Pachyonychia Congenita: A Research Agenda Leading to New Therapeutic Approaches.

Pachyonychia congenita (PC) is a dominantly inherited genetic disorder of cornification. PC stands out among other genodermatoses because despite its rarity, it has been the focus of a very large number of pioneering translational research efforts over the past 2 decades, mostly driven by a patient support organization, the Pachyonychia Congenita Project. These efforts have laid the ground for innovative strategies that may broadly impact approaches to the management of other inherited cutaneous and noncutaneous diseases. This article outlines current avenues of research in PC, expected outcomes, and potential hurdles.

Proteomic profiling reveals KRT6C as a probable hereterodimer partner for KRT9: New insights into re-classifying epidermolytic palmoplantar keratoderma (EPPK) and a milder form of pachyonychia congenita (PC-K6c) as a group of genetic cutaneous disorders.

Epidermolytic palmoplantar keratoderma (EPPK), a highly penetrant autosomal dominant genodermatosis, is characterized by diffuse keratoses on palmplantar epidermis. The keratin 9 gene (KRT9) is responsible for EPPK. To date, phenotypic therapy is the primary treatment for EPPK. Because KRT9 pairs with a type II keratin-binding partner to function in epidermis, identifying the interaction partner is an essential first step in revealing EPPK pathogenesis and its fundamental treatment. In this study, we proved that keratin 6C (KRT6C) is a probable hereterodimer partner for KRT9. In silico model for KRT6C/KRT9 shows a typical coiled-coil structure in their 2B domains. Proteomics analysis shows that KRT6C/KRT9 pair is in a densely connected protein-protein interaction network, where proteins participate jointly in regulating cytoskeleton organization and keratinization. This study shows that co-immunoprecipitation coupled with mass spectroscopy and proteomics analysis provide a sensitive approach, which compensates for inevitable inadequacies of anti-keratin 6C antibody and helps discover the probable hereterodimer partner KRT6C for KRT9. The acknowledgement of KRT6C/KRT9 pairwise relationship may help re-classify EPPK and PC-K6c (a milder form of pachyonychia congenita, caused by KRT6C) as a group of hereditary defects at a molecular-based level, and lay foundation for deciphering the keratin network contributing to EPPK and PC-K6c. SIGNIFICANCE OF THE STUDY: What is already known about this topic? KRT9 and KRT6C are disease-causing factors for epidermolytic palmoplantar keratoderma (EPPK) and a milder form of pachyonychia congenita (PC-K6c), respectively. EPPK and PC-K6c have some symptom similarities. Keratins are the major structural proteins in epithelial cells. Each of the type I keratin is matched by a particular type II keratin to assemble a coiled-coil heterodimer. The hereterodimer partner for KRT9 is unknown. What does this study add? We discovered and proved that KRT6C is a probable hereterodimer partner for KRT9 in palmplantar epidermis in a native endogenous environment by using co-immunoprecipitation coupled with mass spectroscopy and proteomics analysis, etc. The proteomics analysis shows that KRT6C/KRT9 keratin pair is in a densely connected protein-protein interaction network, where proteins participate jointly in regulating intermediate filament-based cytoskeleton organization and keratinization processes. What are the implications of this work? The new understanding of probable KRT6C/KRT9 pairwise correlation may help re-classify the genetic cutaneous disorders EPPK and PC-K6c as a group of hereditary defects at a molecular-based level, and lay foundation for pathogenic mechanism research in EPPK and PC-K6c. The densely related network components derived from the proteomic data using Metascape in the study and pairwise regulation fashion of specific keratin pairs should attract more attention in the further explorations when investigators concern the physiological functions of keratins and the pathogenesis of related skin diseases.

Walking a day in a pachyonychia congenita patient's shoes: Impact on plantar pain and activity levels measured with wristband activity trackers.

Background Plantar keratoderma is a common finding in pachyonychia congenita, significantly impairing ambulation and quality of life. Due to the variation of pain reporting in pachyonychia congenita clinical studies, it is difficult to evaluate the efficacy of treatment outcomes for painful plantar keratodermas. Objectives To objectively analyse associations between plantar pain and activity levels in pachyonychia congenita patients using a wristband tracker. Methods Pachyonychia congenita patients and matched normal controls wore wristband activity trackers and completed a daily digital survey to record their highest and total pain scores (0-10 scale) each day for 28 consecutive days during four different seasons. Results Twenty four participants (12 pachyonychia congenita patients and 12 matched normal controls) completed the study. Pachyonychia congenita patients walked 1801.30 fewer steps/day (95% CI, -3666.4, 64.1) than normal controls (P = 0.072) and had greater average total [5.26; SD, 2.10] and highest (6.92; SD, 2.35) daily pain than normal controls [0.11 (SD, 0.47), 0.30 (SD, 0.22), respectively] (P < 0.001, both). On average, for each one unit increase in daily highest pain level, pachyonychia congenita activity decreased 71.54 steps/day (SE, 38.90, P = 0.066). Limitation The study had a small number of participants, limiting statistical power. Only pachyonychia congenita patients, ages 18 years or older, with keratin 6a, keratin 16, and keratin 17 mutations were included, limiting generalizability. Conclusion Pachyonychia congenita patients were less active with significantly higher pain than normal controls. There was an inverse correlation between pain and activity. Our findings suggest that wristband tracker technology may be used to evaluate treatment efficacy in future trials on severe plantar pain; therapeutic interventions that decrease plantar pain should correlate with significant increases in activity using wristband trackers.

