Different Effect of Various Mutant MITF Encoded by mi,Mior, or Miwh Allele on Phenotype of Murine Mast Cells

The mi locus encodes a member of the basic-helix-loop-helix-leucine zipper protein family of transcription factors (hereafter called MITF). Mutant alleles of mi,Mior, and Miwh are deletion or point mutation of the basic domain by which MITF binds DNA. The basic domain also has nuclear localization potential. In the present study, we compared the mast cell abnormalities ofMior/Mior andMiwh/Miwh mice with those ofmi/mi mice, of which many have been described by us. The number of mast cells in the skin of Mior/Miorsuckling mice was remarkably decreased from that observed inmi/mi suckling mice, but the number was normal in the skin ofMiwh/Miwh suckling mice. The decrease in skin mast cells was more severe in the mi/mi embryos than inmi/mi suckling mice, but the magnitude of the decrease was comparable between Mior/Mior embryos and Mior/Mior suckling mice. The poor mRNA expression of granzyme B and tryptophan hydroxylase genes was observed in all cultured mast cells (CMCs) derived from the spleens ofMiwh/Miwh,Mior/Mior, and mi/mi mice. However, the poor expression of mouse mast cell protease-4 (MMCP-4), MMCP-5, and MMCP-6 was observed only inMior/Mior and mi/mi CMCs. MITF encoded by Miwh mutant allele (Miwh-MITF) showed deficient but demonstratable DNA binding, but mi-MITF and Mior-MITF did not show any DNA binding ability. Although Miwh-MITF and Mior-MITF showed normal nuclear localization potential, the potential was significantly impaired in mi-MITF. The rank order of mast cell abnormality (mi/mi >Mior/Mior >Miwh/Miwh) appears to be related to the functional abnormality of MITF encoded by each mutant gene.