Downless (dl) is a spontaneous recessive mouse mutation that has a phenotype affecting three sets of tissues: the teeth, hair and glands. Downless incisors can be missing or reduced in size, molars are reduced in cusp number and the third molar is often missing (Sofaer 1969). Hair follicle development is defective resulting in only one hair follicle type being formed. Glands that are absent or reduced in size include e.g. sweat glands and Meibomian glands (Gruneberg, 1971, Rao et al, 1994). The phenotype is similar to the Tabby and crinkled mutants. A dominant allele, Sleek (Slk), has also been isolated. The phenotype is similar to the homozygous recessive alleles (Crocker and Cattanach, 1979).
The Downless gene is in chromosome 10 and encodes a TNF receptor (Headon & Overbeek, 1999). It binds ectodysplasin A1, a Tabby isoform and activates the NF?B signalling pathway (Yan et al, 2000).
Downless is expressed in the tooth epithelium, spesifically the enamel knot, from E11 until E17. Its expression is induced in tooth explants by activin?A (Tucker et al, 2000; Laurikkala et al, 2001).
Last edited 28.5.01 by PN