Prader-Willi and Angelman Syndromes: Mechanisms and Management.

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are genetic imprinting disorders resulting from absent or reduced expression of paternal or maternal genes in chromosome 15q11q13 region, respectively. The most common etiology is deletion of the maternal or paternal 15q11q13 region. Methylation is the first line for molecular diagnostic testing; MS-MLPA is the most sensitive test. The molecular subtype of PWS/AS provides more accurate recurrence risk information for parents and for the individual affected with the condition. Management should include a multidisciplinary team by various medical subspecialists and therapists. Developmental and behavioral management of PWS and AS in infancy and early childhood includes early intervention services and individualized education programs for school-aged children. Here, we compare and discuss the mechanisms, pathophysiology, clinical features, and management of the two imprinting disorders, PWS and AS.

Insights into the perinatal phenotype of Kabuki syndrome in infants identified by genome-wide sequencing.

Increasing use of unbiased genomic sequencing in critically ill infants can expand understanding of rare diseases such as Kabuki syndrome (KS). Infants diagnosed with KS through genome-wide sequencing performed during the initial hospitalization underwent retrospective review of medical records. Human phenotype ontology terms used in genomic analysis were aggregated and analyzed. Clinicians were surveyed regarding changes in management and other care changes. Fifteen infants met inclusion criteria. KS was not suspected prior to genomic sequencing. Variants were classified as Pathogenic (n = 10) or Likely Pathogenic (n = 5) by American College of Medical Genetics and Genomics Guidelines. Fourteen variants were de novo (KMT2D, n = 12, KDM6A, n = 2). One infant inherited a likely pathogenic variant in KMT2D from an affected father. Frequent findings involved cardiovascular (14/15) and renal (7/15) systems, with palatal defects also identified (6/15). Three infants had non-immune hydrops. No minor anomalies were universally documented; ear anomalies, micrognathia, redundant nuchal skin, and hypoplastic nails were common. Changes in management were reported in 14 infants. Early use of unbiased genome-wide sequencing enabled a molecular diagnosis prior to clinical recognition including infants with atypical or rarely reported features of KS while also expanding the phenotypic spectrum of this rare disorder.

Atypical presentation of Angelman syndrome with intact expressive language due to low-level mosaicism.

BACKGROUND: Angelman syndrome (AS) occurs due to a lack of expression or function of the maternally inherited UBE3A gene. Individuals with AS typically have significant developmental delay, severe speech impairment with absent to minimal verbal language, gait abnormalities including ataxia, and an incongruous happy demeanor. The majority of individuals with AS also have seizures and microcephaly. Some individuals with mosaic AS have been reported to have expressive language and milder levels of developmental delay. CASE REPORT: We report a male patient presenting with mild to moderate intellectual disability, hyperphagia, obesity, and the ability to communicate verbally. His phenotype was suggestive of Prader-Willi syndrome. However, methylation testing was positive for Angelman syndrome and additional methylation specific multiplex ligation-dependent amplification (MS-MLPA) study revealed low-level mosaicism for AS. CONCLUSION: A broader phenotypic spectrum should be considered for AS as patients with atypical presentations may otherwise elude diagnosis.

Fragile X syndrome in a girl with variant Turner syndrome and an isodicentric X chromosome.

Fragile X (FXS) and Turner (TS) syndromes are X-chromosome-associated disorders. Herein, we report the case of a girl in middle childhood with bicuspid aortic valve in infancy, growth failure, global developmental delay (GDD), visual problems, and coexisting attention-deficit/hyperactivity and anxiety disorders. A high-resolution karyotype in 20 cells revealed 46,X,Idic(X)(p11.21)[19]/45,X[1], suggestive of variant TS. Given her atypical phenotype, subsequent DNA testing was performed. Four FMR1 cytosine-guanine-guanine repeats (30, 410, 580 and 800) were identified, confirming the additional FXS diagnosis. This case study highlights the importance of additional genetic testing in individuals with atypical variant TS, such as unexplained GDD and distinct facial characteristics. The additional FXS diagnosis prompted new therapeutic development for the patient to advance precision healthcare.

Retinal dystrophies: A look beyond the eyes.

