Dermatology. Taylor & Francis.

[6] What is non-melanoma skin cancer? Retrieved March 4, 2008, from Canadian Cancer Society web site: http://www.cancer.ca/ccs/internet/standard/0,2939,3172_10175_87619_langId-en,00.html

[7] McClay, E. F., and Smith, J. (2003). 100 Questions & Answers about Melanoma and Other Skin Cancers. Jones & Bartlett Publishers.

[8] Barnhill, R. L. (2004). Pathology of Malignant Melanoma. Springer.

[9] Colditz, G. A. (2000). Cancer Prevention: The Causes and Prevention of Cancer. Springer.