Murine models of HRAS-mediated cutaneous skeletal hypophosphatemia syndrome suggest bone as the FGF23 excess source.

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is a mosaic RASopathy characterized by the association of dysplastic skeletal lesions, congenital skin nevi of epidermal and/or melanocytic origin, and FGF23-mediated hypophosphatemia. The primary physiological source of circulating FGF23 is bone cells. However, several reports have suggested skin lesions as the source of excess FGF23 in CSHS. Consequently, without convincing evidence of efficacy, many patients with CSHS have undergone painful removal of cutaneous lesions in an effort to normalize blood phosphate levels. This study aims to elucidate whether the source of FGF23 excess in CSHS is RAS mutation-bearing bone or skin lesions. Toward this end, we analyzed the expression and activity of Fgf23 in two mouse models expressing similar HRAS/Hras activating mutations in a mosaic-like fashion in either bone or epidermal tissue. We found that HRAS hyperactivity in bone, not skin, caused excess of bioactive intact FGF23, hypophosphatemia, and osteomalacia. Our findings support RAS-mutated dysplastic bone as the primary source of physiologically active FGF23 excess in patients with CSHS. This evidence informs the care of patients with CSHS, arguing against the practice of nevi removal to decrease circulating, physiologically active FGF23.

Clinical description and genetic analysis of a novel familial skeletal dysplasia characterized by high bone mass and lucent bone lesions.

High bone mass (HBM) disorders are a clinically and genetically heterogeneous subgroup of rare skeletal dysplasias. Here we present a case of a previously unreported familial skeletal dysplasia characterized by HBM and lucent bone lesions that we aimed to clinically characterize and genetically investigate. For phenotyping, we reviewed past clinical records and imaging tests, and performed physical examination (PE), bone densitometry, and mineral panels in affected individuals, including a male proband, his son and daughter, in addition to unaffected controls, including the proband's wife and brother. Affected individuals also underwent impact microindentation (IMI). In an effort to elucidate the disorder's molecular etiology, whole exome sequencing (WES) was performed in all individuals to filter for rare variants present only in affected ones. The cases displayed a unique skeletal phenotype with a mix of sclerotic features and lucent bone lesions, and high IMI values. Bone mineral density was very elevated in the proband and his daughter. The proband's daughter also exhibited idiopathic scoliosis (IS), in addition to mild thrombocytopenia and mild structural thyroid abnormalities, which were the only extra-skeletal abnormalities identified. WES analysis yielded 5 rare putative pathogenic variants in affected members in genes that are associated with bone metabolism including: SEM4AD, TBX18, PTCH1, PTK7, and ADGRE5. The PTK7 variant appeared as possibly implicated in the development of IS while the TBX18 and SEMA4D variants stood out as the strongest candidates for the lucent bone lesions and HBM, respectively, given their high predicted pathogenicity and putative role in bone biology. Variant functionality should be addressed in the future to assess their implication in skeletal metabolism as it is the first time that mutations in TBX18 and SEMA4D have been associated to bone developmental lesions and mineral metabolism in a clinical setting.

Genetic Analysis in a Familial Case With High Bone Mineral Density Suggests Additive Effects at Two Loci.

Osteoporosis is the most common bone disease, characterized by a low bone mineral density (BMD) and increased risk of fracture. At the other end of the BMD spectrum, some individuals present strong, fracture-resistant, bones. Both osteoporosis and high BMD are heritable and their genetic architecture encompasses polygenic inheritance of common variants and some cases of monogenic highly penetrant variants in causal genes. We have investigated the genetics of high BMD in a family segregating this trait in an apparently Mendelian dominant pattern. We searched for rare causal variants by whole-exome sequencing in three affected and three nonaffected family members. Using this approach, we have identified 38 rare coding variants present in the proband and absent in the three individuals with normal BMD. Although we have found four variants shared by the three affected members of the family, we have not been able to relate any of these to the high-BMD phenotype. In contrast, we have identified missense variants in two genes, VAV3 and ADGRE5, each shared by two of out of three affected members, whose loss of function fits with the phenotype of the family. In particular, the proband, a woman displaying the highest BMD (sum Z-score = 7), carries both variants, whereas the other two affected members carry one each. VAV3 encodes a guanine-nucleotide-exchange factor with an important role in osteoclast activation and function. Although no previous cases of VAV3 mutations have been reported in humans, Vav3 knockout (KO) mice display dense bones, similarly to the high-BMD phenotype present in our family. The ADGRE5 gene encodes an adhesion G protein-coupled receptor expressed in osteoclasts whose KO mouse displays increased trabecular bone volume. Combined, these mouse and human data highlight VAV3 and ADGRE5 as novel putative high-BMD genes with additive effects, and potential therapeutic targets for osteoporosis. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Gene Network of Susceptibility to Atypical Femoral Fractures Related to Bisphosphonate Treatment.