Purpose: To illustrate the importance of systemic evaluation in retinal dystrophies through examples of Alstrom syndrome, Bardet Biedl syndrome, and Refsum disease. Observations: Detailed eye evaluations, including visual acuity, visual field, slit lamp examination, and indirect ophthalmoscopy were performed. Retinal imaging included fundus photography and spectral domain optical coherence tomography (SD-OCT). Functional testing of the retina was done using full field electroretinography (ffERG). In addition, molecular genetic testing was performed using a ciliopathy panel, a retinal dystrophy panel, and whole genome sequencing (WGS).We report three individuals who presented with vision concerns first to ophthalmology, noted to have retinal dystrophy, and then referred to genomic medicine for genetic testing. Additional evaluation led to suspicion of specific groups of systemic disorders and guided appropriate genetic testing. The first individual presented with retinal dystrophy, obesity, and short stature with no reported neurocognitive deficits. Genetic testing included a ciliopathy panel that was negative followed by WGS that identified biallelic variants in ALMS: a novel frame-shift pathogenic variant c.6525dupT (p.Gln2176Serfs*17) and a rare nonsense pathogenic variant c.2035C > T (p.Arg679Ter) consistent with Alstrom syndrome. The second individual presented with retinal dystrophy, central obesity, and mild neurocognitive deficits. A ciliopathy genetic testing panel identified a homozygous pathogenic variant in BBS7: c.389_390del (p.Asn130Thrfs*4), confirming the diagnosis of Bardet Biedl syndrome. The third individual presented with progressive vision loss due to retinitis pigmentosa, anosmia, hearing loss, and shortened metatarsals and digits. Genetic testing identified two variants in PHYH: c.375_375del (p.Glu126Argfs*2) a pathogenic variant and c.536A > G (p.His179Arg), a variant of uncertain significance (VUS), suggestive of Refsum disease. Additional biochemical testing revealed markedly elevated phytanic acid with a low concentration of pristanic acid and normal concentrations of very long-chain fatty acids (C22:0, C24:0, C26:0), a pattern consistent with a diagnosis of Refsum disease. Conclusions and importance: In individuals who present with retinal dystrophy to ophthalmologists, additional systemic manifestations such as sensorineural hearing loss, anosmia, or polydactyly, should be sought and a positive history or examination finding should prompt an immediate referral to a clinical geneticist for additional evaluation and appropriate genetic testing. This facilitates pre-test genetic counseling and allows for more accurate diagnosis, prognosis, and management of affected individuals along with better recurrence risk estimates for family members. Identification of an underlying etiology also enhances the understanding of the pathophysiology of disease and expands the genotypic and phenotypic spectrum. Ultimately, successful recognition of these diseases facilitates development of targeted therapies and surveillance of affected individuals.

A novel DPH5-related diphthamide-deficiency syndrome causing embryonic lethality or profound neurodevelopmental disorder.

PURPOSE: Diphthamide is a post-translationally modified histidine essential for messenger RNA translation and ribosomal protein synthesis. We present evidence for DPH5 as a novel cause of embryonic lethality and profound neurodevelopmental delays (NDDs). METHODS: Molecular testing was performed using exome or genome sequencing. A targeted Dph5 knockin mouse (C57BL/6Ncrl-Dph5em1Mbp/Mmucd) was created for a DPH5 p.His260Arg homozygous variant identified in 1 family. Adenosine diphosphate-ribosylation assays in DPH5-knockout human and yeast cells and in silico modeling were performed for the identified DPH5 potential pathogenic variants. RESULTS: DPH5 variants p.His260Arg (homozygous), p.Asn110Ser and p.Arg207Ter (heterozygous), and p.Asn174LysfsTer10 (homozygous) were identified in 3 unrelated families with distinct overlapping craniofacial features, profound NDDs, multisystem abnormalities, and miscarriages. Dph5 p.His260Arg homozygous knockin was embryonically lethal with only 1 subviable mouse exhibiting impaired growth, craniofacial dysmorphology, and multisystem dysfunction recapitulating the human phenotype. Adenosine diphosphate-ribosylation assays showed absent to decreased function in DPH5-knockout human and yeast cells. In silico modeling of the variants showed altered DPH5 structure and disruption of its interaction with eEF2. CONCLUSION: We provide strong clinical, biochemical, and functional evidence for DPH5 as a novel cause of embryonic lethality or profound NDDs with multisystem involvement and expand diphthamide-deficiency syndromes and ribosomopathies.

The seventh international RASopathies symposium: Pathways to a cure-expanding knowledge, enhancing research, and therapeutic discovery.

