Keith A. Choate

Mutations in Kelch-like 3 and Cullin 3 cause hypertension and electrolyte abnormalities

Hypertension affects one billion people and is a principal reversible risk factor for cardiovascular disease. Pseudohypoaldosteronism type II (PHAII), a rare Mendelian syndrome featuring hypertension, hyperkalaemia and metabolic acidosis, has revealed previously unrecognized physiology orchestrating the balance between renal salt reabsorption and K+ and H+ excretion. Here we used exome sequencing to identify mutations in kelch-like 3 (KLHL3) or cullin 3 (CUL3) in PHAII patients from 41 unrelated families. KLHL3 mutations are either recessive or dominant, whereas CUL3 mutations are dominant and predominantly de novo. CUL3 and BTB-domain-containing kelch proteins such as KLHL3 are components of cullin–RING E3 ligase complexes that ubiquitinate substrates bound to kelch propeller domains. Dominant KLHL3 mutations are clustered in short segments within the kelch propeller and BTB domains implicated in substrate and cullin binding, respectively. Diverse CUL3 mutations all result in skipping of exon 9, producing an in-frame deletion. Because dominant KLHL3 and CUL3 mutations both phenocopy recessive loss-of-function KLHL3 mutations, they may abrogate ubiquitination of KLHL3 substrates. Disease features are reversed by thiazide diuretics, which inhibit the Na–Cl cotransporter in the distal nephron of the kidney; KLHL3 and CUL3 are expressed in this location, suggesting a mechanistic link between KLHL3 and CUL3 mutations, increased Na–Cl reabsorption, and disease pathogenesis. These findings demonstrate the utility of exome sequencing in disease gene identification despite the combined complexities of locus heterogeneity, mixed models of transmission and frequent de novo mutation, and establish a fundamental role for KLHL3 and CUL3 in blood pressure, K+ and pH homeostasis.

Mitotic Recombination in Patients with Ichthyosis Causes Reversion of Dominant Mutations in KRT10

Gaining from a Loss Mitotic recombination can cause a cell carrying heterozygous mutations in a tumor suppressor gene to lose the wild-type copy of the gene, setting the cell on the pathway to uncontrolled growth. But can mitotic recombination have beneficial effects in other settings—that is, lead to phenotypic correction of a diseased cell by facilitating loss of the disease-causing mutation? Choate et al. (p. 94, published online 26 August; see the Perspective by Davis and Candotti) now find evidence for this type of event in a rare skin disease called ichthyosis with confetti (IWC). Patients with IWC display severe scaling of the skin but have widespread patches of normal skin that reflect clonal expansion of revertant cells. The revertant cells showed loss of heterozygosity on chromosome 17q and, as a result of mitotic recombination, these cells selectively lost dominant disease-causing mutations in the keratin 10 gene (KRT10), but retained the wild-type copy of the gene. Patches of normal skin in patients with a rare skin disorder are due to cells that have lost the disease-causing mutation. Somatic loss of wild-type alleles can produce disease traits such as neoplasia. Conversely, somatic loss of disease-causing mutations can revert phenotypes; however, these events are infrequently observed. Here we show that ichthyosis with confetti, a severe, sporadic skin disease in humans, is associated with thousands of revertant clones of normal skin that arise from loss of heterozygosity on chromosome 17q via mitotic recombination. This allowed us to map and identify disease-causing mutations in the gene encoding keratin 10 (KRT10); all result in frameshifts into the same alternative reading frame, producing an arginine-rich C-terminal peptide that redirects keratin 10 from the cytokeratin filament network to the nucleolus. The high frequency of somatic reversion in ichthyosis with confetti suggests that revertant stem cell clones are under strong positive selection and/or that the rate of mitotic recombination is elevated in individuals with this disorder.

WNK1, a kinase mutated in inherited hypertension with hyperkalemia, localizes to diverse Cl−-transporting epithelia

Mutations in WNK1 and WNK4, genes encoding members of a novel family of serine–threonine kinases, have recently been shown to cause pseudohypoaldosteronism type II (PHAII), an autosomal dominant disorder featuring hypertension, hyperkalemia, and renal tubular acidosis. The localization of these kinases in the distal nephron and the Cl− dependence of these phenotypes suggest that these mutations increase renal Cl− reabsorption. Although WNK4 expression is limited to the kidney, WNK1 is expressed in many tissues. We have examined the distribution of WNK1 in these extrarenal tissues. Immunostaining using WNK1-specific antibodies demonstrated that WNK1 is not present in all cell types; rather, it is predominantly localized in polarized epithelia, including those lining the lumen of the hepatic biliary ducts, pancreatic ducts, epididymis, sweat ducts, colonic crypts, and gallbladder. WNK1 is also found in the basal layers of epidermis and throughout the esophageal epithelium. The subcellular localization of WNK1 varies among these epithelia. WNK1 is cytoplasmic in kidney, colon, gallbladder, sweat duct, skin, and esophagus; in contrast, it localizes to the lateral membrane in bile ducts, pancreatic ducts, and epididymis. These epithelia are all notable for their prominent role in Cl− flux. Moreover, these sites largely coincide with those involved in the pathology of cystic fibrosis, a disease characterized by deranged epithelial Cl− flux. Together with the known pathophysiology of PHAII, these findings suggest that WNK1 plays a general role in the regulation of epithelial Cl− flux, a finding that suggests the potential of new approaches to the selective modulation of these processes.

