Novel and recurrent mutations in the genes encoding keratins K6a, K16 and K17 in 13 cases of pachyonychia congenita

Keratin 17 mutation in pachyonychia congenita type 2 with early onset sebaceous cysts

BACKGROUND

Pachyonychia congenita (PC) is a group of autosomal dominant ectodermal dysplasias caused by mutations in four differentiation-specific keratin genes. Two major clinical subtypes of PC have been generally recognized. Symmetrically thickened fingernails and toenails are the defining characteristic of PC type 2 (PC-2) with onset at infancy. Pilosebaceous cysts are the best hallmark of PC-2, but they usually occur at puberty.

OBJECTIVES

To report a Chinese pedigree of PC-2 with unusually early onset sebaceous cysts and to explore the genetic mutation and its phenotype.

METHODS

Exon 1 of keratin 17 was amplified by polymerase chain reaction (PCR) from genomic DNA from the three patients in the pedigree, the proband, his half-sister and his younger son, two unaffected members in the pedigree and 50 unrelated and unaffected people. PCR products were directly sequenced to detect the mutation.

RESULTS

Direct sequencing of the PCR products revealed a heterozygous 275A-->G mutation in all three affected members. This mutation predicts the substitution of asparagine by serine in codon 92 (N92S) located in the 1A domain of keratin 17.

CONCLUSIONS

Mutation in the 1A domain of keratin 17 underlies the affected members' phenotype, PC-2 with early onset sebaceous cysts and late-onset thickened fingernails and toenails. The onset of the cysts is very early in some people within this family and the age at onset of thickened fingernails and toenails is variable within the family, implying the existence of modifying factors.

Mutations of the UMOD gene are responsible for medullary cystic kidney disease 2 and familial juvenile hyperuricaemic nephropathy

INTRODUCTION

Medullary cystic kidney disease 2 (MCKD2) and familial juvenile hyperuricaemic nephropathy (FJHN) are both autosomal dominant renal diseases characterised by juvenile onset of hyperuricaemia, gout, and progressive renal failure. Clinical features of both conditions vary in presence and severity. Often definitive diagnosis is possible only after significant pathology has occurred. Genetic linkage studies have localised genes for both conditions to overlapping regions of chromosome 16p11-p13. These clinical and genetic findings suggest that these conditions may be allelic.

AIM

To identify the gene and associated mutation(s) responsible for FJHN and MCKD2.

METHODS

Two large, multigenerational families segregating FJHN were studied by genetic linkage and haplotype analyses to sublocalise the chromosome 16p FJHN gene locus. To permit refinement of the candidate interval and localisation of candidate genes, an integrated physical and genetic map of the candidate region was developed. DNA sequencing of candidate genes was performed to detect mutations in subjects affected with FJHN (three unrelated families) and MCKD2 (one family).

RESULTS

We identified four novel uromodulin (UMOD) gene mutations that segregate with the disease phenotype in three families with FJHN and in one family with MCKD2.

CONCLUSION

These data provide the first direct evidence that MCKD2 and FJHN arise from mutation of the UMOD gene and are allelic disorders. UMOD is a GPI anchored glycoprotein and the most abundant protein in normal urine. We postulate that mutation of UMOD disrupts the tertiary structure of UMOD and is responsible for the clinical changes of interstitial renal disease, polyuria, and hyperuricaemia found in MCKD2 and FJHN.

Psoriasis, PUVA, and skin cancer--molecular epidemiology: the curious question of T-->A transversions

Photochemotherapy with 8-methoxypsoralen and long wavelength ultraviolet radiation (PUVA) is commonly used to treat psoriasis and vitiligo. These vastly different diseases respond to the therapy by different mechanisms even though the immediate effects of the therapy - photoadduct formation - is the same for both. Because psoriasis is not cured by PUVA, patients receive many treatments over their lifetime and develop a significant risk for the development of skin cancers (primarily squamous cell carcinomas). In this review the basic aspects of psoralen photobiology are reviewed briefly. In addition the impact of the analysis of mutations in the tumor suppressor gene, p53, are summarized. An unexpected mutation spectrum (very few T-->A transversions and frequent UVB signature C-->T transitions) suggest that effects other than direct DNA photoadduct formation may be at play. The roles of reactive oxygen species-induced base changes as well as other clastogenic factors are discussed. This analysis suggests that it may be possible to improve the therapeutic efficacy of PUVA by a careful evaluation of the mode of delivery.

DNA photodamage stimulates melanogenesis and other photoprotective responses

Ultraviolet (UV) irradiation is a major source of environmental damage to skin. Melanin pigmentation protects against this damage by absorbing UV photons and UV-generated free radicals before they can react with DNA and other critical cellular components; and UV-induced melanogenesis or tanning is widely recognized as exposed skin's major defense against further UV damage. This article reviews extensive data suggesting DNA damage or DNA repair intermediates directly triggers tanning and other photoprotective responses. Evidence includes the observations that tanning is enhanced in cultured pigment cells by accelerating repair of UV-induced cyclobutane pyrimidine dimers or by treating the cells with UV-mimetic DNA-damaging chemicals. Moreover, small single stranded DNA fragments such as thymidine dinucleotides (pTpT), the substrate for almost all DNA photoproducts, also stimulates tanning when added to cultured pigment cells or applied topically to intact skin. In bacteria, single stranded DNA generated by DNA damage or its repair activates a protease that in turn derepresses over 20 genes whose protein products enhance DNA repair and otherwise promote cell survival, a phenomenon termed the SOS response. Interestingly, pTpT also enhances repair of UV-induced DNA damage in human cells and animal skin, at least in part by activating the tumor suppressor protein and transcription factor p53 and thus upregulating a variety of gene products involved in DNA repair and cell cycle regulation. Together, these data suggest that human cells have an evolutionarily conserved SOS-like response in which UV-induced DNA damage serves as signal to induce photoprotective responses such as tanning and increased DNA repair capacity. The responses can also be triggered in the absence of DNA damage by addition of small single-stranded DNA fragments such as pTpT.