RASopathies are a group of genetic disorders that are caused by genes that affect the canonical Ras/mitogen-activated protein kinase (MAPK) signaling pathway. Despite tremendous progress in understanding the molecular consequences of these genetic anomalies, little movement has been made in translating these findings to the clinic. This year, the seventh International RASopathies Symposium focused on expanding the research knowledge that we have gained over the years to enhance new discoveries in the field, ones that we hope can lead to effective therapeutic treatments. Indeed, for the first time, research efforts are finally being translated to the clinic, with compassionate use of Ras/MAPK pathway inhibitors for the treatment of RASopathies. This biannual meeting, organized by the RASopathies Network, brought together basic scientists, clinicians, clinician scientists, patients, advocates, and their families, as well as representatives from pharmaceutical companies and the National Institutes of Health. A history of RASopathy gene discovery, identification of new disease genes, and the latest research, both at the bench and in the clinic, were discussed.

Quantitative brain MRI morphology in severe and attenuated forms of mucopolysaccharidosis type I.

OBJECTIVE: To assess our hypothesis that brain macrostructure is different in individuals with mucopolysaccharidosis type I (MPS I) and healthy controls (HC), we conducted a comprehensive multicenter study using a uniform quantitative magnetic resonance imaging (qMRI) protocol, with analyses that account for the effects of disease phenotype, age, and cognition. METHODS: Brain MRIs in 23 individuals with attenuated (MPS IA) and 38 with severe MPS I (MPS IH), aged 4-25 years, enrolled under the study protocol NCT01870375, were compared to 98 healthy controls. RESULTS: Cortical and subcortical gray matter, white matter, corpus callosum, ventricular and choroid plexus volumes in MPS I significantly differed from HC. Thicker cortex, lower white matter and corpus callosum volumes were already present at the youngest MPS I participants aged 4-5 years. Age-related differences were observed in both MPS I groups, but most markedly in MPS IH, particularly in cortical gray matter metrics. IQ scores were inversely associated with ventricular volume in both MPS I groups and were positively associated with cortical thickness only in MPS IA. CONCLUSIONS: Quantitatively-derived MRI measures distinguished MPS I participants from HC as well as severe from attenuated forms. Age-related neurodevelopmental trajectories in both MPS I forms differed from HC. The extent to which brain structure is altered by disease, potentially spared by treatment, and how it relates to neurocognitive dysfunction needs further exploration.

Ophthalmic manifestations in Costello syndrome caused by Ras pathway dysregulation during development.

BACKGROUND: Costello syndrome (CS) is a multisystem developmental disorder caused by germline pathogenic variants in HRAS resulting in dysregulation of the Ras pathway. A systematic characterization of ophthalmic manifestations provides a unique opportunity to understand the role of Ras signal transduction in ocular development and guide optimal ophthalmic care in CS individuals. METHODS: Visual function, ocular features and genotype/phenotype correlations were evaluated in CS individuals harboring HRAS pathogenic variants, by cross-sectional and retrospective studies, and were recruited through the Costello Syndrome Family Network (CSFN) between 2007 and 2020. RESULTS: Fifty-six molecularly diagnosed CS individuals including 34 females and 22 males, ages ranging from 0.5 to 37 years were enrolled. The most common ophthalmic manifestations in the cross-sectional study were lack of stereopsis (96%), refractive errors (83%), strabismus (72%), nystagmus (69%), optic nerve hypoplasia or pallor (55%) and ptosis (13.7%) with higher prevalence than in the retrospective data (refractive errors (41%), strabismus (44%), nystagmus (26%), optic nerve hypoplasia or pallor (7%) and ptosis (11%)). Visual acuities were found to ranged from 20/25 to 20/800 and contrast sensitivity from 1.6% to 44%. HRAS pathogenic variants included p.G12S (84%), p.G13C (7%), p.G12A (5.4%), p.G12C (1.8%) and p.A146V (1.8%). CONCLUSION: Majority of individuals with CS have refractive errors, strabismus, nystagmus, absent stereopsis, and optic nerve abnormalities suggesting that HRAS and the Ras pathway play a vital role in visual system development. Ptosis, refractive errors and strabismus are amenable to treatment and early ophthalmic evaluation is crucial to prevent long-term vision impairment and improve overall quality of life in CS.

Semaphorin-Plexin Signaling: From Axonal Guidance to a New X-Linked Intellectual Disability Syndrome.

