Acral lentiginous melanoma (ALM) is a rare subtype of melanoma with aggressive behavior. IMPDH enzyme, involved in de novo GTP biosynthesis, has been reported to assemble into large filamentary structures called rods/rings (RR) or cytoophidium (cellular snakes). RR assembly induces a hyperactive state in IMPDH, usually to supply a high demand for GTP nucleotides, such as in highly proliferative cells. We investigate whether aggressive melanoma tumor cells present IMPDH‐based RR structures. Forty‐five ALM paraffin‐embedded tissue samples and 59 melanocytic nevi were probed with anti‐IMPDH2 antibody. Both the rod‐ and ring‐shaped RR could be observed, with higher frequency in ALM. ROC curve analyzing the proportions of RR‐positive cells in ALM versus nevi yielded a 0.88 AUC. Using the cutoff of 5.5% RR‐positive cells, there was a sensitivity of 80% and specificity of 85% for ALM diagnosis. In ALM, 36 (80%) showed RR frequency above the cutoff, being classified as RR‐positive, compared with only 9 (15%) of the nevi (p < .001). Histopathology showed that 71% of the RR‐positive specimens presented Breslow thickness > 4.0mm, compared with only 29% in the RR‐low/negative (p = .039). We propose that screening for RR structures in biopsy specimens may be a valuable tool helping differentiate ALM from nevi and accessing tumor malignancy.

The choice of appropriate therapeutic strategies may be influenced by intratumor heterogeneity and makes cancer treatment considerably more challenging. We aimed to evaluate the heterogeneity of BRAF exon 15 mutations in different areas of acral lentiginous melanoma (ALM). The entire exon 15 was sequenced in 4 different areas of paraffin-embedded samples from 26 patients with ALM. A total of 26 of 49 cases of ≥1 mm in depth of ALM identified by clinical, anatomical, and pathological data fulfilled the inclusion and exclusion criteria for this study. Tumors had a mean Breslow depth of 7.2 mm and an average mitotic index of 3 mitosis/mm2. Mutations distinct from the common V600E and V600K were detected in 31%, and intratumor heterogeneity was observed in 31% of samples. Interestingly, 63.5% of all mutations had been previously associated with cancer. Most (62.5%) of the missense BRAF exon 15 mutations found in the ALM samples examined here were deemed “detrimental” for protein function according to at least 2 functional prediction programs, and 3 mutations (37.5%) were predicted to be “neutral,” with no effect on protein function. BRAF exon 15 mutations were detected frequently in ALM and displayed heterogeneity, a finding to be further investigated.

Galectin-3 is a member of the lectin family that binds β-galactosides and plays an important role in several types of tumors. Melanoma is an invasive cancer responsible for 80% of deaths associated to skin cancers. There are some evidences that galectin-3 interacts with β-catenin, a molecule involved with Wnt signaling pathway. Here, we evaluate the role of galectin-3 in tumor growth and metastasis, as well as its interaction with β-catenin. Murine melanoma cells (B16F10) were injected subcutaneously and intravenously in male C57BL/6 wild-type (WT) and galectin-3 knockout (KO) mice. Tumor growth and lung melanoma colonies were assessed. The expression of galectin-3 and β-catenin was evaluated by immuno-histochemistry. We observed that tumor growth did not differ between the groups. However, to metastasis, the number of lung colonies in WT mice was significantly increased in comparison to that observed in KO mice. The cytoplasm expression of galectin-3 was observed in subcutaneous and metastatic tumors, in both groups. We observed its nuclear expression in some of subcutaneous tumors of KO mice. The expression of β-catenin was detected in cell membrane of all subcutaneous tumors analyzed, whereas in the metastatic tumors we observed both cytoplasm and cell membrane staining. Altogether, our data suggest that galectin-3 favors the metastasis of melanoma cells and this process is not associated with β-catenin.

We show that marker protein expression, when combined with heterogeneous expression as shown by immunohistochemistry, is a powerful indicator of malignancy in ALMs, especially, when protein pairs are combined.

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