Diagnosis and management in Rubinstein-Taybi syndrome: first international consensus statement.

Rubinstein-Taybi syndrome (RTS) is an archetypical genetic syndrome that is characterised by intellectual disability, well-defined facial features, distal limb anomalies and atypical growth, among numerous other signs and symptoms. It is caused by variants in either of two genes (CREBBP, EP300) which encode for the proteins CBP and p300, which both have a function in transcription regulation and histone acetylation. As a group of international experts and national support groups dedicated to the syndrome, we realised that marked heterogeneity currently exists in clinical and molecular diagnostic approaches and care practices in various parts of the world. Here, we outline a series of recommendations that document the consensus of a group of international experts on clinical diagnostic criteria for types of RTS (RTS1: CREBBP; RTS2: EP300), molecular investigations, long-term management of various particular physical and behavioural issues and care planning. The recommendations as presented here will need to be evaluated for improvements to allow for continued optimisation of diagnostics and care.

From clinical findings to the pathomechanism of poikiloderma with neutropenia.

The clinical problem of a non-healing fistula in ano in a child affected with poikiloderma with neutropenia (PN) was the stimulus for an innovative study by Parajuli et al. that sheds light on the pathological mechanisms in this disease. Multiparametric analyses of the patient's blood mononuclear cells by cell culture, flow cytometry and multiplex cytokine assay suggested a block of monocyte differentiation. Monocyte transcriptome profiling revealed a signature consistent with the haematological picture and the clinical presentation. Commentary on: Parajuli et al. Defective monocyte plasticity and altered cAMP pathway characterize USB1-mutated poikiloderma with neutropenia Clericuzio type. Br J Haematol 2024;204:683-693.

Low-grade parental gonosomal mosaicism in CHD2 siblings with Smith-Magenis-like syndrome.

Loss-of-function CHD2 (chromodomain helicase DNA-binding protein 2) mutations are associated with a spectrum of neurodevelopmental disorders often including early-onset generalized seizures, photosensitivity, and epileptic encephalopathies. Patients show psychomotor delay/intellectual disability (ID), autistic features, and behavior disorders, such as aggression and impulsivity. Most reported cases are sporadic with description of germline mosaicism only in two families. We detect the first case of parental gonosomal CHD2 mosaicism disclosed by two brothers showing mild ID, born to healthy parents. The eldest brother has a history of drug-controlled generalized tonic-clonic seizures and displays sleep disorder and aggressive behavior suggestive of Smith-Magenis syndrome (SMS). Analysis of brothers' DNAs by next-generation sequencing (NGS) custom gene panel for pediatric epilepsy and/or ID disclosed in both the same pathogenic CHD2 variant. Additional NGS experiment on genomic DNA from parents' peripheral blood and from buccal swab raised the suspicion of low-grade gonosomal mosaicism in the unaffected mother subsequently confirmed by digital polymerase chain reaction (dPCR). This report underlines as worthwhile CHD2 screening in individuals presenting ID/developmental delay, with/without epilepsy, and behavior and sleep disorders suggestive of SMS. Detecting a CHD2 variant should prime testing probands' parents by NGS coupled to dPCR on different tissues to exclude/confirm gonosomal mosaicism and define the recurrence risk.

Rare Diseases: Implementation of Molecular Diagnosis, Pathogenesis Insights and Precision Medicine Treatment.

Rare Diseases (RD) do not have an exact definition since local authorities define the criteria in different ways, from fewer than 5 people in 10,000, according to the European Union, to the standard world average of 40 cases per 100,000 people [...].

Germline NUP98 Variants in Two Siblings with a Rothmund-Thomson-Like Spectrum: Protein Functional Changes Predicted by Molecular Modeling.

