Discovery of a novel murine keratin 6 (K6) isoform explains the absence ofhair and nail defects in mice deficient for K6a and K6b

The murine genome is known to have two keratin 6 (K6) genes, mouse K6 (MK6) a and MK6b. These genes display a complex expression pattern with constitut ive expression in the epithelia of oral mucosa, hair follicles, and nail be ds. We generated mice deficient for both genes through embryonic stem cell technology. The majority of MK6a/b(-/-) mice die of starvation within the f irst two weeks of life. This is due to a localized disintegration of the do rsal tongue epithelium, which results in the build up of a plaque of cell d ebris that severely impairs feeding. However, similar to 25% of MK6a/b(-/-) mice survive to adulthood. Remarkably, the surviving MK6a/b(-/-) mice have normal hair and nails. To our surprise, we discovered MK6 staining both in the hair follicle and the nail bed of MK6a/b(-/-) mice, indicating the pre sence of a third MK6 gene. We cloned this previously unknown murine keratin gene and found it to be highly homologous to human K6hf, which is expresse d in hair follicles. We therefore termed this gene MK6 hair follicle (MK6hf ). The presence of MK6hf in the MK6a/b(-/-) follicles and nails offers an e xplanation for the absence of hair and nail defects in MK6a/b(-/-) animals.