Atypical femoral fractures (AFF) are rare fragility fractures in the subtrocantheric or diaphysis femoral region associated with long-term bisphosphonate (BP) treatment. The etiology of AFF is still unclear even though a genetic basis is suggested. We performed whole exome sequencing (WES) analysis of 12 patients receiving BPs for at least 5 years who sustained AFFs and 4 controls, also long-term treated with BPs but without any fracture. After filtration and prioritization of rare variants predicted to be damaging and present in genes shared among at least two patients, a total of 272 variants in 132 genes were identified. Twelve of these genes were known to be involved in bone metabolism and/or AFF, highlighting DAAM2 and LRP5, both involved in the Wnt pathway, as the most representative. Afterwards, we intersected all mutated genes with a list of 34 genes obtained from a previous study of three sisters with BP-related AFF, identifying nine genes. One of these (MEX3D) harbored damaging variants in two AFF patients from the present study and one shared among the three sisters. Gene interaction analysis using the AFFNET web suggested a complex network among bone-related genes as well as with other mutated genes. BinGO biological function analysis highlighted cytoskeleton and cilium organization. In conclusion, several genes and their interactions could provide genetic susceptibility to AFF, that along with BPs treatment and in some cases with glucocorticoids may trigger this so feared complication.

PTH and FGF23 Exert Interdependent Effects on Renal Phosphate Handling: Evidence From Patients With Hypoparathyroidism and Hyperphosphatemic Familial Tumoral Calcinosis Treated With Synthetic Human PTH 1-34.

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) both influence blood phosphate levels by regulating urinary phosphate reabsorption. Clinical data suggest that adequate renal phosphate handling requires the presence of both FGF23 and PTH, but robust evidence is lacking. To investigate whether the phosphaturic effects of PTH and FGF23 are interdependent, 11 patients with hypoparathyroidism, which features high blood phosphate in spite of concomitant FGF23 elevation, and 1 patient with hyperphosphatemic familial tumoral calcinosis (HFTC), characterized by deficient intact FGF23 action and resulting hyperphosphatemia, were treated with synthetic human PTH 1-34 (hPTH 1-34). Biochemical parameters, including blood phosphate, calcium, intact FGF23 (iFGF23), nephrogenic cAMP, 1,25(OH)2 vitamin D (1,25D), and tubular reabsorption of phosphate (TRP), were measured at baseline and after hPTH 1-34 treatment. In patients with hypoparathyroidism, administration of hPTH 1-34 increased nephrogenic cAMP, which resulted in serum phosphate normalization followed by a significant decrease in iFGF23. TRP initially decreased and returned to baseline. In the patient with HFTC, hPTH 1-34 administration also increased nephrogenic cAMP, but this did not produce changes in phosphate or TRP. No changes in calcium were observed in any of the studied patients, although prolonged hPTH 1-34 treatment did induce supraphysiologic 1,25D levels in the patient with HFTC. Our results indicate that PTH and FGF23 effects on phosphate regulation are interdependent and both are required to adequately regulate renal phosphate handling. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

Functional Analyses of Four CYP1A1 Missense Mutations Present in Patients with Atypical Femoral Fractures.

Osteoporosis is the most common metabolic bone disorder and nitrogen-containing bisphosphonates (BP) are a first line treatment for it. Yet, atypical femoral fractures (AFF), a rare adverse effect, may appear after prolonged BP administration. Given the low incidence of AFF, an underlying genetic cause that increases the susceptibility to these fractures is suspected. Previous studies uncovered rare CYP1A1 mutations in osteoporosis patients who suffered AFF after long-term BP treatment. CYP1A1 is involved in drug metabolism and steroid catabolism, making it an interesting candidate. However, a functional validation for the AFF-associated CYP1A1 mutations was lacking. Here we tested the enzymatic activity of four such CYP1A1 variants, by transfecting them into Saos-2 cells. We also tested the effect of commonly used BPs on the enzymatic activity of the CYP1A1 forms. We demonstrated that the p.Arg98Trp and p.Arg136His CYP1A1 variants have a significant negative effect on enzymatic activity. Moreover, all the BP treatments decreased CYP1A1 activity, although no specific interaction with CYP1A1 variants was found. Our results provide functional support to the hypothesis that an additive effect between CYP1A1 heterozygous mutations p.Arg98Trp and p.Arg136His, other rare mutations and long-term BP exposure might generate susceptibility to AFF.