Two adult siblings born to first-cousin parents presented a clinical phenotype reminiscent of Rothmund-Thomson syndrome (RTS), implying fragile hair, absent eyelashes/eyebrows, bilateral cataracts, mottled pigmentation, dental decay, hypogonadism, and osteoporosis. As the clinical suspicion was not supported by the sequencing of RECQL4, the RTS2-causative gene, whole exome sequencing was applied and disclosed the homozygous variants c.83G>A (p.Gly28Asp) and c.2624A>C (p.Glu875Ala) in the nucleoporin 98 (NUP98) gene. Though both variants affect highly conserved amino acids, the c.83G>A looked more intriguing due to its higher pathogenicity score and location of the replaced amino acid between phenylalanine-glycine (FG) repeats within the first NUP98 intrinsically disordered region. Molecular modeling studies of the mutated NUP98 FG domain evidenced a dispersion of the intramolecular cohesion elements and a more elongated conformational state compared to the wild type. This different dynamic behavior may affect the NUP98 functions as the minor plasticity of the mutated FG domain undermines its role as a multi-docking station for RNA and proteins, and the impaired folding can lead to the weakening or the loss of specific interactions. The clinical overlap of NUP98-mutated and RTS2/RTS1 patients, accounted by converging dysregulated gene networks, supports this first-described constitutional NUP98 disorder, expanding the well-known role of NUP98 in cancer.

Modeling RTT Syndrome by iPSC-Derived Neurons from Male and Female Patients with Heterogeneously Severe Hot-Spot MECP2 Variants.

Rett syndrome caused by MECP2 variants is characterized by a heterogenous clinical spectrum accounted for in 60% of cases by hot-spot variants. Focusing on the most frequent variants, we generated in vitro iPSC-neurons from the blood of RTT girls with p.Arg133Cys and p.Arg255*, associated to mild and severe phenotype, respectively, and of an RTT male harboring the close to p.Arg255*, p.Gly252Argfs*7 variant. Truncated MeCP2 proteins were revealed by Western blot and immunofluorescence analysis. We compared the mutant versus control neurons at 42 days for morphological parameters and at 120 days for electrophysiology recordings, including girls' isogenic clones. A precocious reduced morphological complexity was evident in neurons with truncating variants, while in p.Arg133Cys neurons any significant differences were observed in comparison with the isogenic wild-type clones. Reduced nuclear size and branch number show up as the most robust biomarkers. Patch clamp recordings on mature neurons allowed the assessment of cell biophysical properties, V-gated currents, and spiking pattern in the mutant and control cells. Immature spiking, altered cell capacitance, and membrane resistance of RTT neurons, were particularly pronounced in the Arg255* and Gly252Argfs*7 mutants. The overall results indicate that the specific markers of in vitro cellular phenotype mirror the clinical severity and may be amenable to drug testing for translational purposes.

Expanding the Molecular Spectrum of ANKRD11 Gene Defects in 33 Patients with a Clinical Presentation of KBG Syndrome.

KBG syndrome (KBGS) is a neurodevelopmental disorder caused by the Ankyrin Repeat Domain 11 (ANKRD11) haploinsufficiency. Here, we report the molecular investigations performed on a cohort of 33 individuals with KBGS clinical suspicion. By using a multi-testing genomic approach, including gene sequencing, Chromosome Microarray Analysis (CMA), and RT-qPCR gene expression assay, we searched for pathogenic alterations in ANKRD11. A molecular diagnosis was obtained in 22 out of 33 patients (67%). ANKRD11 sequencing disclosed pathogenic or likely pathogenic variants in 18 out of 33 patients. CMA identified one full and one terminal ANKRD11 pathogenic deletions, and one partial duplication and one intronic microdeletion, with both possibly being pathogenic. The pathogenic effect was established by RT-qPCR, which confirmed ANKRD11 haploinsufficiency only for the three deletions. Moreover, RT-qPCR applied to six molecularly unsolved KBGS patients identified gene downregulation in a clinically typical patient with previous negative tests, and further molecular investigations revealed a cryptic deletion involving the gene promoter. In conclusion, ANKRD11 pathogenic variants could also involve the regulatory regions of the gene. Moreover, the application of a multi-test approach along with the innovative use of RT-qPCR improved the diagnostic yield in KBGS suspected patients.

