In our study cohort, the prevalence of the p.E318K germline variant in CDKN2A/CDK4-negative patients was 2.2%, slightly higher than we previously reported in a smaller series of melanoma patients (1.8%) , but in line with Spanish (1.9%) , French (2.8%) , Australian (3.4–3.6%)  and American (2.8%)  studies.
Considering that the p.E318K variant is not common in melanoma patients, attempts to determine its effects on MITF+ patients’ phenotypical features and cancer predisposition are generally limited by sample size. To our best knowledge, the present study describes the largest cohort of MITF+ patients reported to date from a dermoscopic point of view (DN and melanomas), in addition to a genetic, clinical, and pathological perspective.
Concerning the histotype of the first diagnosed melanoma, we validated the association between the p.E318K variant and the nodular subtype previously reported by our group . Indeed, seven out of 22 p.E318K patients (32%) developed a first melanoma with nodular histotype, a significantly higher percentage than the one observed in MITF− patients (16%). Our results differ from those of previous studies by other groups, which did not find significant associations of p.E318K with pathological features, possibly due to underpowered study samples [14, 16].
However, Potrony and colleagues reported that during 10 years of dermatological surveillance of patients at high risk of melanoma, the only two fast-growing melanomas (growth rate greater than 0.4 mm per month) were diagnosed in MITF+ patients. Of these two lesions, one was a nodular melanoma and the other one was a superficial spreading melanoma (SSM) . However, in our MITF+ study group, all nodular melanomas were first diagnosed melanomas, identified during dermatological screening with digital follow-up or clinical examination. Conversely, all subsequent melanomas diagnosed in our MITF+ cohort during dermatological follow-up were SSM, and Breslow thickness of melanomas in patients with MPM was always lower than that of the preceding ones, except for one patient, possibly reflecting the intensive dermatological follow-up after the first melanoma diagnosis. However, further investigations with larger series of patients are needed to confirm the association between the p.E318K variant and nodular-type melanoma, and to study the prognostic role of this variant.
Concerning the role of the p.E318K variant in the predisposition to tumors other than melanoma, we confirm the association with renal cell carcinoma (RCC) previously described [6, 7, 13,14,15]. The association with pancreatic cancer we previously observed in a smaller series of patients  was not confirmed here, and therefore remains to be further explored. Although none of our p.E318K patients developed RCC, 18% of them reported a positive family history, as opposed to 4% of MITF− patients. Apart from melanoma, the most frequent tumor in MITF+ patients was basal cell carcinoma (14% of the patients), in line with previous data reported by Potrony et al. .
The finding that MITF+ p.E318K was associated with a higher number of histopathologically confirmed DN in our cohort was never reported to date, differently from CDKN2A variants, whose possible role in influencing the development of dysplastic melanocytic lesions has already been described [26, 27].
Our study confirms that MITF+ patients have an increased risk of developing multiple melanomas and a higher total nevi count compared to MITF− patients, as previously reported [6, 7, 13, 14]. Indeed, 28% of MITF+ patients in our cohort had more than 50 nevi with > 2 mm diameter, ascompared to 19% of MITF− patients. Similarly, previous studies reported high nevi counts in MITF+ patients, corroborating the hypothesis that MITF is involved in nevogenesis [11, 14, 16].
Of course, as MITF p.E318K is considered an intermediate penetrance allele, the possibility that other additional gene’s effects may have affected our results cannot be completely ruled out. However, patients with CDKN2A pathogenic variants were excluded from this study, in order to avoid a confounding effect by this gene. Moreover, MC1R variants, which influence phototype and are associated with melanoma risk [19, 28], had a similar distribution in the two study groups, therefore not affecting our analyses. MC1R RHC variants have also been associated with the likelihood of developing amelanotic/hypomelanotic melanomas . In our cohort, both MITF+ patients with amelanotic/hypomelanotic melanomas carried one RHC variants. However, due to the retrospective nature of this study, standardized information on pigmentation was not available, and therefore we could not assess the impact of RHC variants on melanoma pigmentation according to MITF germline status.
Although dermoscopic patterns of melanocytic nevi in MITF+ and MITF− patients have already been reported [11, 14, 16], our study is the first to assess the dermoscopic characterization of DN and melanomas in MITF+ patients compared to MITF− patients. Previous studies [11, 14, 16] found that the predominant dermoscopic pattern of nevi was the reticular one, both in MITF+ and in MITF− patients. Moreover, Sturm et al. reported that the frequency of globular nevi was greater in MITF+ patients, albeit not significant . In DN and melanomas of our series of MITF+ patients, we found 3 prevalent dermoscopic patterns: unspecific, globular-homogeneous and reticular-homogeneous. The unspecific pattern was defined as devoid of structures or with too few structures to identify a pattern, except for the presence of blood vessels. This latter pattern is most frequently found in amelanotic/hypomelanotic melanocytic lesions including amelanotic/hypomelanotic nodular melanomas where it can be associated with polymorphous atypical vessels .
While the reticular pattern is suggestive of photoinduced nevogenesis, the globular-homogeneous one, with globules at the periphery of the lesion, expression of lesion growth, suggests that p.E318K variant may also act to force the continuous growth of the nevi/melanomas .
Considering only melanomas, the prevalent pattern among the MITF+ patients was the unspecific one, a finding that has never been associated with the MITF+ variant to date.
Conversely, the multicomponent pattern was prevalent among the MITF− patients, as already reported in the literature [32, 33].
Noteworthy, as a rule, all lesions with unspecific patterns should be biopsied, also in the context of lesions clinically appearing benign, to avoid missing melanomas .
Therefore, the detection of this pattern in MITF+ patients should alert dermatologist raising the level of suspicion of malignancy.
Since among the 22 MITF+ patients one patient developed 10 melanomas (of which 7 dermoscopic images were available), and the different distribution of clinico-pathological-dermoscopic features between the two groups could have been influenced by this single patient, we repeated the analysis excluding this patient. Even though the observed patterns were actually influenced by this patient, the unspecific pattern remained prevalent in MITF+ patients and the association remained significant. Dermoscopically, the most common patterns of DN and melanomas (multicomponent, reticular-globular) were almost absent in MITF+ patients, while the multicomponent was the most frequent pattern among MITF− patients.
The major limitation in this study is the small number of images included for assessment of dermoscopic pattern in relation to MITF variant which may influence the reliability of these results (only 23 dermoscopic images belonging to four MITF+ patients were available).