Calcifediol Treatment and COVID-19-Related Outcomes.

CONTEXT: COVID-19 is a major health problem because of saturation of intensive care units (ICU) and mortality. Vitamin D has emerged as a potential treatment able to reduce the disease severity. OBJECTIVE: This work aims to elucidate the effect of 25(OH)D3 (calcifediol) treatment on COVID-19-related outcomes. METHODS: This observational cohort study was conducted from March to May 2020, among patients admitted to COVID-19 wards of Hospital del Mar, Barcelona, Spain. A total of 930 patients with COVID-19 were included; 92 were excluded because of previous calcifediol intake. Of the remaining 838, a total of 447 received calcifediol (532 µg on day 1 plus 266 µg on days 3, 7, 15, and 30), whereas 391 were not treated at the time of hospital admission (intention-to-treat). Of the latter, 53 patients were treated later during ICU admission and were allocated in the treated group in a second analysis. In healthy individuals, calcifediol is about 3.2-fold more potent on a weight basis than cholecalciferol. Main outcome measures were ICU admission and mortality. RESULTS: ICU assistance was required by 102 (12.2%) participants. Out of 447 patients treated with calcifediol at admission, 20 (4.5%) required the ICU, compared to 82 (21%) out of 391 nontreated (P < .001). Logistic regression of calcifediol treatment on ICU admission, adjusted by age, sex, linearized 25-hydroxyvitamin D levels at baseline, and comorbidities showed that treated patients had a reduced risk of requiring the ICU (odds ratio [OR] 0.13; 95% CI 0.07-0.23). Overall mortality was 10%. In the intention-to-treat analysis, 21 (4.7%) out of 447 patients treated with calcifediol at admission died compared to 62 patients (15.9%) out of 391 nontreated (P = .001). Adjusted results showed a reduced mortality risk with an OR of 0.21 (95% CI, 0.10-0.43). In the second analysis, the obtained OR was 0.52 (95% CI, 0.27-0.99). CONCLUSION: In patients hospitalized with COVID-19, calcifediol treatment significantly reduced ICU admission and mortality.

Genetics and Genomics of SOST: Functional Analysis of Variants and Genomic Regulation in Osteoblasts.

SOST encodes the sclerostin protein, which acts as a key extracellular inhibitor of the canonical Wnt pathway in bone, playing a crucial role in skeletal development and bone homeostasis. The objective of this work was to assess the functionality of two variants previously identified (the rare variant rs570754792 and the missense variant p.Val10Ile) and to investigate the physical interactors of the SOST proximal promoter region in bone cells. Through a promoter luciferase reporter assay we show that the minor allele of rs570754792, a variant located in the extended TATA box motif, displays a significant decrease in promoter activity. Likewise, through western blot studies of extracellular and intracellular sclerostin, we observe a reduced expression of the p.Val10Ile mutant protein. Finally, using a circular chromosome conformation capture assay (4C-seq) in 3 bone cell types (MSC, hFOB, Saos-2), we have detected physical interactions between the SOST proximal promoter and the ECR5 enhancer, several additional enhancers located between EVT4 and MEOX1 and a distant region containing exon 18 of DHX8. In conclusion, SOST presents functional regulatory and missense variants that affect its expression and displays physical contacts with far reaching genomic sequences, which may play a role in its regulation within bone cells.

Cutaneous mosaicism: Special considerations for women.