Histone Deacetylase Inhibitors Ameliorate Morphological Defects and Hypoexcitability of iPSC-Neurons from Rubinstein-Taybi Patients.

Rubinstein-Taybi syndrome (RSTS) is a rare neurodevelopmental disorder caused by mutations in CREBBP or EP300 genes encoding CBP/p300 lysine acetyltransferases. We investigated the efficacy of the histone deacetylase inhibitor (HDACi) Trichostatin A (TSA) in ameliorating morphological abnormalities of iPSC-derived young neurons from P149 and P34 CREBBP-mutated patients and hypoexcitability of mature neurons from P149. Neural progenitors from both patients' iPSC lines were cultured one week with TSA 20 nM and, only P149, for 6 weeks with TSA 0.2 nM, in parallel to neural progenitors from controls. Immunofluorescence of MAP2/TUJ1 positive cells using the Skeletonize Image J plugin evidenced that TSA partially rescued reduced nuclear area, and decreased branch length and abnormal end points number of both 45 days patients' neurons, but did not influence the diminished percentage of their neurons with respect to controls. Patch clamp recordings of TSA-treated post-mitotic P149 neurons showed complete/partial rescue of sodium/potassium currents and significant enhancement of neuron excitability compared to untreated replicas. Correction of abnormalities of P149 young neurons was also affected by valproic acid 1 mM for 72 h, with some variation, with respect to TSA, on the morphological parameter. These findings hold promise for development of an epigenetic therapy to attenuate RSTS patients cognitive impairment.

Spontaneous chromosomal instability in peripheral blood lymphocytes from two molecularly confirmed Italian patients with Hereditary Fibrosis Poikiloderma: insights into cancer predisposition.

Two Italian patients with the initial clinical diagnosis of Rothmund-Thomson syndrome were negative for RECQL4 mutations but showed in peripheral blood cells a spontaneous chromosomal instability significantly higher than controls. Revisiting after time their clinical phenotype, the suggestive matching with the autosomal dominant syndrome Poikiloderma, Hereditary Fibrosing with Tendon Contracture, Myopathy and Pulmonary fibrosis (POIKTMP) was confirmed by identification of the c.1879A>G (p.Arg627Gly) alteration in FAM111B. We compare the overall clinical signs of our patients with those of reported carriers of the same mutation and present the up-to-date mutational repertoire of FAM111B and the related phenotypic spectrum. Our snapshot highlights the age-dependent clinical expressivity of POIKTMP and the need to follow-up patients to monitor the multi-tissue impairment caused by FAM111B alterations. We link our chromosomal instability data to the role of FAM111B in cancer predisposition, pointed out by its implication in DNA-repair pathways and the outcome of pancreatic cancer in 2 out of 17 adult POIKTMP patients. The chromosomal instability herein highlighted well connects POIKTMP to cancer-predisposing syndromes, such as Rothmund-Thomson which represents the first hereditary poikiloderma entering in differential diagnosis with POIKTMP.

Mutations in CREBBP and EP300 genes affect DNA repair of oxidative damage in Rubinstein-Taybi syndrome cells.

Rubinstein-Taybi syndrome (RSTS) is an autosomal-dominant disorder characterized by intellectual disability, skeletal abnormalities, growth deficiency and an increased risk of tumors. RSTS is predominantly caused by mutations in CREBBP or EP300 genes encoding for CBP and p300 proteins, two lysine acetyl-transferases (KAT) playing a key role in transcription, cell proliferation and DNA repair. However, the efficiency of these processes in RSTS cells is still largely unknown. Here, we have investigated whether pathways involved in the maintenance of genome stability are affected in lymphoblastoid cell lines (LCLs) obtained from RSTS patients with mutations in CREBBP or in EP300 genes. We report that RSTS LCLs with mutations affecting CBP or p300 protein levels or KAT activity, are more sensitive to oxidative DNA damage and exhibit defective base excision repair (BER). We have found reduced OGG1 DNA glycosylase activity in RSTS compared to control cell extracts, and concomitant lower OGG1 acetylation levels, thereby impairing the initiation of the BER process. In addition, we report reduced acetylation of other BER factors, such as DNA polymerase ß and Proliferating Cell Nuclear Antigen (PCNA), together with acetylation of histone H3. We also show that complementation of CBP or p300 partially reversed RSTS cell sensitivity to DNA damage. These results disclose a mechanism of defective DNA repair as a source of genome instability in RSTS cells.

