Novel proline substitution mutations in keratin 16 in two cases of pachyonychia congenita type 1

Mutation p.Arg127Pro in the 1A Domain of KRT16 Causes Pachyonychia Congenita in Chinese Patient: A Case Report of PC Associated with Acral Melanoma

Pachyonychia congenita (PC) is a group of rare hereditary disorders, characterised by hypertrophic nails and palmoplantar keratoderma (PPK), particularly localised to the pressure areas of the feet. At a molecular level, it is caused by mutations in genes encoding KRT6A, KRT6B, KRT6C, KRT16, or KRT17. To identify the underlying gene mutation in a Chinese family with PC presenting with disabling palmoplantar keratoderma and subsequent associated acral melanoma. Genomic DNA was extracted from peripheral blood samples of three available individuals in the Chinese family, which included the patient and his two unaffected sisters. The index patient presented with severe palmoplantar keratoderma as well as a newly diagnosed acral malignant melanoma (MM). Whole-exome sequencing (WES) was carried out with amplification of exon 1 of KRT16 by polymerase chain reaction (PCR). PCR products were then sequenced to identify potential mutations. We identified the proline substitution mutation p.Arg127Pro (c.380G>C) in our patient's 1A domain of KRT16. The same mutation was not found in his sisters or unrelated healthy controls. The mutation (p.Arg127Pro (c.380G>C)) in KRT16 has been reported in Dutch patients with PC. However, it is the first such report of a patient with a PC of Chinese origin. In addition, the acral MM occurred under the background of genetic PPK caused by KRT16 mutation in this patient.

Severe dermatitis, multiple allergies, and metabolic wasting syndrome caused by a novel mutation in the N-terminal plakin domain of desmoplakin

Background

Severe dermatitis, multiple allergies, and metabolic wasting (SAM) syndrome is a recently recognized syndrome caused by mutations in the desmoglein 1 gene (DSG1). To date, only 3 families have been reported.

Objective

We studied a new case of SAM syndrome known to have no mutations in DSG1 to detail the clinical, histopathologic, immunofluorescent, and ultrastructural phenotype and to identify the underlying molecular mechanisms in this rare genodermatosis.

Methods

Histopathologic, electron microscopy, and immunofluorescent studies were performed. Whole-exome sequencing data were interrogated for mutations in desmosomal and other skin structural genes, followed by Sanger sequencing of candidate genes in the patient and his parents.

Results

No mutations were identified in DSG1; however, a novel de novo heterozygous missense c.1757A>C mutation in the desmoplakin gene (DSP) was identified in the patient, predicting the amino acid substitution p.His586Pro in the desmoplakin polypeptide.

Conclusions

SAM syndrome can be caused by mutations in both DSG1 and DSP. Knowledge of this genetic heterogeneity is important for both analysis of patients and genetic counseling of families. This condition and these observations reinforce the importance of heritable skin barrier defects, in this case desmosomal proteins, in the pathogenesis of atopic disease.

Proline: the distribution, frequency, positioning, and common functional roles of proline and polyproline sequences in the human proteome

Proline is an anomalous amino acid. Its nitrogen atom is covalently locked within a ring, thus it is the only proteinogenic amino acid with a constrained phi angle. Sequences of three consecutive prolines can fold into polyproline helices, structures that join alpha helices and beta pleats as architectural motifs in protein configuration. Triproline helices are participants in protein-protein signaling interactions. Longer spans of repeat prolines also occur, containing as many as 27 consecutive proline residues. Little is known about the frequency, positioning, and functional significance of these proline sequences. Therefore we have undertaken a systematic bioinformatics study of proline residues in proteins. We analyzed the distribution and frequency of 687,434 proline residues among 18,666 human proteins, identifying single residues, dimers, trimers, and longer repeats. Proline accounts for 6.3% of the 10,882,808 protein amino acids. Of all proline residues, 4.4% are in trimers or longer spans. We detected patterns that influence function based on proline location, spacing, and concentration. We propose a classification based on proline-rich, polyproline-rich, and proline-poor status. Whereas singlet proline residues are often found in proteins that display recurring architectural patterns, trimers or longer proline sequences tend be associated with the absence of repetitive structural motifs. Spans of 6 or more are associated with DNA/RNA processing, actin, and developmental processes. We also suggest a role for proline in Kruppel-type zinc finger protein control of DNA expression, and in the nucleation and translocation of actin by the formin complex.