Genetic mosaicism results from postzygotic mutations during embryogenesis. Cells harboring pathogenic mutations distribute throughout the developing embryo and can cause clinical disease in the tissues they populate. Cutaneous mosaicism is readily visualized since affected tissue often follows predetermined patterns, such as lines of Blaschko. Due to its clinical accessibility, cutaneous mosaicism is well suited for genetic analysis. An individual's unaffected tissue can be used as an intrapatient genetic control, a technique that has yielded insight into the genetic etiologies of many disorders, several of which bear mutations in genes that would otherwise be embryonic-lethal. Particular mosaic diseases can also disproportionally impact women. Two such diseases, incontinentia pigmenti (IP) and congenital hemidysplasia with ichthyosiform erythroderma and limb defects (CHILD) syndrome, arise from mutations on the X chromosome. Both diseases result in fetal demise in males in most cases, thus making the two diseases largely specific to women. Women with McCune-Albright Syndrome, caused by somatic mutations in GNAS, often experience precocious puberty and infertility as a result of uncontrolled cAMP regulation in affected tissue. Women with cutaneous mosaicism carry a risk of transmission to offspring when gonosomal mosaicism is present, yet cutaneous disease burden does not correlate with germline transmission risk. Cutaneous mosaic disease represents a biologically unique set of disorders that can warrant special clinical attention in women.

Functional Assessment of Coding and Regulatory Variants From the DKK1 Locus.

The DKK1 gene encodes an extracellular inhibitor of the Wnt pathway with an important role in bone tissue development, bone homeostasis, and different critical aspects of bone biology. Several BMD genome-wide association studies (GWASs) have consistently found association with SNPs in the DKK1 genomic region. For these reasons, it is important to assess the functionality of coding and regulatory variants in the gene. Here, we have studied the functionality of putative regulatory variants, previously found associated with BMD in different studies by others and ourselves, and also six missense variants present in the general population. Using a Wnt-pathway-specific luciferase reporter assay, we have determined that the variants p.Ala41Thr, p.Tyr74Phe, p.Arg120Leu, and p.Ser157Ile display a reduced DKK1 inhibitory capacity as compared with WT. This result agrees with the high-bone-mass (HBM) phenotype of two women from our cohort who carried mutations p.Tyr74Phe or p.Arg120Leu. On the other hand, by means of a circularized chromosome conformation capture- (4C-) sequencing experiment, we have detected that the region containing 24 BMD-GWA variants, located 350-kb downstream of DKK1, interacts both with DKK1 and the LNCAROD (LncRNA-activating regulator of DKK1, AKA LINC0148) in osteoblastic cells. In conclusion, we have shown that some rare coding variants are partial loss-of-function mutations that may lead to a HBM phenotype, whereas the common SNPs associated with BMD in GWASs belong to a putative long-range regulatory region, through a yet unknown mechanism involving LNCAROD. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

DIAGNOSIS OF ENDOCRINE DISEASE: Mosaic disorders of FGF23 excess: Fibrous dysplasia/McCune-Albright syndrome and cutaneous skeletal hypophosphatemia syndrome.

Fibrous dysplasia/McCune-Albright Syndrome (FD/MAS), arising from gain-of-function mutations in Gαs, and cutaneous skeletal hypophosphatemia syndrome (CSHS), arising from gain-of-function mutations in the Ras/MAPK pathway, are strikingly complex, mosaic diseases with overlapping phenotypes. Both disorders are defined by mosaic skin and bone involvement, and both are complicated by increased FGF23 production. These similarities have frequently led to mis-diagnoses, primarily in patients with CSHS who are often assumed to have FD/MAS. The intriguing similarities in skeletal involvement in these genetically distinct disorders have led to novel insights into FGF23 physiology, making an understanding of FD/MAS and CSHS relevant to both clinicians and researchers interested in bone and endocrine disorders. This review will give an overview of FD/MAS and CSHS, focusing on the roles of mosaicism and FGF23 in the pathogenesis and clinical presentation of these disorders.

Both glycaemic control and insulin dose during pregnancy in women with type 1 diabetes are associated with neonatal anthropometric measures and placental weight.

BACKGROUND: To investigate longitudinal associations of maternal glucose/HbA1c and insulin dose with birthweight-related outcomes in women with type 1 diabetes. METHODS: We performed a cohort study including 473 pregnant women with type 1 diabetes with singleton pregnancies. We investigated maternal self-monitored blood glucose (SMBG, mmol/L), HbA1c (%, mmol/mol) and insulin dose (IU/kg/day) in the three trimesters as potential independent variables, while adjusting for potential confounders. Outcomes of interest were birthweight, birthweight SD score, neonatal length, weight/length index, ponderal index and placental weight. Multiple linear regression analysis was performed with separate analyses for SMBG and HbA1c . RESULTS: Maternal glucose and insulin dose were independently associated with birthweight-related outcomes. In the main analysis, in the first trimester most associations were positive for insulin dose, in the second the associations were positive for glucose and inverse for insulin while in the third there were no associations. Most sensitivity analyses produced consistent results. In a sensitivity analysis splitting the first trimester in two periods, positive associations of maternal insulin with birthweight-related outcomes were observed in weeks 0+ to 6+. CONCLUSIONS: Early in pregnancy in women with type 1 diabetes, maternal insulin dose is positively associated with birthweight-related outcomes, whereas in the second trimester, a positive association with SMBG emerges and the association with maternal insulin becomes inverse. If confirmed in other cohorts, these results would have implications in the management of women with type 1 diabetes.

