The Longer Your Telomeres, the Larger Your Nevus?:

Review: Are moles senescent?

Melanocytic nevi (skin moles) have been regarded as a valuable example of cell senescence occurring in vivo. However, a study of induced nevi in a mouse model reported that the nevi were arrested by cell interactions rather than a cell-autonomous process like senescence, and that size distributions of cell nests within nevi could not be accounted for by a stochastic model of oncogene-induced senescence. Moreover, others reported that some molecular markers used to identify cell senescence in human nevi are also found in melanoma cells-not senescent. It has thus been questioned whether nevi really are senescent, with potential implications for melanoma diagnosis and therapy. Here I review these areas, along with the genetic, biological, and molecular evidence supporting senescence in nevi. In conclusion, there is strong evidence that cells of acquired human benign (banal) nevi are very largely senescent, though some must contain a minor non-senescent cell subpopulation. There is also persuasive evidence that this senescence is primarily induced by dysfunctional telomeres rather than directly oncogene-induced.

Pathways from senescence to melanoma: focus on MITF sumoylation

Cutaneous melanoma is a deadly skin cancer that originates from melanocytes. The development of cutaneous melanoma involves a complex interaction between environmental factors, mainly ultraviolet radiation from sunlight, and genetic alterations. Melanoma can also occur from a pre-existing nevus, a benign lesion formed from melanocytes harboring oncogenic mutations that trigger proliferative arrest and senescence entry. Senescence is a potent barrier against tumor progression. As such, the acquisition of mutations that suppress senescence and promote cell division is mandatory for cancer development. This topic appears central to melanoma development because, in humans, several somatic and germline mutations are related to the control of cellular senescence and proliferative activity. Consequently, primary melanoma can be viewed as a paradigm of senescence evasion. In support of this notion, a sumoylation-defective germline mutation in microphthalmia-associated transcription factor (MITF), a master regulator of melanocyte homeostasis, is associated with the development of melanoma. Interestingly, this MITF variant has also been recently reported to negatively impact the program of senescence. This article reviews the genetic alterations that have been shown to be involved in melanoma and that alter the process of senescence to favor melanoma development. Then, the transcription factor MITF and its sumoylation-defective mutant are described. How sumoylation misregulation can change MITF activity and impact the process of senescence is discussed. Finally, the contribution of such information to the development of anti-malignant melanoma strategies is evaluated.

Melanocytic nevi and melanoma: unraveling a complex relationship

Approximately 33% of melanomas are derived directly from benign, melanocytic nevi. Despite this, the vast majority of melanocytic nevi, which typically form as a result of BRAF^V600E-activating mutations, will never progress to melanoma. Herein, we synthesize basic scientific insights and data from mouse models with common observations from clinical practice to comprehensively review melanocytic nevus biology. In particular, we focus on the mechanisms by which growth arrest is established after BRAF^V600E mutation. Means by which growth arrest can be overcome and how melanocytic nevi relate to melanoma are also considered. Finally, we present a new conceptual paradigm for understanding the growth arrest of melanocytic nevi in vivo termed stable clonal expansion. This review builds upon the canonical hypothesis of oncogene-induced senescence in growth arrest and tumor suppression in melanocytic nevi and melanoma.

Genetics of melanoma progression: the rise and fall of cell senescence

There are many links between cell senescence and the genetics of melanoma, meaning both familial susceptibility and somatic-genetic changes in sporadic melanoma. For example, CDKN2A, the best-known melanoma susceptibility gene, encodes two effectors of cell senescence, while other familial melanoma genes are related to telomeres and their maintenance. This article aimed to analyze our current knowledge of the genetic or epigenetic driver changes necessary to generate a cutaneous metastatic melanoma, the commonest order in which these occur, and the relation of these changes to the biology and pathology of melanoma progression. Emphasis is laid on the role of cell senescence and the escape from senescence leading to cellular immortality, the ability to divide indefinitely.

Teenage acne and cancer risk in US women: A prospective cohort study

BACKGROUND

Acne reflects hormone imbalance and is a key component of several systemic diseases. We hypothesized that diagnosis of acne as a teenager might predict subsequent risk of hormone-related cancers.

METHODS

We followed 99,128 female nurses in the Nurses' Health Study II cohort for 20 years (1989-2009) and used Cox proportional hazards models to estimate the hazard ratios (HRs) of 8 specific cancers (breast, thyroid, colorectal, ovarian, cervical, and endometrial cancers, melanoma, and non-Hodgkin lymphoma) for women with a history of severe teenage acne.

RESULTS

After thoroughly adjusting for the previously known risk factors for each cancer, we found that among women with a history of severe teenage acne, the relative risk increased, with a multivariable-adjusted HR of 1.44 (95% confidence interval [CI], 1.03-2.01) for melanoma. We replicated this association in an independent melanoma case-control study of 930 cases and 1026 controls (multivariable-adjusted odds ratio, 1.27; 95% CI, 1.03-1.56). We also found that in both studies the individuals with teenage acne were more likely to have moles (52.7% vs 50.1%, P < .001 in the cohort study; and 55.2% vs 45.1%, P = .004 in the case-control study).