Genotype-phenotype correlations among pachyonychia congenita patients with K16 mutations

Pachyonychia congenita (PC) is a rare, autosomal dominant keratin disorder caused by mutations in four genes (KRT6A, KRT6B, KRT16, or KRT17). The International PC Research Registry is a database with information on patients' symptoms as well as genotypes. We sought to describe the heterogeneity of clinical symptoms and to investigate possible genotype-phenotype correlations in patients with two types of K16 mutations, p.Asn125 and p.Arg127, causing the PC-16 subtype of PC. We found that clinical symptoms depended on the type of amino-acid substitution. Patients with p.Asn125Asp and p.Arg127Pro mutations exhibited more severe disease than patients carrying p.Asn125Ser and p.Arg127Cys mutations in terms of age of onset of symptoms, extent of nail involvement, and impact on daily quality of life. We speculate that amino-acid substitutions causing larger, more disruptive changes to the K16 protein structure, such as a change in amino-acid charge in the p.Asn125Asp mutation or a bulky proline substitution in the p.Arg127Pro mutation, may also lead to more severe disease phenotypes. The variation in phenotypes seen with different substitutions at the same mutation site suggests a genotype-phenotype correlation. Knowledge of the exact gene defect is likely to assist in predicting disease prognosis and clinical management.

Disadhesion of epidermal keratinocytes: a histologic clue to palmoplantar keratodermas caused by DSG1 mutations

BACKGROUND

Recent developments in molecular genetics may lead to re-examination of the histopathology of inherited palmoplantar keratodermas (PPKs) based on more precise groupings of the various entities and syndromes.

OBJECTIVE

We sought to characterize the histopathological findings in PPKs associated with mutations in DSG1, which encodes desmoglein 1.

METHODS

We studied the histopathology of 3 cases of keratosis palmoplantaris striata type I and one case of diffuse PPK, all associated with autosomal-dominant mutations in DSG1. Our cases for comparison included 4 cases with Mal de Meleda PPK associated with autosomal-recessive SLURP1 mutations, one case with pachyonychia congenita type II PPK associated with an autosomal-dominant KRT17 mutation, and one case with focal PPK associated with an autosomal-dominant KRT16 mutation.

RESULTS

The distinguishing histopathological features of the 3 keratosis palmoplantaris striata type I cases and the diffuse PPK case associated with DSG1 mutation were: varying degrees of widening of the intercellular spaces and partial disadhesion of keratinocytes in the mid and upper epidermal spinous cell layers, often extending to the granular cell layer. These findings, which are associated with haploinsufficiency of desmoglein 1, were not observed in any of the other 6 PPK cases. Mild perinuclear eosinophilic condensations and cytoplasmic vacuolizations were observed in the spinous cell layer keratinocytes of the pachyonychia congenita type II PPK and the nonspecified focal PPK cases.

LIMITATIONS

There were a limited number of patients and control patients with hereditary PPKs.

CONCLUSION

Widening of the intercellular spaces and disadhesion of epidermal keratinocytes may serve as a histologic clue to PPKs caused by DSG1 mutations.

A spectrum of mutations in keratins K6a, K16 and K17 causing pachyonychia congenita

BACKGROUND

Pachyonychia congenita (PC) is a rare autosomal dominant keratin disorder, subdivided into two major variants, PC-1 and PC-2. Predominant characteristics include hypertrophic nail dystrophy, focal palmoplantar keratoderma and oral leukokeratosis. Multiple steatocystomas that develop during puberty are a useful feature distinguishing PC-2 from PC-1. At the molecular level it has been shown that mutations in keratin K6a or K16 cause PC-1 whereas those in K6b or K17 lead to PC-2.

OBJECTIVE

To identify mutations in 22 families presenting with clinical symptoms of either PC-1/focal non-epidermolytic palmoplantar keratoderma (FNEPPK) or PC-2.

METHODS

Mutation analysis was performed on genomic DNA from PC patients by direct sequencing.

RESULTS

Here, we report four new missense and five known mutations in K6a; one new deletion and three previously identified missense mutations in K16; plus one known mutation in K17.

CONCLUSION

With one exception, all these heterozygous mutations are within the highly conserved helix boundary motif regions at either end of the keratin rod domain. In one sporadic case, a unique mutation in K16 resulting in deletion of 24bp was found within the central rod domain, in a child with a phenotype predominantly consisting of focal plantar keratoderma. The identification of mutations in cases of PC is prerequisite for future development of gene-specific and/or mutation-specific therapies.

