An in vivo method for the detection of somatic mutations at the cellular level in mice

This new method for detection of somatic mutations is based on the fact that recessive mutations at the dilute (d) and leaden (ln) loci change the shape of pigment-cells in the hair follicles from their normal highly dendritic appearance (nucleofugal) to a rounded-up almost non-dendritic form (nucleopetal). Gene action is autonomous and individual mutant melanocytes can be recognised in whole mounts of dorsal skin made after the post-natal onset of melanogenesis. Foetal heterozygotes for d and ln were treated with X-rays (0-200 rad) or procarbazine (0-300 mg/kg body mass by maternal i.p. injection) 17 days after detection of a vaginal plug; whole mounts of dorsal skin (fixed 3 days after birth) were microscopically scanned for mutant melanocytes in the hair follicles.About half a million follicular melanocytes were present in the 2 mounts made per mouse. Mutant cells were mainly in the form of small intrafollicular clones; the few mount containing large interfollicular clones were excluded from the analysis. For both treatments, dose-response relationships showed good fits to both a quadratic and a power-law curve. Equations for the latter were: For X-rays, y = 3.2 x 10(-5) + 3.12 x 10(-9) D2.07; for procarbazine, y = 3.6 x 10(-5) + 0.4 x 10(-10) D2.60. However, neither of these differed significantly from a square-law relationship. The estimated mutation rate at the 100 rad X-ray level of 4.7 x 10(-7)/rad/locus was in good agreement with that obtained by Russell and Major's (1957) pioneer studies of somatic mutation in mice, by looking for mutant patches in the coat ("spot test"). Possible ways in which the present method might be improved and extended are discussed.