Recombinant Chromosome 7 Driven by Maternal Chromosome 7 Pericentric Inversion in a Girl with Features of Silver-Russell Syndrome.

Maternal uniparental disomy of chromosome 7 is present in 5-10% of patients with Silver-Russell syndrome (SRS), and duplication of 7p including GRB10 (Growth Factor Receptor-Bound Protein 10), an imprinted gene that affects pre-and postnatal growth retardation, has been associated with the SRS phenotype. Here, we report on a 17 year old girl referred to array-CGH analysis for short stature, psychomotor delay, and relative macrocephaly. Array-CGH analysis showed two copy number variants (CNVs): a ~12.7 Mb gain in 7p13-p11.2, involving GRB10 and an ~9 Mb loss in 7q11.21-q11.23. FISH experiments performed on the proband's mother showed a chromosome 7 pericentric inversion that might have mediated the complex rearrangement harbored by the daughter. Indeed, we found that segmental duplications, of which chromosome 7 is highly enriched, mapped at the breakpoints of both the mother's inversion and the daughter's CNVs. We postulate that pairing of highly homologous sequences might have perturbed the correct meiotic chromosome segregation, leading to unbalanced outcomes and acting as the putative meiotic mechanism that was causative of the proband's rearrangement. Comparison of the girl's phenotype to those of patients with similar CNVs supports the presence of 7p in a locus associated with features of SRS syndrome.

9q34.3 microduplications lead to neurodevelopmental disorders through EHMT1 overexpression.

Both copy number losses and gains occur within subtelomeric 9q34 region without common breakpoints. The microdeletions cause Kleefstra syndrome (KS), whose responsible gene is EHMT1. A 9q34 duplication syndrome (9q34 DS) had been reported in literature, but it has never been characterized by a detailed molecular analysis of the gene content and endpoints. To the best of our knowledge, we report on the first patient carrying the smallest 9q34.3 duplication containing EHMT1 as the only relevant gene. We compared him with 21 reported patients described here as carrying 9q34.3 duplications encompassing the entire gene and extending within ~ 3 Mb. By surveying the available clinical and molecular cytogenetic data, we were able to discover that similar neurodevelopmental disorders (NDDs) were shared by patient carriers of even very differently sized duplications. Moreover, some facial features of the 9q34 DS were more represented than those of KS. However, an accurate in silico analysis of the genes mapped in all the duplications allowed us to support EHMT1 as being sufficient to cause a NDD phenotype. Wider patient cohorts are needed to ascertain whether the rearrangements have full causative role or simply confer the susceptibility to NDDs and possibly to identify the cognitive and behavioral profile associated with the increased dosage of EHMT1.

Exploring by whole exome sequencing patients with initial diagnosis of Rubinstein-Taybi syndrome: the interconnections of epigenetic machinery disorders.

Rubinstein-Taybi syndrome (RSTS) is an autosomal-dominant neurodevelopmental disease affecting 1:125,000 newborns characterized by intellectual disability, growth retardation, facial dysmorphisms and skeletal abnormalities. RSTS is caused by mutations in genes encoding for writers of the epigenetic machinery: CREBBP (~ 60%) or its homologous EP300 (~ 10%). No causative mutation is identified in up to 30% of patients. We performed whole-exome sequencing (WES) on eight RSTS-like individuals who had normal high-resolution array CGH testing and were CREBBP- and EP300-mutation -negative, to identify the molecular cause. In four cases, we identified putatively causal variants in three genes (ASXL1, KMT2D and KMT2A) encoding members of the epigenetic machinery known to be associated with the Bohring-Opitz, Kabuki and Wiedemann-Steiner syndromes. Each variant is novel, de novo, fulfills the ACMG criteria and is predicted to result in loss-of-function leading to haploinsufficiency of the epi-gene. In two of the remaining cases, homozygous/compound heterozygous variants in XYLT2 and PLCB4 genes, respectively, associated with spondyloocular and auriculocondylar 2 syndromes and in the latter an additional candidate variant in XRN2, a gene yet unrelated to any disease, were detected, but their pathogenicity remains uncertain. These results underscore the broad clinical spectrum of Mendelian disorders of the epigenetic apparatus and the high rate of WES disclosure of the genetic basis in cases which may pose a challenge for phenotype encompassing distinct syndromes. The overlapping features of distinct intellectual disability syndromes reflect common pathogenic molecular mechanisms affecting the complex regulation of balance between open and closed chromatin.

