A histopathological classification system of Tyr::NRASQ61K murine melanocytic lesions: A reproducible simplified classification.

Genetically engineered mouse models offer essential opportunities to investigate the mechanisms of initiation and progression in melanoma. Here, we report a new simplified histopathology classification of mouse melanocytic lesions in Tyr::NRASQ61K derived models, using an interactive decision tree that produces homogeneous categories. Reproducibility for this classification system was evaluated on a panel of representative cases of murine melanocytic lesions by pathologists and basic scientists. Reproducibility, measured as inter-rater agreement between evaluators using a modified Fleiss' kappa statistic, revealed a very good agreement between observers. Should this new simplified classification be adopted, it would create a robust system of communication between researchers in the field of mouse melanoma models.

NRAS, NRAS, Which Mutation Is Fairest of Them All?

In 28% of melanomas, NRAS is mutated in one of two hotspots: G12 or Q61. Phosphoproteomic analysis of primary human melanocytes transduced with G12 and Q61 showed different phosphorylation events in the phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways. Surprisingly, NRAS(G12) modulates the PI3K pathway and overexpresses the kinase PIM2, whereas NRAS(Q61) is associated with the MAPK pathway and overexpression of CK2α.

A polymorphism in IRF4 affects human pigmentation through a tyrosinase-dependent MITF/TFAP2A pathway.

Sequence polymorphisms linked to human diseases and phenotypes in genome-wide association studies often affect noncoding regions. A SNP within an intron of the gene encoding Interferon Regulatory Factor 4 (IRF4), a transcription factor with no known role in melanocyte biology, is strongly associated with sensitivity of skin to sun exposure, freckles, blue eyes, and brown hair color. Here, we demonstrate that this SNP lies within an enhancer of IRF4 transcription in melanocytes. The allele associated with this pigmentation phenotype impairs binding of the TFAP2A transcription factor that, together with the melanocyte master regulator MITF, regulates activity of the enhancer. Assays in zebrafish and mice reveal that IRF4 cooperates with MITF to activate expression of Tyrosinase (TYR), an essential enzyme in melanin synthesis. Our findings provide a clear example of a noncoding polymorphism that affects a phenotype by modulating a developmental gene regulatory network.

MITF mutations associated with pigment deficiency syndromes and melanoma have different effects on protein function.

The basic-helix-loop-helix-leucine zipper (bHLHZip) protein MITF (microphthalmia-associated transcription factor) is a master regulator of melanocyte development. Mutations in the MITF have been found in patients with the dominantly inherited hypopigmentation and deafness syndromes Waardenburg syndrome type 2A (WS2A) and Tietz syndrome (TS). Additionally, both somatic and germline mutations have been found in MITF in melanoma patients. Here, we characterize the DNA-binding and transcription activation properties of 24 MITF mutations found in WS2A, TS and melanoma patients. We show that most of the WS2A and TS mutations fail to bind DNA and activate expression from melanocyte-specific promoters. Some of the mutations, especially R203K and S298P, exhibit normal activity and may represent neutral variants. Mutations found in melanomas showed normal DNA-binding and minor variations in transcription activation properties; some showed increased potential to form colonies. Our results provide molecular insights into how mutations in a single gene can lead to such different phenotypes.

Activation of beta-catenin is a late event in the pathogenesis of nephroblastomas and rarely correlated with genetic changes of the APC gene.

AIMS: Activation of ß-catenin has been identified as a possible mechanism for the development of nephroblastomas. In our study we investigated whether this activation occurs already in precursor lesions of nephroblastomas, called nephrogenic rests (NRs). Inactivation of the adenomatous polyposis coli (APC) protein is an important regulatory mechanism of activating ß-catenin. We clarified the role of APC by assessing loss of heterozygosity (LOH) and possible mutations within the genomic region. METHODS: Activation of ß-catenin was examined by immunohistochemistry identifying nuclear translocation. Two polymorphic loci of the APC gene were investigated for LOH and sequence analysis was performed for the mutation cluster region of the APC gene on formalin fixed, paraffin embedded samples. RESULTS: Four of the 18 nephroblastomas available for immunohistochemistry exhibited nuclear staining of ß-catenin, but none of the NRs. Analysis of LOH revealed 14 homozygous samples, 10 heterozygous tumours and six tumours exhibiting LOH of the APC gene. One blastema-type nephroblastoma showed nuclear localisation of ß-catenin in conjunction with LOH of the APC gene. Analysis of 12 nephroblastomas revealed no sequence aberration. CONCLUSION: Our results indicate that nuclear activation of ß-catenin is a late event in the tumorigenesis of nephroblastomas coinciding in some tumours with LOH of the APC gene.

Loss of PTEN/MMAC1 activity is a rare and late event in the pathogenesis of nephroblastomas.

Recent genetic investigations of nephroblastomas point to an activation of the Wnt pathway. Data indicate however that activation might be partly due to cross talk of different signaling pathways including the tumor suppressor gene PTEN (phosphatase and tensin homolog on chromosome 10). Therefore, we examined expression and chromosomal aberrations of PTEN in nephroblastomas of different subtypes and the corresponding nephrogenic rests. Loss of heterozygosity was analyzed by high-resolution melting analysis of 4 different single nucleotide polymorphisms. Results were confirmed by sequence analysis of the polymerase chain reaction products. In addition, an intragenic insertion-deletion polymorphism of the PTEN gene was investigated. Protein expression was assessed by immunohistochemistry. Twenty-two nephroblastomas and their corresponding nephrogenic rests were included in the study. In the high-resolution melting analysis, 15 samples were homozygous, 6 were heterozygous, and for 1 sample results could not be obtained for technical reasons. None of the samples showed loss of heterozygosity. Nineteen of the tumors and corresponding nephrogenic rests were also examined immunohistochemically. All tumors showed cytoplasmic positivity, with the exception of 1 tumor that showed complete loss of staining. In 1 tumor, the epithelial component showed distinct cytoplasmic staining, whereas the immature muscle and hyaline cartilage were negative. All nephrogenic rests exhibited positive cytoplasmic staining of all components. Our results establish that inactivation of PTEN is a rare and late event in the pathogenesis of nephroblastomas.