Clinical and Pathological Features of Pachyonychia Congenita

Pachyonychia congenita (PC) is a rare genodermatosis affecting the nails, skin, oral mucosae, larynx, hair, and teeth. Pathogenic mutations in keratins K6a or K16 are associated with the PC-1 phenotype whereas K6b and K17 mutations are associated with the PC-2 phenotype. Analysis of clinical, pathological, and genetic data from the literature and two research registries reveal that >97% of PC cases exhibit fingernail and toenail thickening, and painful plantar keratoderma. Prospective evaluation of 57 PC patients from 41 families revealed variable clinical findings: hyperhidrosis (79%), oral leukokeratosis (75%), follicular keratosis (65%), palmar keratoderma (60%), cutaneous cysts (35%), hoarseness or laryngeal involvement (16%), coarse or twisted hair (26%), early primary tooth loss (14%), and presence of natal or prenatal teeth (2%). Stratification of these data by keratin mutation confirmed the increased incidence of cyst formation and natal teeth among PC-2 patients, although cysts were more commonly seen in PC-1 than previously reported (25%-33%). Previously unreported clinical features of PC include development of painful oral and nipple lesions during breastfeeding, copious production of waxy material in ears, and inability to walk without an ambulatory aid (50%). Possible pathogenic mechanisms are discussed with respect to the clinicopathologic and genetic correlations observed.

Focal palmoplantar and gingival keratosis: A distinct palmoplantar ectodermal dysplasia with epidermolytic alterations but lack of mutations in known keratins

Focal palmoplantar and gingival keratosis is a rare autosomal dominant disease whose clinical features, and in particular, pathologic alterations and molecular etiology remain to be well defined. Recently we observed a German family affected by the disease in at least 3 consecutive generations. The 4 patients examined showed circumscribed and painful hyperkeratosis at the weight-bearing plantar skin since infancy, rather mild palmar hyperkeratosis, and continuous leukokeratosis confined to the maxillary and mandibulary attached gingiva. There were no nail changes, subungeal keratoses, or follicular hyperkeratosis. Light and electron microscopy of the plantar and gingival lesions revealed alterations of epidermolytic hyperkeratosis. Mutations in the known keratin genes were excluded by linkage analysis using microsatellite markers. We conclude that focal palmoplantar and gingival keratosis is a clinically distinct palmoplantar ectodermal dysplasia that is pathologically characterized by epidermolytic alterations, but is most probably not caused by a mutation in a keratin gene.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

The molecular genetics of keratin disorders

Keratins are the type I and II intermediate filament proteins which form a cytoskeletal network within all epithelial cells. They are expressed in pairs in a tissue- and differentiation-specific fashion. Epidermolysis bullosa simplex (EBS) was the first human disorder to be associated with keratin mutations. The abnormal keratin filament aggregates observed in basal cell keratinocytes of some EBS patients are composed of keratins K5 and K14. Dominant mutations in the genes encoding these proteins were shown to disrupt the keratin filament cytoskeleton resulting in cells that are less resilient and blister with mild physical trauma. Identification of mutations in other keratin genes soon followed with attention focussed on disorders showing abnormal clumping of keratin filaments in specific cells. For example, in bullous congenital ichthyosiform erythroderma, clumping of filaments in the suprabasal cells led to the identification of mutations in the suprabasal keratins, K1 and K10. Mutations have now been identified in 18 keratins, all of which produce a fragile cell phenotype. These include ichthyosis bullosa of Siemens (K2e), epidermolytic palmoplantar keratoderma (K1, K9), pachyonychia congenita (K6a, K6b, K16, K17), white sponge nevus (K4, K13), Meesmann's corneal dystrophy (K3, K12), cryptogenic cirrhosis (K8, K18) and monilethrix (hHb6, hHb1).In general, these disorders are inherited as autosomal dominant traits and the mutations act in a dominant-negative manner. Therefore, treatment in the form of gene therapy is difficult, as the mutant gene needs to be inactivated. Ways of achieving this are actively being studied. Reliable mutation detection methods from genomic DNA are now available. This enables rapid screening of patients for keratin mutations. For some of the more severe phenotypes, prenatal diagnosis may be requested and this can now be performed from chorionic villus samples at an early stage of the pregnancy. This review article describes the discovery of, to date, mutations in 18 keratin genes associated with inherited human diseases.

