A Pearly Bump That Would Not Heal: The Pathophysiology of Basal Cell Carcinoma From Ultraviolet Damage to the Hedgehog Pathway
[Student Name]
University of Phoenix
NSG/521: Advanced Pathophysiology
Week 2 Assignment
[Instructor Name]
[Date]
Composite patient written as a model document. No real patient is described.
A composite 68-year-old man who spent 35 years as a commercial fisherman came to his nurse practitioner about a small bump beside his left nostril that had bled on and off for eight months. He thought it was a pimple that would not heal. The lesion was a 7 mm pearly, translucent papule with a rolled border, fine branching blood vessels on its surface and a small central crust. Biopsy confirmed nodular basal cell carcinoma. The lesion was small, but it was the visible end of a chain of events that began with decades of sunlight on the water and ended with a single signaling pathway that could no longer be switched off. This paper explains the pathophysiology of basal cell carcinoma as it applies to his case.
Ultraviolet Radiation and DNA Damage
Basal cell carcinoma is the most common cancer in people with light skin, and ultraviolet radiation is its main environmental cause (Cameron et al., 2019). Ultraviolet B radiation is absorbed directly by DNA in keratinocytes and causes adjacent pyrimidine bases, cytosine and thymine, to bond abnormally, forming cyclobutane pyrimidine dimers and related lesions. Cells repair most of this damage through nucleotide excision repair. When repair fails or is incomplete, replication across the damaged site introduces characteristic mutations, typically cytosine changed to thymine at sites where two pyrimidines sit side by side. These signature mutations mark the tumor as a product of sunlight.
The patient's history fits. Years on open water exposed him to both direct and reflected ultraviolet light, and the central face, especially the nose, receives some of the highest cumulative exposure of any skin surface. Risk rises with fair skin, a tendency to burn, older age and immune suppression; intermittent intense exposure and cumulative exposure both contribute.
The Hedgehog Pathway
The critical target of those mutations in basal cell carcinoma is the hedgehog signaling pathway. In normal adult skin, the pathway is largely inactive. The receptor PTCH1 restrains a second membrane protein, smoothened (SMO), preventing it from signaling. When a hedgehog ligand binds PTCH1, the restraint is released, SMO becomes active and a cascade leads to activation of GLI transcription factors, which turn on genes that drive cell proliferation (Epstein, 2008).
In most basal cell carcinomas, the pathway is switched on without any ligand. The most common cause is loss of function of PTCH1, often through ultraviolet-signature mutations in one copy and loss of the other copy, which removes the brake on SMO. A smaller group of tumors carries activating mutations in SMO itself. Mutations in TP53, the gene that normally halts the cycle of damaged cells, are also common and add to genomic instability (Epstein, 2008). The result is constitutive GLI activity in cells of the basal layer of the epidermis or hair follicle, which proliferate without their normal limits.
People with nevoid basal cell carcinoma syndrome, also called Gorlin syndrome, inherit one nonfunctional copy of PTCH1 and develop many basal cell carcinomas from a young age, which provided early evidence that the hedgehog pathway was central to this cancer.
From Pathway to Lesion
The mechanism explains the lesion's appearance and behavior. Tumor cells resemble basal keratinocytes and grow in nests with a characteristic palisading arrangement at their edges, surrounded by a specialized stroma. The translucent, pearly quality of a nodular lesion reflects these densely packed nests beneath a thinned epidermis. The rolled border comes from tumor growth at the periphery. Surface blood vessels, seen as fine branching telangiectasias, reflect the vascular supply the tumor induces. As the center outgrows its support, the overlying skin breaks down, producing the crusting and intermittent bleeding the patient noticed (Cameron et al., 2019).
The tumor's dependence on its stroma helps explain why basal cell carcinoma, despite its local invasiveness, very rarely metastasizes. Left untreated, however, it continues to invade surrounding tissue, and on the nose it can destroy cartilage and extend along embryonic fusion planes, which is why lesions on the central face are considered high risk for recurrence and are often treated with tissue-sparing surgery that examines all margins.
Distinguishing It From Look-Alike Lesions
The mechanism also helps separate basal cell carcinoma from lesions that can look similar. Squamous cell carcinoma arises from more superficial keratinocytes, often through TP53 mutations and precursor actinic keratoses; it tends to be scaly or keratotic rather than pearly, grows faster and carries a small but real risk of metastasis. Sebaceous hyperplasia, common on the nose of older adults, produces yellowish papules with a central dimple and does not bleed or ulcerate. Amelanotic melanoma can be pink and vascular and is the look-alike that must not be missed, since its behavior is far more aggressive. An intradermal nevus can be dome-shaped and skin-colored but is usually present for years without change or bleeding. The features that pointed to basal cell carcinoma, translucency, a rolled border, arborizing vessels and a nonhealing central crust, each follow from the tumor's structure described above, and the biopsy confirmed what the examination suggested.
