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Phenotypic reversions at the W/Kit locus mediated by mitotic recombination in mice
P De Sepulveda1, J L Guenet, J J Panthier
1URA-INRA de Génétique Moléculaire, Ecole Nationale Vétérinaire d'Alfort, France.
Abstract:
The mouse W locus encodes Kit, the receptor tyrosine kinase for stem cell factor (SCF). Kit is required for several developmental processes, including the proliferation and survival of melanoblasts. Because of the nearly complete failure of Wrio/+ melanoblasts to colonize the skin, the costs of Wrio/+ mice are characterized by a majority of white hairs interspersed among pigmented hairs, giving a roan effect. However, 3.6% of Wrio/+ mice exhibit phenotypic reversions, i.e., spots of wild-type color on their coats with an otherwise mutant phenotype. Melanocyte cell lines were derived from each of six independent reversion spots on the skin of (C57BL/6 x DBA/2)F1 Wrio/+ mice. All six melanocyte cell lines exhibited the general characteristics common to normal, nonimmortal mouse melanocytes. Of these, three revertant cell lines had lost the dominant-negative Wrio allele following mitotic recombination between the centromere and the W locus. One of the cell lines remained Wrio/+ but showed (i) stimulation in response to SCF and (ii) increased Kit expression, suggesting that the Wrio mutation can be rescued by increased endogenous expression of the c-kit proto-oncogene. Finally, two cell lines showed no detectable genetic change at the W/Kit locus and failed to respond to SCF stimulation in vitro. These results demonstrate that mitotic recombination can create large patches of wild-type hair on the coats of Wrio/+ mutant mice. This shows that mitotic recombination occurs spontaneously in normal healthy tissue in vivo. Moreover, these experiments confirm that other mechanisms, not associated with loss of heterozygosity, may account for the coat color reversion phenotype.
Insights
Mitotic recombination can spontaneously occur in mice, leading to patches of normal hair color in Wrio/+ mutants. This phenomenon, observed in vivo, highlights alternative mechanisms for coat color reversion beyond simple gene loss.
Area of Science:
- Developmental biology
- Genetics
- Cell biology
Background:
- The W locus in mice encodes Kit, a receptor tyrosine kinase crucial for melanoblast development and survival.
- Wrio/+ mice exhibit a 'roan' coat color due to impaired melanoblast colonization of the skin.
- A small percentage of Wrio/+ mice display spontaneous coat color reversion, showing patches of wild-type pigmentation.
Purpose of the Study:
- To investigate the genetic and molecular mechanisms underlying coat color reversion in Wrio/+ mice.
- To determine if mitotic recombination is responsible for the observed phenotypic reversions.
- To explore alternative explanations for coat color reversion in the absence of Wrio allele loss.
Main Methods:
- Derivation and characterization of melanocyte cell lines from spontaneous reversion spots on Wrio/+ mouse coats.
- Analysis of genetic changes at the W/Kit locus, including loss of heterozygosity via mitotic recombination.
- Assessment of cellular response to stem cell factor (SCF) and Kit expression levels in revertant cell lines.
Main Results:
- Three out of six revertant cell lines showed loss of the Wrio allele, consistent with mitotic recombination.
- One cell line remained Wrio/+ but exhibited increased Kit expression and SCF responsiveness, suggesting rescue by elevated c-kit proto-oncogene expression.
- Two cell lines had no detectable genetic alteration at the W/Kit locus and remained unresponsive to SCF in vitro.
Conclusions:
- Mitotic recombination is a spontaneous in vivo mechanism capable of generating wild-type hair patches in Wrio/+ mutant mice.
- Phenotypic reversion in Wrio/+ mice can occur through mechanisms other than loss of heterozygosity, including increased endogenous Kit expression.
- These findings underscore the complexity of genetic regulation in coat color development and the occurrence of spontaneous genetic events in somatic tissues.