Pathogenic Variants in STXBP1 and in Genes for GABAa Receptor Subunities Cause Atypical Rett/Rett-like Phenotypes.

Rett syndrome (RTT) is a neurodevelopmental disorder, affecting 1 in 10,000 girls. Intellectual disability, loss of speech and hand skills with stereotypies, seizures and ataxia are recurrent features. Stringent diagnostic criteria distinguish classical Rett, caused by a MECP2 pathogenic variant in 95% of cases, from atypical girls, 40-73% carrying MECP2 variants, and rarely CDKL5 and FOXG1 alterations. A large fraction of atypical and RTT-like patients remain without genetic cause. Next Generation Sequencing (NGS) targeted to multigene panels/Whole Exome Sequencing (WES) in 137 girls suspected for RTT led to the identification of a de novo variant in STXBP1 gene in four atypical RTT and two RTT-like girls. De novo pathogenic variants-one in GABRB2 and, for first time, one in GABRG2-were disclosed in classic and atypical RTT patients. Interestingly, the GABRG2 variant occurred at low rate percentage in blood and buccal swabs, reinforcing the relevance of mosaicism in neurological disorders. We confirm the role of STXBP1 in atypical RTT/RTT-like patients if early psychomotor delay and epilepsy before 2 years of age are observed, indicating its inclusion in the RTT diagnostic panel. Lastly, we report pathogenic variants in Gamma-aminobutyric acid-A (GABAa) receptors as a cause of atypical/classic RTT phenotype, in accordance with the deregulation of GABAergic pathway observed in MECP2 defective in vitro and in vivo models.

Insights into Mutation Effect in Three Poikiloderma with Neutropenia Patients by Transcript Analysis and Disease Evolution of Reported Patients with the Same Pathogenic Variants.

PURPOSE: Poikiloderma with neutropenia (PN) is a genodermatosis currently described in 77 patients, all presenting with early-onset poikiloderma, neutropenia, and several additional signs. Biallelic loss-of-function mutations in USB1 gene are detected in all molecularly tested patients but genotype-phenotype correlation remains elusive. Cancer predisposition is recognized among PN features and pathogenic variants found in patients who developed early in life myelodysplasia (n = 12), acute myeloid leukemia (n = 2), and squamous cell carcinoma (n = 2) should be kept into account in management and follow-up of novel patients. This will hopefully allow achieving data clustered on specific mutations relevant to oncological surveillance of the carrier patients. METHODS: We describe the clinical features of three unreported PN patients and characterize their USB1 pathogenic variants by transcript analysis to get insights into the effect on the overall phenotype and disease evolution. RESULTS: A Turkish boy is homozygous for the c.531delA deletion, a recurrent mutation in Turkey; an adult Italian male is compound heterozygous for two nonsense mutations, c.243G>A and c.541C>T, while an Italian boy is homozygous for the splicing c.683_693+1del variant. The identified mutations have already been reported in PN patients who developed hematologic or skin cancer. Aberrant mRNAs of all four mutated alleles could be identified confirming that transcripts of USB1 main isoform either carrying stop codons or mis-spliced may at least partially escape nonsense-mediated decay. CONCLUSIONS: Our study addresses the need of gathering insights on genotype-phenotype correlations in newly described PN patients, by transcript analysis and information on disease evolution of reported patients with the same pathogenic variants.