BACKGROUND: Semaphorins and plexins are ligands and cell surface receptors that regulate multiple neurodevelopmental processes such as axonal growth and guidance. PLXNA3 is a plexin gene located on the X chromosome that encodes the most widely expressed plexin receptor in fetal brain, plexin-A3. Plexin-A3 knockout mice demonstrate its role in semaphorin signaling in vivo. The clinical manifestations of semaphorin/plexin neurodevelopmental disorders have been less widely explored. This study describes the neurological and neurodevelopmental phenotypes of boys with maternally inherited hemizygous PLXNA3 variants. METHODS: Data-sharing through GeneDx and GeneMatcher allowed identification of individuals with autism or intellectual disabilities (autism/ID) and hemizygous PLXNA3 variants in collaboration with their physicians and genetic counselors, who completed questionnaires about their patients. In silico analyses predicted pathogenicity for each PLXNA3 variant. RESULTS: We assessed 14 boys (mean age, 10.7 [range 2 to 25] years) with maternally inherited hemizygous PLXNA3 variants and autism/ID ranging from mild to severe. Other findings included fine motor dyspraxia (92%), attention-deficit/hyperactivity traits, and aggressive behaviors (63%). Six patients (43%) had seizures. Thirteen boys (93%) with PLXNA3 variants showed novel or very low allele frequencies and probable damaging/disease-causing pathogenicity in one or more predictors. We found a genotype-phenotype correlation between PLXNA3 cytoplasmic domain variants (exons 22 to 32) and more severe neurodevelopmental disorder phenotypes (P < 0.05). CONCLUSIONS: We report 14 boys with maternally inherited, hemizygous PLXNA3 variants and a range of neurodevelopmental disorders suggesting a novel X-linked intellectual disability syndrome. Greater understanding of PLXNA3 variant pathogenicity in humans will require additional clinical, computational, and experimental validation.

Retinal degeneration in mice and humans with neuronal ceroid lipofuscinosis type 8.

BACKGROUND: Ceroid lipofuscinosis type 8 belongs to a heterogenous group of vision and life-threatening neurodegenerative diseases, neuronal ceroid lipofuscinosis (NCL). Effective therapy is limited to a single drug for treatment of ceroid lipofuscinosis type 2, necessitating animal disease models to facilitate further therapeutic development. Murine models are advantageous for therapeutic development due to easy genetic manipulation and rapid breeding, however appropriate genetic models need to be identified and characterized before being used for therapy testing. To date, murine models of ocular disease associated with ceroid lipofuscinosis type 8 have only been characterized in motor neuron degeneration mice. METHODS: Cln8-/- mice were produced by CRISPR/Cas9 genome editing through the International Mouse Phenotyping Consortium. Ophthalmic examination, optical coherence tomography, electroretinography, and ocular histology was performed on Cln8-/- mice and controls at 16 weeks of age. Quantification of all retinal layers, retinal pigmented epithelium, and the choriocapillaris was performed using images acquired with ocular coherence tomography and planimetry of histologic sections. Necropsy was performed to investigate concurrent systemic abnormalities. Clinical correlation with human patients with CLN8-associated retinopathy is provided. RESULTS: Retinal degeneration characterized by retinal pigment epithelium mottling, scattered drusen, and retinal vascular attenuation was noted in all Cln8-/- mice. Loss of inner and outer photoreceptor segment demarcation was noted on optical coherence tomography, with significant thinning of the whole retina (P=1e-9), outer nuclear layer (P=1e-9), and combined photoreceptor segments (P=1e-9). A global reduction in scotopic and photopic electroretinographic waveforms was noted in all Cln8-/- mice. Slight thickening of the inner plexiform layer (P=0.02) and inner nuclear layer (P=0.004), with significant thinning of the whole retina (P=0.03), outer nuclear layer (P=0.01), and outer photoreceptor segments (P=0.001) was appreciated on histologic sections. Scattered lipid vacuoles were noted in splenic red pulp of all Cln8-/- mice, though no gross systemic abnormalities were detected on necropsy. Retinal findings are consistent with those seen in patients with ceroid lipofuscinosis type 8. CONCLUSIONS: This study provides detailed clinical characterization of retinopathy in adult Cln8-/- mice. Findings suggest that Cln8-/- mice may provide a useful murine model for development of novel therapeutics needed for treating ocular disease in patients with ceroid lipofuscinosis type 8.

Genetic testing for inherited colorectal cancer and polyposis, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG).