GFAP mutations in Alexander disease

Alexander disease is a rare but often fatal disease of the central nervous system. Infantile, juvenile and adult forms have been described that present with different clinical signs, but are unified by the characteristic presence in astrocytes of Rosenthal fibers-protein aggregates that contain glial fibrillary acidic protein (GFAP) and small stress proteins. The chance discovery that mice expressing a human GFAP transgene formed abundant Rosenthal fibers suggested that mutations in the GFAP gene are a cause of Alexander disease. Sequencing results from several laboratories have indeed now identified GFAP coding mutations in most cases of the disease, including both the infantile and juvenile forms. These mutations have been found in the 1A, 2A and 2B segments of the conserved central rod domain of GFAP, and also in the variable tail region. All changes detected are heterozygous missense mutations, and none has been found in any parent of a patient that has been tested. This indicates that most cases of Alexander disease arise through de novo, dominant, GFAP mutations. Many of these mutations are homologous to ones described in other intermediate filament diseases. These other diseases have been attributed to a dominant loss of function, as the intermediate filament network is usually disrupted and a similar phenotype is observed in mice in which the corresponding intermediate filament gene has been inactivated. However, astrocytes of Alexander disease patients have normal appearing intermediate filaments, and GFAP null mice do not display the symptoms or pathology of Alexander disease. Thus, Alexander disease likely results from a dominant gain of function. Drawing upon the homology of many of the Alexander disease mutations to those found in other intermediate filament diseases, it is suggested that the gain of function is due to a partial block of filament assembly that leads to accumulation of an intermediate that participates in toxic interactions.

Discovery of a novel murine keratin 6 (K6) isoform explains the absence of hair 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 constitutive expression in the epithelia of oral mucosa, hair follicles, and nail beds. We generated mice deficient for both genes through embryonic stem cell technology. The majority of MK6a/b-/- mice die of starvation within the first two weeks of life. This is due to a localized disintegration of the dorsal tongue epithelium, which results in the build up of a plaque of cell debris that severely impairs feeding. However, approximately 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 presence 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 expressed 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 explanation for the absence of hair and nail defects in MK6a/b-/- animals.

Delayed-onset pachyonychia congenita associated with a novel mutation in the central 2B domain of keratin 16

A young girl with clinical features of pachyonychia congenita type 1 was unusual in that the typical skin and nail changes were not noted until the age of 6 years. Direct sequencing of the KRT16A gene, encoding keratin K16, revealed a novel mutation K354N in the central 2B domain of the K16 polypeptide. The mutation created a new BsmI restriction site and therefore, the mutation was confirmed in the patient and excluded from both parents and 50 normal, unrelated individuals by BsmI digestion of KRT16A polymerase chain reaction products. This is the first time a mutation has been described in this location in a keratin other than K14, where similar mutations cause the milder Weber-Cockayne and/or Köbner types of epidermolysis bullosa simplex.

A novel keratin 12 mutation in a German kindred with Meesmann's corneal dystrophy

AIM

To study a kindred with Meesmann's corneal dystrophy (MCD) to determine if a mutation within the cornea specific K3 or K12 genes is responsible for the disease phenotype.

METHODS

Slit lamp examination of the cornea in four members of the kindred was carried out to confirm the diagnosis of MCD. The region encoding the helix initiation motif (HIM) of the K12 polypeptide was polymerase chain reaction (PCR) amplified from genomic DNA derived from affected individuals in the kindred. PCR products generated were subjected to direct automated sequencing. Restriction enzyme analysis employing Ban I was used to confirm the presence of the mutation in affected individuals of the family.

RESULTS

Sequencing of the K12 gene in an affected individual from the family revealed a novel heterozygous missense mutation (413A-->C), predicting the substitution of a proline for a glutamine at codon 130 (Q130P) in the HIM of the K12 protein. The mutation was excluded from 50 normal, unaffected individuals by restriction enyzme analysis and was therefore unlikely to be a common polymorphism.

CONCLUSION

A novel missense mutation in the K12 gene leads to MCD in a German kindred. Missense mutations have now been identified within the region encoding the helix initiation motif of the K12 protein in eight of 11 MCD kindreds analysed at the molecular level.

DNA repair, DNA replication, and UV mutagenesis

Cells that have been irradiated with ultraviolet light (UV) suffer damage to their DNA, primarily in the form of covalent linkage between adjacent pyrimidines. Such photoproducts represent blocks to RNA and DNA polymerases and are potentially mutagenic. Blockage of RNA polymerase II by a photoproduct in the transcribed strand of an active gene leads to induction of the p53 protein, which induces pleiotropic responses that may include apoptotic cell death. If a cell survives, the blocked polymerase targets the nucleotide excision repair machinery to the site of the lesion, which is repaired in an error-free manner. Repair coupled to transcription in this manner strongly influences the mutation spectrum induced by UV, reducing the proportion of base substitutions that arise from photoproducts on the transcribed strand. If the damage persists when the DNA is replicated in S-phase, either because the cell is unable to repair the damage or because there is insufficient time between the induction of damage and the onset of S-phase. To do so, the replicative DNA polymerase complex may be blocked. In this situation, lesion bypass can be accomplished using an error-free mechanism, or using an error-prone mechanism that involves the newly described, non-processive DNA polymerase zeta encoded by the human homolog of the yeast REV3 gene.