From Mechanism to Treatment
Most basal cell carcinomas are cured by surgical excision or, for high-risk sites such as the nose, by Mohs micrographic surgery, which removes the tumor layer by layer while checking every margin. The patient was referred for Mohs surgery.
Understanding the hedgehog pathway also produced targeted therapy for the minority of patients with locally advanced or metastatic disease that surgery or radiation cannot treat. Vismodegib, an oral inhibitor of SMO, blocks the pathway downstream of the defective PTCH1 brake. Sekulic et al. (2012), in the trial that led to its approval for patients with advanced basal cell carcinoma, found meaningful tumor responses in both locally advanced and metastatic disease. The drug's side effects, muscle cramps, loss of taste and hair loss, reflect the roles of the hedgehog pathway in normal tissues, and tumors can become resistant through new SMO mutations, another demonstration of the pathway's central role.
Prevention and Follow-Up
Because the damage is cumulative, a person with one basal cell carcinoma has a substantially higher risk of developing others. The patient will need a full skin examination at regular intervals, instruction in self-examination and counseling about sun protection, including broad-spectrum sunscreen, a wide-brimmed hat and protective clothing if he returns to the water as a hobby. Prevention cannot undo past damage, but it can reduce the new ultraviolet mutations that add to it.
Conclusion
Decades of ultraviolet exposure produced signature DNA mutations in this fisherman's skin, and when those mutations disabled PTCH1, the hedgehog pathway stayed switched on in basal keratinocytes. The resulting tumor grew slowly in nests supported by its own stroma, producing a pearly papule with a rolled border, telangiectasias and a crusting center. The same mechanism explains why the tumor rarely spreads, why surgery cures most cases and why a drug that blocks SMO can treat the few that surgery cannot.
References
Cameron, M. C., Lee, E., Hibler, B. P., Barker, C. A., Mori, S., Cordova, M., Nehal, K. S., & Rossi, A. M. (2019). Basal cell carcinoma: Epidemiology; pathophysiology; clinical and histological subtypes; and disease associations. Journal of the American Academy of Dermatology, 80(2), 303-317. https://doi.org/10.1016/j.jaad.2018.03.060
Epstein, E. H. (2008). Basal cell carcinomas: Attack of the hedgehog. Nature Reviews Cancer, 8(10), 743-754. https://doi.org/10.1038/nrc2503
Sekulic, A., Migden, M. R., Oro, A. E., Dirix, L., Lewis, K. D., Hainsworth, J. D., Solomon, J. A., Yoo, S., Arron, S. T., Friedlander, P. A., Marmur, E., Rudin, C. M., Chang, A. L. S., Low, J. A., Mackey, H. M., Yauch, R. L., Graham, R. A., Reddy, J. C., & Hauschild, A. (2012). Efficacy and safety of vismodegib in advanced basal-cell carcinoma. New England Journal of Medicine, 366(23), 2171-2179. https://doi.org/10.1056/NEJMoa1113713
How this NSG 521 Week 2 example is structured
The University of Phoenix library guide for NSG/521 lists Week 2 as EENT, Respiratory and Dermatology, with skin cancer among the research topics. The paper connects each level of the mechanism, DNA damage, a signaling pathway, tissue behavior and clinical appearance, to the patient's lesion, so the reader can see why this cancer looks and behaves as it does. The treatment section closes the loop by showing that understanding the pathway produced a drug that blocks it. Students search this week as NSG 521 Week 2, NSG521 Wk 2 or NSG/521 Wk 2; all three are the same assignment.
NSG/521 Week 2 questions, answered
What does NSG/521 Week 2 usually ask for?
The library guide for NSG/521 lists Week 2 as EENT, respiratory and dermatology, with topics including asthma, COPD, pneumonia, EENT conditions and skin cancer. Many sections ask for a paper explaining the pathophysiology of one condition from these systems and relating it to a clinical presentation.
Why does basal cell carcinoma rarely spread?
Basal cell carcinoma grows slowly and depends closely on the surrounding stroma, the supporting tissue around the tumor cells. It invades locally and can destroy nearby tissue, but metastasis is rare, which is why local treatment is usually curative.
What is the hedgehog pathway?
It is a cell signaling pathway important in embryonic development that is normally switched off in most adult skin cells. In most basal cell carcinomas, mutations leave the pathway switched on, driving the growth of tumor cells that resemble the basal layer of the epidermis.
Write yours, or have the desk draft it
This paper is an original model document written by our desk, not a submitted student paper and not an official University of Phoenix document. Read it for the moves, then write your own to the instructions in your classroom. If you want one built to your exact prompt and rubric, the first custom sample is free and arrives in 24 to 48 hours.