Colorectal cancer (CRC) is the fourth most frequently diagnosed cancer and 30% of all cases of CRC are believed to have a familial component and up to one-third of these (10%) are hereditary. Pathogenic germline variants in multiple genes have been associated with predisposition to hereditary CRC or polyposis. Lynch syndrome (LS) is the most common hereditary CRC syndrome, caused by variants in the mismatch repair (MMR) genes MLH1, MSH2, MSH6, and PMS2 and is inherited in a dominant manner. Heritable conditions associated with colonic polyposis include familial adenomatous polyposis (FAP) associated with APC pathogenic variants, MUTYH-associated polyposis (MAP) caused by biallelic MUTYH pathogenic variants, and polymerase proofreading-associated polyposis (PPAP) caused by POLE or POLD1 pathogenic variants. Given the overlapping phenotypes of the cancer syndromes along with the limited sensitivity of using clinical criteria alone, a multigene panel testing approach to diagnose these conditions using next-generation sequencing (NGS) is effective and efficient. This technical standard is not recommended for use in the clinic for patient evaluation. Please refer to National Comprehensive Cancer Network (NCCN) clinical practice guidelines to determine an appropriate testing strategy and guide medical screening and management. This 2021 edition of the American College of Medical Genetics and Genomics (ACMG) technical standard supersedes the 2013 edition on this topic.

Clinical outcomes after 4.5 years of eliglustat therapy for Gaucher disease type 1: Phase 3 ENGAGE trial final results.

Eliglustat, an oral substrate reduction therapy, is approved for eligible adults with Gaucher disease type 1. In the Phase 3 ENGAGE trial of previously untreated adults with Gaucher disease type 1, eliglustat-treated patients had statistically significant improvements in organ volumes and hematologic parameters compared with placebo in the 9-month primary analysis. We report final outcomes by time on eliglustat among all patients who participated in the ENGAGE trial and extension. No patient deteriorated clinically or withdrew due to adverse events; 39/40 patients entered the open-label extension period and 34/40 (85%) remained in the trial until completion or switching to commercial eliglustat after its approval (2.3-6 years). Clinically meaningful improvements in Gaucher disease manifestations were seen in all patients concomitant with reductions in pathological lipid substrate levels (glucosylceramide and glucosylsphingosine). Among patients with 4.5 years of eliglustat exposure, mean spleen volume decreased by 66% (from 17.1 to 5.8 multiples of normal [MN], n = 13), mean liver volume decreased by 23% (from 1.5 to 1.1 MN, n = 13), mean hemoglobin increased 1.4 g/dl (from 11.9 to 13.4 g/dl, n = 12), mean platelet count increased by 87% (from 67.6 to 122.6 × 109 /L, n = 12), median chitotriosidase decreased by 82% (from 13 394 to 2312 nmol/h/ml, n = 11), median glucosylceramide decreased by 79% (from 11.5 to 2.4 µg/ml, n = 11), median glucosylsphingosine decreased by 84% (from 518.5 to 72.1 ng/ml, n = 10), and mean spine T-score increased from -1.07 (osteopenia) to -0.53 (normal) (n = 9). The magnitude of improvement in Gaucher disease manifestations and biomarkers over time was similar among the full trial cohort. Eliglustat was well-tolerated and led to clinically significant improvements in previously untreated patients with Gaucher disease type 1 during 4.5 years of treatment.

Project Baby Bear: Rapid precision care incorporating rWGS in 5 California children's hospitals demonstrates improved clinical outcomes and reduced costs of care.

Genetic disorders are a leading contributor to mortality in neonatal and pediatric intensive care units (ICUs). Rapid whole-genome sequencing (rWGS)-based rapid precision medicine (RPM) is an intervention that has demonstrated improved clinical outcomes and reduced costs of care. However, the feasibility of broad clinical deployment has not been established. The objective of this study was to implement RPM based on rWGS and evaluate the clinical and economic impact of this implementation as a first line diagnostic test in the California Medicaid (Medi-Cal) program. Project Baby Bear was a payor funded, prospective, real-world quality improvement project in the regional ICUs of five tertiary care children's hospitals. Participation was limited to acutely ill Medi-Cal beneficiaries who were admitted November 2018 to May 2020, were <1 year old and within one week of hospitalization, or had just developed an abnormal response to therapy. The whole cohort received RPM. There were two prespecified primary outcomes-changes in medical care reported by physicians and changes in the cost of care. The majority of infants were from underserved populations. Of 184 infants enrolled, 74 (40%) received a diagnosis by rWGS that explained their admission in a median time of 3 days. In 58 (32%) affected individuals, rWGS led to changes in medical care. Testing and precision medicine cost $1.7 million and led to $2.2-2.9 million cost savings. rWGS-based RPM had clinical utility and reduced net health care expenditures for infants in regional ICUs. rWGS should be considered early in ICU admission when the underlying etiology is unclear.