KRIT1, a gene mutated in cerebral cavernous malformation, encodes a microtubule-associated protein

Mutations in Krev1 interaction trapped gene 1 (KRIT1) cause cerebral cavernous malformation, an autosomal dominant disease featuring malformation of cerebral capillaries resulting in cerebral hemorrhage, strokes, and seizures. The biological functions of KRIT1 are unknown. We have investigated KRIT1 expression in endothelial cells by using specific anti-KRIT1 antibodies. By both microscopy and coimmunoprecipitation, we show that KRIT1 colocalizes with microtubules. In interphase cells, KRIT1 is found along the length of microtubules. During metaphase, KRIT1 is located on spindle pole bodies and the mitotic spindle. During late phases of mitosis, KRIT1 localizes in a pattern indicative of association with microtubule plus ends. In anaphase, the plus ends of the interpolar microtubules show strong KRIT1 staining and, in late telophase, KRIT1 stains the midbody remnant most strongly; this is the site of cytokinesis where plus ends of microtubules from dividing cells overlap. These results establish that KRIT1 is a microtubule-associated protein; its location at plus ends in mitosis suggests a possible role in microtubule targeting. These findings, coupled with evidence of interaction of KRIT1 with Krev1 and integrin cytoplasmic domain-associated protein-1 alpha (ICAP1 α), suggest that KRIT1 may help determine endothelial cell shape and function in response to cell–cell and cell–matrix interactions by guiding cytoskeletal structure. We propose that the loss of this targeting function leads to abnormal endothelial tube formation, thereby explaining the mechanism of formation of cerebral cavernous malformation (CCM) lesions.

Multilineage somatic activating mutations in HRAS and NRAS cause mosaic cutaneous and skeletal lesions, elevated FGF23 and hypophosphatemia

Pathologically elevated serum levels of fibroblast growth factor-23 (FGF23), a bone-derived hormone that regulates phosphorus homeostasis, result in renal phosphate wasting and lead to rickets or osteomalacia. Rarely, elevated serum FGF23 levels are found in association with mosaic cutaneous disorders that affect large proportions of the skin and appear in patterns corresponding to the migration of ectodermal progenitors. The cause and source of elevated serum FGF23 is unknown. In those conditions, such as epidermal and large congenital melanocytic nevi, skin lesions are variably associated with other abnormalities in the eye, brain and vasculature. The wide distribution of involved tissues and the appearance of multiple segmental skin and bone lesions suggest that these conditions result from early embryonic somatic mutations. We report five such cases with elevated serum FGF23 and bone lesions, four with large epidermal nevi and one with a giant congenital melanocytic nevus. Exome sequencing of blood and affected skin tissue identified somatic activating mutations of HRAS or NRAS in each case without recurrent secondary mutation, and we further found that the same mutation is present in dysplastic bone. Our finding of somatic activating RAS mutation in bone, the endogenous source of FGF23, provides the first evidence that elevated serum FGF23 levels, hypophosphatemia and osteomalacia are associated with pathologic Ras activation and may provide insight in the heretofore limited understanding of the regulation of FGF23.

Whole-Exome Sequencing Reveals Somatic Mutations in HRAS and KRAS, which Cause Nevus Sebaceus

ACKNOWLEDGMENTS We thank the patients and their families for taking part in this project. We also thank S Aasi, R Khosla, and P Lorenz for their valuable assistance. Bryan K. Sun, Andrea Saggini, Kavita Y. Sarin, Jinah Kim, Latanya Benjamin, Philip E. LeBoit and Paul A. Khavari Department of Dermatology, Stanford University School of Medicine, Stanford, California, USA; Department of Dermatology, University of California, San Francisco, San Francisco, California, USA and Department of Dermatology, University of Rome Tor Vergata, Rome, Italy E-mail: bryansun@stanford.edu