Phenotype and genotype features of Vietnamese children with pachyonychia congenita.

BACKGROUND: Pachyonychia congenita (PC) is a group of autosomal dominant disorders caused by mutations in one of five keratin genes (KRT6A, KRT6B, KRT6C, KRT16, or KRT17). PC is an extremely rare condition. To our knowledge, this is the largest genotype-phenotype study of PC in a Vietnamese population to date. MATERIALS AND METHODS: We investigated keratin gene mutations and clinical features of seven Vietnamese children with PC. RESULTS: The seven Vietnamese patients were from six different families (two patients in the same family) from across Northern, Central, and Southern Vietnam. All children displayed PC symptoms before 1 year of age, but diagnosis was delayed in 4/7 patients. Thick fingernails, thick toenails, oral leukokeratosis, and follicular hyperkeratosis were the most common features recorded by all seven patients. Plantar keratoderma and thick fingernails were the clinical features associated with the most significant effect on daily function. All patients had mutations in KRT6A (PC-K6a) focused on the 1A and 2B domains. We found three distinct types of mutations (K6a R466P, K6a N171K, and K6a N172del). One mutation (N172del) was common to 5/7 (71.4%) of the patients. CONCLUSIONS: Individuals displaying nail dystrophy, oral leukokeratosis, follicular hyperkeratosis, and plantar keratoderma should be referred for genetic testing given the high likelihood of a PC-K6a-related mutation in patients with this constellation of clinical signs.

EGFR Signaling Is Overactive in Pachyonychia Congenita: Effective Treatment with Oral Erlotinib.

Pachyonychia congenita (PC) is a rare keratinizing disorder characterized by painful palmoplantar keratoderma for which there is no standard current treatment. PC is caused by dominant mutations in keratin (K) K6A, K6B, K6C, K16, or K17 genes involved in stress, wound healing, and epidermal barrier formation. Mechanisms leading to pain and painful palmoplantar keratoderma in PC remain elusive. In this study, we show overexpression of EGFR ligands epiregulin and TGF-α as well as HER1‒EGFR and HER2 in the upper spinous layers of PC lesions. EGFR activation was confirmed by upregulated MAPK/ERK and mTOR signaling. Abnormal late terminal keratinization was associated with elevated TGM1 activity. In addition, the calcium ion permeable channel TRPV3 was significantly increased in PC-lesional skin, suggesting a predominant role of the TRPV3/EGFR signaling complex in PC. We hypothesized that this complex contributes to promoting TGM1 activity and induces the expression and shedding of EGFR ligands. To counteract this biological cascade, we treated three patients with PC with oral erlotinib for 6‒8 months. The treatment was well-tolerated and led to an early, drastic, and sustained reduction of neuropathic pain with a major improvement of QOL. Our study provides evidence that targeted pharmacological inhibition of EGFR is an effective strategy in PC.

KERATIN 17-related recessive atypical pachyonychia congenita with variable hair and tooth anomalies.

We present the first pachyonychia congenita (PC) to involve all ectodermal derivatives and the first recessive KRT17-related PC in total seven members of two consanguineous Pakistani families. This atypical PC is characterized by an unusual combination of pachyonychia, plantar keratoderma, folliculitis, alopecia, sparse eyebrows, dental anomalies and variable acanthosis nigricans of neck, dry skin, palmoplantar hyperhidrosis, recurrent blisters on soles and/or arms, rough sparse hair on scalp and keratosis pilaris. By exome sequencing we detected homozygous KRT17 c.281G>A (p.(Arg94His)) in affected individuals, and linkage mapping indicated a single locus. Heterozygous variants in KRT17 cause PC2 (PC-K17) with main characteristics of pachyonychia, subungual keratosis, palmoplantar keratoderma, hyperhidrosis, oral leukokeratosis and epidermal cysts, or steatocystoma multiplex, both with dominant inheritance. The causative variant has been reported in heterozygous state in a family afflicted with severe steatocystoma multiplex and in a sporadic PC2 case, and thus we also define a third phenotype related to the variant. Both exome sequencing and linkage mapping demonstrated recessive inheritance whereas Sanger sequencing indicated heterozygosity for the causal variant, reiterating caution for simple targeted sequencing for genetic testing. Testing parents for variants found in sibs could uncover recessive inheritance also in other KRT genes.