CONCLUSIONS

Our findings suggest that a history of teenage acne might be a novel risk factor for melanoma independent from the known factors, which supports a need for continued investigation of these relationships.

Delving into somatic variation in sporadic melanoma

Melanoma, the most aggressive form of skin cancer, has increased in incidence more rapidly than any other cancer. The completion of the human genome project and advancements in genomics technologies has allowed us to investigate genetic alterations of melanoma at a scale and depth that is unprecedented. Here, we survey the history of the different approaches taken to understand the genomics of melanoma - from early candidate genes, to gene families, to genome-wide studies. The new era of whole-exome and whole-genome sequencing has paved the way for an in-depth understanding of melanoma biology, identification of new therapeutic targets, and development of novel personalized therapies for melanoma.

Shorter telomeres associate with a reduced risk of melanoma development

Epidemiologic studies have linked shortened telomeres with the development of many cancers. However, recent studies have suggested that longer telomeres may lead to prolonged senescence in melanocytes, providing increased opportunity for malignant transformation. We therefore examined whether shorter prediagnostically measured relative telomere length in peripheral blood leukocytes (PBL) was associated with a decreased risk of cutaneous melanoma. Telomere length in prospectively collected PBLs was measured in incident melanoma cases and age-matched controls selected from participants in three large prospective cohorts: the Women's Health Initiative Observational Study (WHI-OS), the Health Professionals Follow-up Study (HPFS), and the Nurses' Health Study (NHS). Shorter telomere lengths were associated with decreased risk of melanoma in each cohort. The P(trend) across quartiles was 0.03 in the WHI-OS and 0.008 in the HPFS. When combining these two datasets with published data in the NHS (P(trend), 0.09), compared with individuals in the fourth quartile (the longest telomere lengths), those in the first quartile had an OR of 0.43 (95% CI: 0.28-0.68; P(trend), 0.0003). Unlike findings for other tumors, shorter telomeres were significantly associated with a decreased risk of melanoma in this study, suggesting a unique role of telomeres in melanoma development.

A prospective study of telomere length and the risk of skin cancer

Telomere length is important in tumorigenesis. Using quantitative real-time PCR, we prospectively measured relative telomere length in a nested case-control study within the Nurses' Health Study: 218 melanoma cases, 285 squamous-cell carcinoma (SCC) cases, 300 basal-cell carcinoma (BCC) cases, and 870 controls. We observed that shorter telomeres were associated with a decreased number of moles (P=0.002) and a decreased risk of melanoma. Women in the second and first quartiles, those with the shortest telomere length, had an odds ratio (OR) for melanoma of 0.54 (95% confidence interval (CI), 0.29-1.01) and 0.59 (95% CI, 0.31-1.13), respectively, compared with those in the fourth quartile (P, trend=0.09). There was no clear trend between telomere length and SCC risk. In contrast, we found that shorter telomere length was associated with an increased risk of BCC. Compared with those in the fourth quartile, women in the first quartile had an OR of 1.85 (95% CI, 0.94-3.62) (P, trend=0.09). The opposing associations observed should be interpreted with caution, and further research is needed to confirm these possible associations.

Cellular senescence in naevi and immortalisation in melanoma: a role for p16?

Cellular senescence, the irreversible proliferative arrest seen in somatic cells after a limited number of divisions, is considered a crucial barrier to cancer, but direct evidence for this in vivo was lacking until recently. The best-known form of human cell senescence is attributed to telomere shortening and a DNA-damage response through p53 and p21. There is also a more rapid form of senescence, dependent on the p16-retinoblastoma pathway. p16 (CDKN2A) is a known melanoma susceptibility gene. Here, we use retrovirally mediated gene transfer to confirm that the normal form of senescence in cultured human melanocytes involves p16, since disruption of the p16/retinoblastoma pathway is required as well as telomerase activation for immortalisation. Expression (immunostaining) patterns of senescence mediators and markers in melanocytic lesions provide strong evidence that cell senescence occurs in benign melanocytic naevi (moles) in vivo and does not involve p53 or p21 upregulation, although p16 is widely expressed. In comparison, dysplastic naevi and early (radial growth-phase, RGP) melanomas show less p16 and some p53 and p21 immunostaining. All RGP melanomas expressed p21, suggesting areas of p53-mediated senescence, while most areas of advanced (vertical growth-phase) melanomas lacked both p16 and p21, implying escape from both forms of senescence (immortalisation). Moreover, nuclear p16 but not p21 expression can be induced in human melanocytes by oncogenic BRAF, as found in around 80% of naevi. We conclude that cell senescence can form a barrier to melanoma development. This also provides a potential explanation of why p16 is a melanoma suppressor gene.