Frequent somatic reversion of KRT1 mutations in ichthyosis with confetti

Widespread reversion of genetic disease is rare; however, such events are particularly evident in some skin disorders in which normal clones develop on a background of affected skin. We previously demonstrated that mutations in keratin 10 (KRT10) cause ichthyosis with confetti (IWC), a severe dominant disorder that is characterized by progressive development of hundreds of normal skin spots via revertant mosaicism. Here, we report on a clinical and histological IWC subtype in which affected subjects have red, scaly skin at birth, experience worsening palmoplantar keratoderma in childhood, and develop hundreds of normal skin spots, beginning at around 20 years of age, that increase in size and number over time. We identified a causal de novo mutation in keratin 1 (KRT1). Similar to IWC-causing KRT10 mutations, this mutation in KRT1 resulted in a C-terminal frameshift, replacing 22 C-terminal amino acids with an alternate 30-residue peptide. Mutant KRT1 caused partial collapse of the cytoplasmic intermediate filament network and mislocalized to the nucleus. As with KRT10 mutations causing IWC, reversion of KRT1 mutations occurred via mitotic recombination. Because reversion is not observed with other disease-causing keratin mutations, the results of this study implicate KRT1 and KRT10 C-terminal frameshift mutations in the high frequency of revertant mosaicism in IWC.

Somatic Activating RAS Mutations Cause Vascular Tumors Including Pyogenic Granuloma

Young H. Lim1,2,5, Stephanie R. Douglas3, Christine J. Ko1,2, Richard J. Antaya1,4, Jennifer M. McNiff1,2, Jing Zhou1,2,5, Yale Center for Genome Analysis, Keith A. Choate1,2,5,*, and Deepak Narayan3,* 1Department of Dermatology, Yale University School of Medicine, New Haven, Connecticut, USA 2Department of Pathology, Yale University School of Medicine, New Haven, Connecticut, USA 3Section of Plastic Surgery, Department of Surgery, Yale University School of Medicine, New Haven, Connecticut, USA

Capillary Malformation—Arteriovenous Malformation Syndrome: Review of the Literature, Proposed Diagnostic Criteria, and Recommendations for Management

Capillary malformation–arteriovenous malformation syndrome is an autosomal dominant disorder caused by mutations in the RASA1 gene and characterized by multiple small, round to oval capillary malformations with or without arteriovenous malformations. Ateriovenous malformations occur in up to one‐third of patients and may involve the brain and spine. Although making the diagnosis is straightforward in some patients, there are other patients for whom diagnostic criteria may be helpful in their evaluation. Here we review the literature regarding capillary malformation−arteriovenous malformation syndrome, propose diagnostic criteria, and discuss the care of patients with this condition.

Somatic HRAS p.G12S mutation causes woolly hair and epidermal nevi.