Coexistence of pachyonychia congenita and hidradenitis suppurativa: more than a coincidence.

BACKGROUND: The coexistence of pachyonychia congenita (PC) and hidradenitis suppurativa (HS) has been described in case reports. However, the pathomechanism underlying this association and its true prevalence are unknown. OBJECTIVES: To determine the genetic defect underlying the coexistence of PC and HS in a large kindred, to delineate a pathophysiological signalling defect jointly leading to both phenotypes, and to estimate the prevalence of HS in PC. METHODS: We used direct sequencing and a NOTCH luciferase reporter assay to characterize the pathophysiological basis of the familial coexistence of HS and PC. A questionnaire was distributed to patients with PC registered with the International Pachyonychia Congenita Research Registry (IPCRR) to assess the prevalence of HS among patients with PC. RESULTS: Direct sequencing of DNA samples obtained from family members displaying both PC and HS demonstrated a missense variant (c.275A>G) in KRT17, encoding keratin 17. Abnormal NOTCH signalling has been suggested to contribute to HS pathogenesis. Accordingly, the KRT17 c.275A>G variant resulted in a significant decrease in NOTCH activity. To ascertain the clinical importance of the association of HS with PC, we distributed a questionnaire to all patients with PC registered with the IPCRR. Seventy-two of 278 responders reported HS-associated clinical features (25·9%). Disease-causing mutations in KRT17 were most prevalent among patients with a dual phenotype of PC and HS (43%). CONCLUSIONS: The coexistence of HS and KRT17-associated PC is more common than previously thought. Impaired NOTCH signalling as a result of KRT17 mutations may predispose patients with PC to HS. What is already known about this topic? The coexistence of pachyonychia congenita (PC) and hidradenitis suppurativa (HS) has been described in case reports. However, the pathomechanism underlying this association and its true prevalence are unknown. What does this study add? A dual phenotype consisting of PC and HS was found to be associated with a pathogenic variant in KRT17. This variant was found to affect NOTCH signalling, which has been previously implicated in HS pathogenesis. HS was found to be associated with PC in a large cohort of patients with PC, especially in patients carrying KRT17 variants, suggesting that KRT17 variants causing PC may also predispose to HS. What is the translational message? These findings suggest that patients with PC have a higher prevalence of HS than previously thought, and hence physicians should have a higher level of suspicion of HS diagnosis in patients with PC.

Scrutinising the role of simvastatin in a patient of Pachyonychia Congenita with KRT6A gene mutation.

A 25-year-old male patient presented with palmoplantar keratoderma, dystrophic nails, severe plantar pain and oral leukokeratosis since birth. On genetic analysis, a heterozygous KRT6A gene missense mutation (c.1381G > A, p.Glu461Lys in exon 7) was identified by next-generation sequencing technology, consistent with pachyonychia congenita 6a. Oral simvastatin 40 mg was started once daily, and after 16 weeks of therapy, excellent improvement was noted in palmoplantar keratoderma and plantar pain. The maximum thickness of his foot callosity reduced by 4 mm on ultrasonography, and the Dermatology Life Quality Index score dropped significantly by eight points. These benefits may be attributed to inhibition of KRT6A gene expression, modulation of autophagy and mitophagy and Keap1-Nrf2 signalling activation; the latter two mechanisms of statins previously undiscussed in the context of pachyonychia congenita. Simvastatin and other statins are pathogenesis-targeted, disease-modifying therapy in pachyonychia congenita, therefore qualifying as a promising treatment avenue and warranting further clinical trials.

A KRT6A mutation p.Ile462Asn in a Chinese family with pachyonychia congenita, and identification of maternal mosaicism: a case report.

BACKGROUND: Pachyonychia congenita (PC, OMIM #167200, #167210, #615726, #615728, and #615735) is a rare autosomal dominant disorder caused by keratin gene mutations in KRT6A,KRT6B,KRT6C,KRT16 or KRT17. It is characterized with nail dystrophy and palmoplantar keratoderma (PPK). The most prominent manifestation is plantar pain. This is a further unusual case of parental mosaicism in PC. Although very rare, germ cell mosaicism should be considered when providing genetic counselling for unaffected parents of a child with PC. CASE PRESENTATION: We report the case of a 5-year-old boy with thickening nails and oral leukokeratosis at birth. He began to develop palmoplantar keratoderma at 2 years old and his sister has similar clinical manifestation characterized with nail discoloration and thickening. A previously reported heterozygous mutation, p.Ile462Asn, was identified in KRT6A in the proband and his affected sister. SNaPshot sequencing revealed mosaicism at a level of 2.5% and 4.7% in DNA from blood and hair bulbs from the unaffected mother. HiSeq deep sequencing demonstrated low-grade mosaicism in the patient's younger sister and parents. CONCLUSION: These findings indicate the ability of WES and SNaPshot sequencing to detect low-frequency mosaic mutations. Although very rare, germinal mosaicism should be considered when genetic counseling is given to families with presumed spontaneous cases of PC.