Functional characterization of the C7ORF76 genomic region, a prominent GWAS signal for osteoporosis in 7q21.3.

Genome-wide association studies (GWAS) have repeatedly identified genetic variants associated with bone mineral density (BMD) and osteoporotic fracture in non-coding regions of C7ORF76, a poorly studied gene of unknown function. The aim of the present study was to elucidate the causality and molecular mechanisms underlying the association. We re-sequenced the genomic region in two extreme BMD groups from the BARCOS cohort of postmenopausal women to search for functionally relevant variants. Eight selected variants were tested for association in the complete cohort and 2 of them (rs4342521 and rs10085588) were found significantly associated with lumbar spine BMD and nominally associated with osteoporotic fracture. cis-eQTL analyses of these 2 SNPs, together with SNP rs4727338 (GWAS lead SNP in Estrada et al., Nat Genet. 44:491-501, 2012), performed in human primary osteoblasts, disclosed a statistically significant influence on the expression of the proximal neighbouring gene SLC25A13 and a tendency on the distal SHFM1. We then studied the functionality of a putative upstream regulatory element (UPE), containing rs10085588. Luciferase reporter assays showed transactivation capability with a strong allele-dependent effect. Finally, 4C-seq experiments in osteoblastic cell lines showed that the UPE interacted with different tissue-specific enhancers and a lncRNA (LOC100506136) in the region. In summary, this study is the first one to analyse in depth the functionality of C7ORF76 genomic region. We provide functional regulatory evidence for the rs10085588, which may be a causal SNP within the 7q21.3 GWAS signal for osteoporosis.

Bone Matrix Mineralization in Patients With Gain-of-Function Calcium-Sensing Receptor Mutations Is Distinctly Different From that in Postsurgical Hypoparathyroidism.

The role of the calcium-sensing receptor (CaSR) as a regulator of parathyroid hormone secretion is well established, but its function in bone is less well defined. In an effort to elucidate the CaSR's skeletal role, bone tissue and material characteristics from patients with autosomal dominant hypocalcemia (ADH), a genetic form of primary hypoparathyroidism caused by CASR gain-of-function mutations, were compared to patients with postsurgical hypoparathyroidism (PSH). Bone structure and formation/resorption indices and mineralization density distribution (BMDD), were examined in transiliac biopsy samples from PSH (n = 13) and ADH (n = 6) patients by histomorphometry and quantitative backscatter electron imaging, respectively. Bone mineral density (BMD by DXA) and biochemical characteristics were measured at the time of the biopsy. Because both study groups comprised children and adults, all measured biopsy parameters and BMD outcomes were converted to Z-scores for comparison. Histomorphometric indices were normal and not different between ADH and PSH, with the exception of mineral apposition rate Z-score, which was higher in the ADH group. Similarly, average BMD Z-scores were normal and not different between ADH and PSH. Significant differences were observed for the BMDD: average Z-scores of mean and typical degree of mineralization (CaMean, CaPeak, respectively) were lower (p = 0.02 and p = 0.03, respectively), whereas the heterogeneity of mineralization (CaWidth) and percentage of lower mineralized areas (CaLow) were increased in ADH versus PSH (p = 0.01 and p = 0.002, respectively). The BMDD outcomes point toward a direct, PTH-independent role of the CaSR in the regulation of bone mineralization. © 2018 American Society for Bone and Mineral Research.

Expression of an active Gαs mutant in skeletal stem cells is sufficient and necessary for fibrous dysplasia initiation and maintenance.

