Mosaicism of tyrosinase‐locus transcription and chromatin structure in dark vs. light melanocyte clones of homozygous chinchilla‐mottled mice

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Abstract
The chinchilla‐mottled (cm) mutation at the mouse tyrosinase‐encoding locus leads to a transversely striped pattern of dark‐ and light‐grey coat colors in homozygotes. The same basic pattern occurs in various other genotypes and has previously been found to represent the clonal developmental history of melanocytes. In a homozygote such as cm/cm, cis‐acting mechanisms would be expected to account for the color differences. To search for these mechanisms, the genomic structure of the mutation was examined and compared with the wild‐type, and its function was compared in cultured melanocyte clones of the respective colors. Evidence from restriction mapping indicated that the coding region of the mutant gene resembles that of the fully and uniformly pigmented wild‐type. However, the upstream sequences are rearranged in the mutation. The rearrangement begins 5 kb 5′ of the transcription initiation site and is estimated to encompass at least 30 kb of distal upstream sequence. At least two stable functional states of the cm gene were detectable: Light‐cell clones have low levels of tyrosinase‐specific transcription, reduced DNAase I sensitivity of tyrosinase chromatin, and no detectable hypersensitive sites near the gene; dark‐cell clones have higher (but subnormal) levels of transcription, greater sensitivity of chromatin to DNAase I, and a hypersensitive site in the promoter region. The changed relation between the structural gene and its upstream region may separate it from cis‐acting control elements, resulting in reduced and variable ability to achieve the appropriate chromatin configuration near the time of melanocyte determination; differences in expression among clonal initiator cells are then mitotically perpetuated.