The evolving role of genetics in ophthalmology.

Advances in molecular genetics over the past three decades have helped identify a substantial number of genetic variants causing inherited eye diseases that can be identified rapidly by appropriate genetic tests in a clinically useful window. With this progression of knowledge, the roles of genetics and ophthalmology in patient care have become increasingly intertwined, and the necessity for subspecialists in the field of ophthalmic genetics is of paramount importance. As a result of continual medical specialization, technological progress in genetics and knowledge garnered by over a century and a half of cataloguing eye pathology, ophthalmic genetics has become an emerging subspecialty within ophthalmology. By virtue of its rapidly changing advances, genetics and genomics serves a large role within ophthalmology, and subspecialists with the same level of detailed and broad knowledge as any other ophthalmology subspecialty are now required in order to meet the growing needs of the expanding population.

Long-term efficacy and safety of migalastat treatment in Fabry disease: 30-month results from the open-label extension of the randomized, phase 3 ATTRACT study.

Results from the 18-month randomized treatment period of the phase 3 ATTRACT study demonstrated the efficacy and safety of oral migalastat compared with enzyme replacement therapy (ERT) in patients with Fabry disease who previously received ERT. Here, we report data from the subsequent 12-month, migalastat-only, open-label extension (OLE) period. ATTRACT (Study AT1001-012; NCT01218659) was a randomized, open-label, active-controlled study in patients aged 16-74 years with Fabry disease, an amenable GLA variant, and an estimated glomerular filtration rate (eGFR) ≥30 mL/min/1.73 m2. During the OLE, patients who received migalastat 150 mg every other day (QOD) during the randomized period continued receiving migalastat (Group 1 [MM]); patients who received ERT every other week discontinued ERT and started migalastat treatment (Group 2 [EM]). Outcome measures included eGFR, left ventricular mass index (LVMi), composite clinical outcome (renal, cardiac or cerebrovascular events), and safety. Forty-six patients who completed the randomized treatment period continued into the OLE (Group 1 [MM], n = 31; Group 2 [EM], n = 15). eGFR remained stable in both treatment groups. LVMi decreased from baseline at month 30 in Group 1 (MM) in patients with left ventricular hypertrophy at baseline. Only 10% of patients experienced a new composite clinical event with migalastat treatment during the OLE. No new safety concerns were reported. In conclusion, in patients with Fabry disease and amenable GLA variants, migalastat 150 mg QOD was well tolerated and demonstrated durable, long-term stability of renal function and reduction in LVMi.

Ectopia Lentis et Pupillae Caused by ADAMTSL4 Pathogenic Variants and an Algorithm for Work-up.

Ectopia lentis is displacement of the lens from its original position. It can be inherited or acquired with isolated or systemic findings. The authors describe a 4-year-old girl with isolated ectopia lentis et pupillae caused by pathogenic variants in the ADAMTSL4 gene and discuss the molecular genetic work-up of individuals with ectopia lentis. [J Pediatr Ophthalmol Strabismus. 2019;56:e45-e48.].

Costello syndrome: Clinical phenotype, genotype, and management guidelines.

Costello syndrome (CS) is a RASopathy caused by activating germline mutations in HRAS. Due to ubiquitous HRAS gene expression, CS affects multiple organ systems and individuals are predisposed to cancer. Individuals with CS may have distinctive craniofacial features, cardiac anomalies, growth and developmental delays, as well as dermatological, orthopedic, ocular, and neurological issues; however, considerable overlap with other RASopathies exists. Medical evaluation requires an understanding of the multifaceted phenotype. Subspecialists may have limited experience in caring for these individuals because of the rarity of CS. Furthermore, the phenotypic presentation may vary with the underlying genotype. These guidelines were developed by an interdisciplinary team of experts in order to encourage timely health care practices and provide medical management guidelines for the primary and specialty care provider, as well as for the families and affected individuals across their lifespan. These guidelines are based on expert opinion and do not represent evidence-based guidelines due to the lack of data for this rare condition.