TO THE EDITOR Woolly hair nevus (WHN) is a mosaic disorder characterized by distinct patterns of tightly curled scalp hair which can appear concurrently with epidermal nevi (EN) at other sites (Peteiro et al., 1989; Venugopal et al., 2012). Woolly hair is also found in congenital disorders resulting from mutations affecting diverse cellular components including intermediate filament, adherens junction, and signal transduction proteins (Harel and Christiano, 2012). Embryonic somatic mutation causes mosaic disorders which appear in patterns of ectodermal progenitor dorsovental migration. Somatic mutations causing mosaic disorders including Proteus syndrome (Lindhurst et al., 2011), port-wine stains (Shirley et al., 2013), and EN (Levinsohn et al., 2013; Sun et al., 2013) have been found using exome sequencing. Recognizing that exome sequencing would permit identification of mutations causing WHN, we ascertained two cases. Our first (WHN100, Figure 1a-d) was a 10 year-old girl without history of developmental delay who had regions of slightly curly hair over her occipital scalp from infancy which progressively curled with no scalp surface change and lie alongside areas of straight hair. She has hyperpigmented patches on her neck, trunk, and arms, with more keratotic lesions on her distal extremities, and acanthosis nigricans in both axillae. There was linear palmar keratoderma (PPK) and hyperkeratosis over most metacarpophalangeal and some proximal interphalangeal joints. Given concurrent PPK and woolly hair, clinical concern for Naxos or Carvajal syndromes led to regular cardiology evaluations that found no abnormalities. Figure 1 Clinical features of index cases with woolly hair nevi. On the scalp, woolly hair nevus presents with a portion of the scalp exhibiting patches of curly, thin, hair intermixed with regions of normal, straight hair, as observed in WHN100 and WHN101. On ... Our second case (WHN101, Figure 1e-h) was a 6 year-old girl whose hair developed at age one and consisted of a mixture of poker-straight hair and curly, thin hair. In infancy, she developed linear dyspigmentation on the right arm and trunk, which became more raised and scaly on the distal extremities over time. She had normal development, with no cardiac or ophthalmic abnormalities found on routine physical examination, cardiac MRI and serial electrocardiograms. Clinical suspicion of mosaic Naxos or Caravajal syndrome motivated clinical sequencing of DSP, DSC1, DSG1, JUP, PKP2, and TMEM43; no mutations were found. To determine the genetic basis of WHN, we performed paired whole exome sequencing of DNA isolated from affected tissue and blood in both cases (Supplementary Figure 2). Data was analyzed to identify somatic single nucleotide variants (SNVs), deletions and insertions (Supplementary Methods). A somatic heterozygous HRAS c.34G>A, p.G12S substitution was found in each (Figure 2a). There was no evidence of loss of heterozygosity (LOH) (Supplementary Figure 3) or secondary mutation somatic mutation, suggesting that HRAS mutation alone is sufficient to cause WHN. Sanger sequencing confirmed mutation presence in affected tissue (Figure 2b, c). To determine if this mutation causes woolly hair, we prepared DNA from hair bulbs of straight and curly hair obtained from affected individual WHN101, finding the HRAS p.G12S mutation in curly hair only (Figure 2d, Supplementary Figure 1). Figure 2 Somatic HRAS p.G12S mutation causes WHN. (a) In WHN100 and WHN101, exome sequencing of affected tissue and blood was performed. Tissue-specific SNVs are annotated bychromosome, position, base change, protein consequence, and numbers of reference and non-reference ... Consistent with somatic mosaicism in an epidermal progenitor, prior cases of WHN have been reported with concurrent keratinocytic epidermal nevi (KEN). KEN result from somatic mutations in HRAS, KRAS, PIK3CA, FGFR3, and NRAS (Hafner et al., 2012) including the HRAS p.G12S mutation found in WHN (Hafner et al., 2011). Furthermore, Costello syndrome (CS), in which patients present with developmental delay, high birth weight, feeding difficulties, failure to thrive, cardiac anomalies, and curly hair, results from germline heterozygous HRAS mutations, including p.G12S (Gripp and Lin, 2012; Siegel et al., 2012). The timing of somatic mutation during embryonic development determines extent of cutaneous involvement and presence of other systemic abnormalities (Moss et al., 1993). Notably, somatic activating HRAS mutations are found in most cases of nevus sebaceus (NS), a mosaic lesion which typically appears on the scalp and features alopecia, papillomatosis, and marked sebaceus hyperplasia (Groesser et al., 2012; Levinsohn et al., 2013; Sun et al., 2013). These features contrast with those of WHN in which hair is present but curly, and sebaceous hyperplasia is absent. Given that WHN and NS are both caused by somatic HRAS mutations, we hypothesize that their phenotypic divergence may derive from relative potency of the mutant allele with respect to MAP kinase activation. HRAS mutations in WHN and NS fall within the finger loop of HRAS, replacing glycine residues with larger amino acids which prevent GTP hydrolysis (Malumbres and Barbacid, 2003). Though comparison of the WHN p.G12S mutation and the common NS p.G13R mutation has not been performed, HRAS codon 12 serine substitutions have been shown to be less activating than arginine, aspartic acid or valine substitutions (Fasano et al., 1984). To evaluate the frequency of HRAS mutation in NS, we screened 116 archival scalp NS lesions for HRAS and KRAS mutation. We found 88 HRAS and 9 KRAS mutations. HRAS p.G13R was present in 85 NS and p.G12S was not found (Supplementary Table 2). In prior reports, 64 additional samples were screened, and HRAS p.G12S mutations were not found (Levinsohn et al., 2013; Sun et al., 2013). In one report, 3 specimens with HRAS p.G12S mutations were identified; in 2 there was a concurrent HRAS p.G13R mutation, and in one, the lesion was on the ear, a site at which it could be difficult to distinguish EN and NS (Groesser et al., 2012). These data combined with evidence from CS suggest that more strongly activating RAS mutations may cause the alopecia and sebaceous hyperplasia found in NS, and the more mildly activating p.G12S mutation causes woolly hair phenotypes. In summary, we find somatic HRAS c.34G>A, p.G12S mutation in affected tissue from two cases with mosaic woolly hair and EN. Consistent with reports of WHN and in KEN, the identified p.G12S mutation causes an EN phenotype on the body, but the finding of curly hair on the scalp suggests that WHN represents a mosaic RASopathy with phenotype determined by location, either due to distinct epidermal progenitor types or site-specific mesenchymal interactions. We hypothesize that in contrast to strongly activating RAS mutations found in NS which drive hair follicle progenitors toward sebocyte differentiation, the more weakly activating mutation found in WHN permits an intermediate phenotype with abnormal curly hair growth but without sebaceous hyperplasia.