Epidermolysis bullosa simplex (EBS) is a group of autosomal dominant skin diseases characterized by blistering, due to mechanical stress-induced degeneration of basal epidermal cells. It is now well-established that the three major subtypes of EBS are genetic disorders of the basal epidermal keratins, keratin 5 (K5) and keratin 14 (K14). Here we show that a rare subtype, referred to as EBS with mottled pigmentation (MP), is also a disorder of these keratins.Affected members of two seemingly unrelated families with EBS-MP had a C to T point mutation in the second base position of codon 24 of one of two KS alleles, leading to a Pro:Leu mutation. This mutation was not present in unaffected members nor in 100 alleles from normal individuals.Linkage analyses mapped the defect to this type H keratin gene (peak logarithm of odds score at 4) = 0 of 3.9), which is located on chromosome 12q11-q13. This provides strong evidence that this mutation is responsible for the EBS-MP phenotype. Only conserved between K5 and K6, and not among any of the other type H keratins, Pro-24 is in the nonhelical head domain of K5, and only mildly perturbs the length of 10-nm keratin filaments assembled in vitro. However, this part of the K5 head domain is likely to protrude on the filament surface, perhaps leading to additional aberrations in intermediate filament architecture and/or in melanosome distribution that are seen ultrastructurally in patients with the mutation.Epidermolysis bullosa simplex (EBS) has been subdivided into three major types (1-3). (i) EBS Dowling-Meara (EBS-DM) is the most severe type, typified by blistering over whole body regions and keratin filament clumping and cytolysis in basal cells. (ii) EBS Kobner (EBS-K) is also characterized by generalized blistering and basal cell cytolysis, but with fewer abnormalities in basal cell keratin networks. (iii) EBS WeberCockayne (EBS-WC) is the mildest form, with blistering concentrated primarily in palmar and plantar regions, and very minor keratin filament perturbations. An unusual and rare form of EBS with generalized blistering and hyper-and hypopigmented spots over the body surface was first reported by Fischer and Gedde-Dahl (4). The unifying feature of all of these EBS subtypes is basal cell cytolysis, with a largely normal differentiation process in the suprabasal layers.

Phenotypic reversion of the rubber-band, muscle-defective phenotype conferred by unc-93(e1500) was used to determine the utility of N-ethyl-N-nitrosourea (ENU) as a mutagen for genetic research in Caenorhabditis elegans. In this system, ENU produces revertants at a frequency of 3 × 10–4, equivalent to that of the commonly used mutagen, EMS. The gene identity of 154 ENU-induced revertants shows that the distribution of alleles between three possible suppressor genes differs from that induced by EMS. A higher percentage of revertants are alleles of unc-93 and many fewer are alleles of sup-9 and sup-10. Three revertants complement the three known suppressor genes; they may therefore identify a new gene product(s) involved in this system of excitation-contraction coupling in C. elegans. Molecular characterization of putative unc-93 null alleles reveals that the base changes induced by ENU are quite different from those induced by EMS; specifically we see an increased frequency of A/T → G/C transitions. The frequency of ENU-induced intragenic deletions is found to be 13%. We suggest that ENU, at concentrations below 5 mm, will be a superior mutagen for studies of protein function in C. elegans.