This increased risk has been linked to the level of immunosuppression and UV exposure. As the duration and dosage of immunosuppressive agents increases, so does the risk of cutaneous malignancy; this effect is reversed with decreasing the dosage of, or taking a break from, immunosuppressive agents. Heart transplant recipients, requiring the highest rates of immunosuppression, are at much higher risk of cutaneous malignancy than liver transplant recipients in whom much lower levels of immunosuppression are needed to avoid rejection.[23,28] The risk appears to be highest in geographic areas of high UV radiation exposure: when comparing Australian and Dutch organ transplant populations, the Australian patients carried a fourfold increased risk of developing SCC and a fivefold increased risk of developing BCC. This speaks to the importance of rigorous sun avoidance among high-risk immunosuppressed individuals.
Individuals with BCCs and/or SCCs report a higher frequency of these cancers in their family members than do controls. The importance of this finding is unclear. Apart from defined genetic disorders with an increased risk of BCC, a positive family history of any skin cancer is a strong predictor of the development of BCC.
Previous personal history of nonmelanoma skin cancer
A personal history of BCC or SCC is strongly associated with subsequent BCC or SCC. There is an approximate 20% increased risk of a subsequent lesion within the first year after a skin cancer has been diagnosed. The mean age of occurrence for these nonmelanoma skin cancers is the mid-60s.[30,31,32,33,34,35] In addition, several studies have found that individuals with a history of skin cancer have an increased risk of a subsequent diagnosis of a noncutaneous cancer;[36,37,38,39] however, other studies have contradicted this finding.[40,41,42,43] In the absence of other risk factors or evidence of a defined cancer susceptibility syndrome, as discussed below, skin cancer patients are encouraged to follow screening recommendations for the general population for sites other than the skin.
Major Genes for Basal Cell Carcinoma
Mutations in the gene coding for the transmembrane receptor protein PTCH, or PTCH1, are associated with basal cell nevus syndrome (BCNS) and sporadic cutaneous BCCs. PTCH, the human homolog of the Drosophila segment polarity gene patched (ptc), is an integral component of the hedgehog signaling pathway, which serves many developmental (appendage development, embryonic segmentation, neural tube differentiation) and regulatory (maintenance of stem cells) roles.
In the resting state, the transmembrane receptor protein PTCH acts catalytically to suppress the seven-transmembrane protein Smoothened (Smo), preventing further downstream signal transduction. Stoichiometric binding of the hedgehog ligand to PTCH releases inhibition of Smo, with resultant activation of transcription factors (GLI1, GLI2), cell proliferation genes (cyclin D, cyclin E, myc), and regulators of angiogenesis.[45,46] Thus, the balance of PTCH (inhibition) and Smo (activation) manages the essential regulatory downstream hedgehog signal transduction pathway. Loss-of-function mutations of PTCH or gain-of-function mutations of Smo tip this balance toward constitutive activation, a key event in potential neoplastic transformation.