Genotype‒Structurotype‒Phenotype Correlations in Patients with Pachyonychia Congenita.

Pachyonychia congenita (PC) is a genetic disorder of keratin that presents with nail dystrophy, painful palmoplantar keratoderma, and other clinical manifestations. We investigated the genotype‒structurotype‒phenotype correlations seen with mutations in keratin genes (keratin [K]6A, K6B, K6C, K16, K17) and utilized protein structure modeling of high-frequency mutations to examine the functional importance of keratin structural domains in PC pathogenesis. Participants of the International PC Research Registry underwent genetic testing and completed a standardized survey on their symptoms. Our results support previous reports associating oral leukokeratosis with K6A mutations and cutaneous cysts, follicular hyperkeratosis, and natal teeth with K17 mutations. Painful keratoderma was prominent with K6A and K16 mutations. Nail involvement was most common in patients with K6A mutation and least common in those with K6C mutation. Across keratin subtypes, patients with coil 2B mutations had the greatest impairment in ambulation, and patients with coil 1A mutations reported more emotional issues. Molecular modeling demonstrated that hotspot missense mutations in PC largely disrupted hydrophobic interactions or surface charge. The former may destabilize keratin dimers/tetramers, whereas the latter likely interferes with higher-order keratin filament formation. Understanding the pathologic alterations in keratin structure improves our knowledge of how PC genotype correlates with clinical phenotype, advancing insight into disease pathogenesis and therapeutic development.

Identification of clinically useful predictive genetic variants in pachyonychia congenita.

BACKGROUND: Pachyonychia congenita (PC) refers to a group of autosomal dominant disorders caused by mutations in five keratin genes (KRT16,KRT6A,KRT17,KRT6B or KRT6C). Current disease classification is based on the gene harbouring disease-causing variants. AIMS: We harnessed the International Pachyonychia Congenita Research Registry (IPCRR) containing both clinical and molecular data on patients with PC worldwide, to identify genetic variants predicting disease severity. METHODS: We ascertained 815 individuals harbouring keratin mutations registered in the IPCRR. We looked for statistically significant associations between genetic variants and clinical manifestations in a subgroup of patients carrying mutations found in at least 10% of the cohort. Data were analysed using χ2 and Kruskal-Wallis tests. RESULTS: We identified five mutations occurring in at least 10% of the patients registered in the IPCRR. The KRT16 p.L132P mutation was significantly associated with younger age of onset, presence of palmar keratoderma oral leucokeratosis and a higher number of involved nails. By contrast, the KRT16 p.N125S and p.R127C mutations resulted in a milder phenotype featuring a decreased number of involved nails and older age of onset. Patients carrying the p.N125S mutation were less likely to develop palmar keratoderma while p.R127C was associated with an older age of palmoplantar keratoderma onset. Moreover, the KRT17 p.L99P mutation resulted in an increased number of involved fingernails and patients demonstrating 20-nail dystrophy, while the opposite findings were observed with KRT17 p.N92S mutation. CONCLUSIONS: We have identified novel and clinically useful genetic predictive variants in the largest cohort of patients with PC described to date.

Molecular epidemiology of pachyonychia congenita in the Israeli population.

BACKGROUND: Pachyonychia congenita (PC) is a rare autosomal dominant disorder featuring palmoplantar keratoderma, nail dystrophy, oral leucokeratosis, pilosebaceous cysts and natal teeth. PC results from dominant mutations in one of five genes (KRT6A, KRT6B, KRT6C, KRT16, KRT17) encoding keratin proteins. AIM: To delineate the clinical and genetic features of PC in a series of Israeli patients. METHODS: We used direct sequencing of genomic DNA, and also used cDNA sequencing where applicable. RESULTS: We collected clinical information and molecular data in a cohort of Israeli families diagnosed with PC (n = 16). Most of the patients were Ashkenazi Jews and had a family history of PC. The most common clinical findings were painful focal plantar keratoderma (94%) accompanied by nail dystrophy (81%), pilosebaceous cysts (31%) and prenatal/natal teeth (13%). In contrast to the high prevalence of KRT6A mutations in other populations, we found that KRT16 mutations were the most common type among Israeli patients with PC (56%). Most (77%) of the Israeli patients with PC with KRT16 mutation carried the same variant (c.380G>A; p.R127H) and shared the same haplotype around the KRT16 locus, suggestive of a founder effect. CONCLUSION: The data gleaned from this study emphasizes the importance of population-specific tailored diagnostic strategies.