An Unstable Targeted Allele of the Mouse Mitf Gene With a High Somatic and Germline Reversion Rate

The mouse Mitf gene encodes a transcription factor that is regulated by serine phosphorylation and is critical for the development of melanin-containing pigment cells. To test the role of phosphorylation at a particular serine, S73 in exon 2 of Mitf, we used a standard targeting strategy in mouse embryonic stem cells to change the corresponding codon into one encoding an alanine. By chance, we generated an allele in which 85,222 bp of wild-type Mitf sequence are duplicated and inserted into an otherwise correctly targeted Mitf gene. Depending on the presence or absence of a neomycin resistance cassette, this genomic rearrangement leads to animals with a white coat with or without pigmented spots or a gray coat with obligatory white and black spots. Several independent, genetically stable germline revertants that lacked the duplicated wild-type sequence but retained the targeted codon were then derived. These animals were normally pigmented, indicating that the serine-to-alanine mutation is not deleterious to melanocyte development. The fact that mosaic coat reversions occur in all mice lacking the neo-cassette and that ∼1% of these transmit a reverted allele to their offspring places this mutation among those with the highest spontaneous reversion rates in mammals

Lack of the mesodermal homeodomain protein MEOX1 disrupts sclerotome polarity and leads to a remodeling of the cranio-cervical joints of the axial skeleton

AbstractMeox1 and Meox2 are two related homeodomain transcription factor genes that together are essential for the development of all somite compartments. Here we show that mice homozygous for Meox1 mutations alone have abnormalities that are restricted to the sclerotome and its derivatives. A prominent and consistent phenotype of these mutations is a remodeling of the cranio-cervical joints whose major feature is the assimilation of the atlas into the basioccipital bone so that the skull rests on the axis. These abnormalities can be traced back to changes in the relative rates of cell proliferation in the rostral and caudal sclerotome compartments, and they are associated with alterations in the expression of at least three transcription factor genes, Tbx18, Uncx, and Bapx1. As previously observed for Bapx1, MEOX1 protein occupies evolutionarily conserved promoter regions of Tbx18 and Uncx, suggesting that Meox1 regulates these genes at least in part directly. Hence, Meox1 is part of a regulatory circuit that serves an essential, non-redundant function in the maintenance of rostro-caudal sclerotome polarity and axial skeleton formation