Target Discovery and Validation Reviews and Protocols

Keratin Transgenics and Knockouts 219
caused by keratin K6a, K6b, or keratin K16 mutations, whereas type 2 is generally
evoked by keratin K17 mutations (see www.interfil.org).
Initially, it seemed easy to mimic the corresponding pathology in mice after
targeted disruption of the corresponding keratin orthologs. However, K6a-deficient
mice do not show any spontaneous skin or oral mucosa lesions (92), and
K17-deficient mice only develop a strain-dependent (C57BL/6) and reversible
alopecia, but they histologically fail abnormalities in oral mucosa, glands, footpad
epidermis, and nails (36,93). Even mice being double deficient for both K6
gene isoforms a and b contain histologically normal skin epithelial appendages
(94). This obvious discrepancy between the mentioned null mouse models and
the PC disease led to the idea that other keratins, such as K5, K6hf, K14, K16,
or K17n known to be expressed in nail bed epithelium (36,95,96), might compensate
their functional loss. Support for this concept of functional redundancy
was provided after targeted deletion of three keratin genes (K6a, K6b, and K17)
by intercrossing the existing keratin-deficient strains to generate the corresponding
K6a/K6b/K17 triple-null mouse model (97). These mice also died, as
expected from the double-deficient K6a and K6b mice, shortly after birth but
they exhibited severe lysis to the nail bed epithelium, where all the three genes
are abundantly expressed. Moreover, a decrease in the level of a fourth keratin
protein, K16, was observed (97). Conclusively, the loss of these three keratin
genes strongly suggests that the nail bed epithelium is initially targeted in PC
(see Fig. 3E).
Wound healing, proliferation, and migration of keratinocytes harvested from
K6a(–/–) animals seemed unaffected compared with wild-type controls (92).
Targeted deletion of both keratin genes K6a and K6b in mice with different genetic
background produced conflicting but interesting results. All K6a(–/–)/K6b(–/–)double
deficient mice with a mixed 129Sv-Ck35/C57BL/6 background die about
1 wk after birth, showing growth delay and weight loss associated with reduced
milk intake, white plaques, and extensive cell blistering in the posterior region
of the dorsal tongue and the upper palate epithelia combined with traumatic filiform
papillae in the anterior compartment (94). The anterior compartment of
filiform papillae, normally expressing K6a and K6b, is devoid of keratin IFs in
these double knockouts (94). In contrast, 25% of K6a(–/–)/K6b(–/–) doubledeficient
mice with a mixed AB2.2/129SvEv-C57BL/6N background survived
to adulthood with normal hair and nails (see Tables 1 and 2). Nevertheless, 75%
of these double-deficient mice die within the first 2 wk of life because of disintegration
of the dorsal tongue epithelium causing a plaque of cell debris that
severely impairs feeding (96). These authors cloned a third, and at that time
unidentified, murine keratin 6 gene (designated K6hf) likely to be the ortholog
to human K6hf expressed in hair follicles. Redundancy of this novel K6hf gene
in K6a/K6b-deficient follicles and nails is an attractive explanation for the