Fibrous dysplasia (FD) is a disease caused by postzygotic activating mutations of GNAS (R201C and R201H) that encode the α-subunit of the Gs stimulatory protein. FD is characterized by the development of areas of abnormal fibroosseous tissue in the bones, resulting in skeletal deformities, fractures, and pain. Despite the well-defined genetic alterations underlying FD, whether GNAS activation is sufficient for FD initiation and the molecular and cellular consequences of GNAS mutations remains largely unresolved, and there are no currently available targeted therapeutic options for FD. Here, we have developed a conditional tetracycline (Tet)-inducible animal model expressing the GαsR201C in the skeletal stem cell (SSC) lineage (Tet-GαsR201C/Prrx1-Cre/LSL-rtTA-IRES-GFP mice), which develops typical FD bone lesions in both embryos and adult mice in less than 2 weeks following doxycycline (Dox) administration. Conditional GαsR201C expression promoted PKA activation and proliferation of SSCs along the osteogenic lineage but halted their differentiation to mature osteoblasts. Rather, as is seen clinically, areas of woven bone admixed with fibrous tissue were formed. GαsR201C caused the concomitant expression of receptor activator of nuclear factor kappa-B ligand (Rankl) that led to marked osteoclastogenesis and bone resorption. GαsR201C expression ablation by Dox withdrawal resulted in FD-like lesion regression, supporting the rationale for Gαs-targeted drugs to attempt FD cure. This model, which develops FD-like lesions that can form rapidly and revert on cessation of mutant Gαs expression, provides an opportunity to identify the molecular mechanism underlying FD initiation and progression and accelerate the development of new treatment options.

Octreotide Is Ineffective in Treating Tumor-Induced Osteomalacia: Results of a Short-Term Therapy.

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome in which unregulated hypersecretion of fibroblast growth factor 23 (FGF23) by phosphaturic mesenchymal tumors (PMT) causes renal phosphate wasting, hypophosphatemia, and osteomalacia. The resulting mineral homeostasis abnormalities and skeletal manifestations can be reversed with surgical resection of the tumor. Unfortunately, PMTs are often difficult to locate, and medical treatment with oral phosphate and vitamin D analogues is either insufficient to manage the disease or not tolerated. Octreotide has been proposed as a potential treatment for TIO due to the presence of somatostatin receptors (SSTR) on PMTs; however, the role of somatostatin signaling in PMTs and the efficacy of treatment of TIOs with somatostatin analogues is not clear. In an effort to evaluate the efficacy of octreotide therapy in TIO, five subjects with TIO were treated with octreotide for 3 days. Blood intact FGF23, phosphate, and 1,25(OH)2 D3 , and tubular reabsorption of phosphate (TRP) were measured at frequent time points during treatment. Octreotide's effects were assessed by comparing group means of the biochemical parameters at each time-point to mean baseline values. There were no significant changes in blood phosphate, FGF23, 1,25(OH)2 D3 , or TRP during octreotide treatment, consistent with a lack of efficacy of octreotide in treating TIO. © 2017 American Society for Bone and Mineral Research.

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is a multilineage somatic mosaic RASopathy.

BACKGROUND: We recently demonstrated multilineage somatic mosaicism in cutaneous skeletal hypophosphatemia syndrome (CSHS), which features epidermal or melanocytic nevi, elevated fibroblast growth factor (FGF)-23, and hypophosphatemia, finding identical RAS mutations in affected skin and bone. OBJECTIVE: We sought to: (1) provide an updated overview of CSHS; (2) review its pathobiology; (3) present a new patient with CSHS; and (4) discuss treatment modalities. METHODS: We searched PubMed for "nevus AND rickets," and "nevus AND hypophosphatemia," identifying cases of nevi with hypophosphatemic rickets or elevated serum FGF-23. For our additional patient with CSHS, we performed histopathologic and radiographic surveys of skin and skeletal lesions, respectively. Sequencing was performed for HRAS, KRAS, and NRAS to determine causative mutations. RESULTS: Our new case harbored somatic activating HRAS p.G13 R mutation in affected tissue, consistent with previous findings. Although the mechanism of FGF-23 dysregulation is unknown in CSHS, interaction between FGF and MAPK pathways may provide insight into pathobiology. Anti-FGF-23 antibody KRN-23 may be useful in managing CSHS. LIMITATIONS: Multilineage RAS mutation in CSHS was recently identified; further studies on mechanism are unavailable. CONCLUSION: Patients with nevi in association with skeletal disease should be evaluated for serum phosphate and FGF-23. Further studies investigating the role of RAS in FGF-23 regulation are needed.

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.