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Spontaneous malignant transformation of melanocytes explanted from Wf/Wf mice with a Kit kinase-domain mutation
L Larue1, N Dougherty, S Porter
1Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, PA 19111.
Abstract:
The W/Kit mouse locus, affecting proliferation and survival of pigment cells, blood cells, and germ cells, is known to encode a tyrosine kinase growth factor receptor and is considered a protooncogene; yet it has not heretofore been causally implicated in any malignancies of those cells. The Wf/Wf mutant mouse coat comprises viable and inviable melanoblast clones, seen ultimately as pigmented and white transverse stripes--the latter more prominent. Judging from the pattern, all clones initially expand, and the inviable ones then undergo programmed cell death prenatally. To observe skin melanocytes of the viable clones during extended proliferation, the cells were explanted from individual young mice. An unusually large number of primary explants failed to survive--a result consistent with a growth handicap. In 3 of the 10 surviving cell lines, many cells spontaneously underwent a series of striking changes with the classic features of transformation. The two transformed lines that have been tested by grafting to immunosuppressed hosts formed undifferentiated invasive tumors compatible with malignant amelanotic melanoma. None of our 52 other melanocyte lines of the coisogenic wild-type strain and 13 other natural genotypes have become transformed under the same culture conditions. Molecular analysis of the Wf gene revealed a single change from wild-type: a point mutation affecting the catalytic region in the kinase domain of the Kit protein. The apparent growth disadvantage due to the mutation may allow selection for melanocytes mobilizing more efficient pathways, thus leading to neoplasia. Production of both viable and inviable melanoblast clones is unlikely to be due only to the kinase mutation; possibly the degree, duration, and consistency of expression of this locus may be controlled by cis elements outside the coding region.
Insights
The W/Kit mouse locus mutation, affecting pigment cells, can lead to cancer. This study found that a specific Kit protein mutation in W/Kit mice can cause malignant melanoma, implicating the locus in cancer development.
Area of Science:
- Cell Biology
- Genetics
- Oncology
Background:
- The W/Kit mouse locus encodes a tyrosine kinase growth factor receptor implicated in cell proliferation and survival.
- While considered a protooncogene, the W/Kit locus has not been causally linked to malignancies in pigment, blood, or germ cells.
- Wf/Wf mutant mice exhibit coat patterns reflecting viable and inviable melanoblast clones, with inviable clones undergoing programmed cell death.
Purpose of the Study:
- To investigate the role of the W/Kit locus in the development of malignancies.
- To analyze the transformation of melanocytes in W/Kit mice under extended proliferation conditions.
- To identify the molecular basis of the W/Kit mutation and its effect on cell behavior.
Main Methods:
- Explantaion of skin melanocytes from young W/Kit mice for extended proliferation culture.
- Observation of spontaneous cellular transformation in surviving melanocyte lines.
- Grafting of transformed cell lines into immunosuppressed hosts to assess tumor formation.
- Molecular analysis of the W/Kit gene to identify mutations.
Main Results:
- A significant number of primary melanocyte explants from W/Kit mice failed to survive, indicating a growth handicap.
- In 3 out of 10 surviving cell lines, melanocytes exhibited classic features of transformation.
- Grafting of two transformed lines resulted in undifferentiated invasive tumors, consistent with malignant amelanotic melanoma.
- Molecular analysis revealed a point mutation in the kinase domain of the Kit protein within the W/Kit gene.
Conclusions:
- The W/Kit locus, specifically a mutation in the Kit protein's kinase domain, can be causally implicated in the development of malignant melanoma.
- The observed growth disadvantage may select for melanocytes with alternative pathways, leading to neoplasia.
- Regulation of W/Kit locus expression by cis elements outside the coding region might influence melanoblast clone viability.