Rings and Bricks: Expression of Cohesin Components is Dynamic during Development and Adult Life.

Cohesin complex components exert fundamental roles in animal cells, both canonical in cell cycle and non-canonical in gene expression regulation. Germline mutations in genes coding for cohesins result in developmental disorders named cohesinopaties, of which Cornelia de Lange syndrome (CdLS) is the best-known entity. However, a basic description of mammalian expression pattern of cohesins in a physiologic condition is still needed. Hence, we report a detailed analysis of expression in murine and human tissues of cohesin genes defective in CdLS. Using both quantitative and qualitative methods in fetal and adult tissues, cohesin genes were found to be ubiquitously and differentially expressed in human tissues. In particular, abundant expression was observed in hematopoietic and central nervous system organs. Findings of the present study indicate tissues which should be particularly sensitive to mutations, germline and/or somatic, in cohesin genes. Hence, this expression analysis in physiological conditions may represent a first core reference for cohesinopathies.

Rothmund-Thomson Syndrome: Insights from New Patients on the Genetic Variability Underpinning Clinical Presentation and Cancer Outcome.

Biallelic mutations in RECQL4 gene, a caretaker of the genome, cause Rothmund-Thomson type-II syndrome (RTS-II) and confer increased cancer risk if they damage the helicase domain. We describe five families exemplifying clinical and allelic heterogeneity of RTS-II, and report the effect of pathogenic RECQL4 variants by in silico predictions and transcripts analyses. Complete phenotype of patients #39 and #42 whose affected siblings developed osteosarcoma correlates with their c.[1048_1049del], c.[1878+32_1878+55del] and c.[1568G>C;1573delT], c.[3021_3022del] variants which damage the helicase domain. Literature survey highlights enrichment of these variants affecting the helicase domain in patients with cancer outcome raising the issue of strict oncological surveillance. Conversely, patients #29 and #19 have a mild phenotype and carry, respectively, the unreported homozygous c.3265G>T and c.3054A>G variants, both sparing the helicase domain. Finally, despite matching several criteria for RTS clinical diagnosis, patient #38 is heterozygous for c.2412_2414del; no pathogenic CNVs out of those evidenced by high-resolution CGH-array, emerged as contributors to her phenotype.

Mutations in chromatin regulators functionally link Cornelia de Lange syndrome and clinically overlapping phenotypes.

The coordinated tissue-specific regulation of gene expression is essential for the proper development of all organisms. Mutations in multiple transcriptional regulators cause a group of neurodevelopmental disorders termed "transcriptomopathies" that share core phenotypical features including growth retardation, developmental delay, intellectual disability and facial dysmorphism. Cornelia de Lange syndrome (CdLS) belongs to this class of disorders and is caused by mutations in different subunits or regulators of the cohesin complex. Herein, we report on the clinical and molecular characterization of seven patients with features overlapping with CdLS who were found to carry mutations in chromatin regulators previously associated to other neurodevelopmental disorders that are frequently considered in the differential diagnosis of CdLS. The identified mutations affect the methyltransferase-encoding genes KMT2A and SETD5 and different subunits of the SWI/SNF chromatin-remodeling complex. Complementary to this, a patient with Coffin-Siris syndrome was found to carry a missense substitution in NIPBL. Our findings indicate that mutations in a variety of chromatin-associated factors result in overlapping clinical phenotypes, underscoring the genetic heterogeneity that should be considered when assessing the clinical and molecular diagnosis of neurodevelopmental syndromes. It is clear that emerging molecular mechanisms of chromatin dysregulation are central to understanding the pathogenesis of these clinically overlapping genetic disorders.

Nomenclature and definition in asymmetric regional body overgrowth.

We designate a novel term "isolated lateralized overgrowth" (ILO) for the findings previously described as "isolated hemihypertrophy" and "isolated hemihyperplasia." ILO is defined as lateralized overgrowth in the absence of a recognized pattern of malformations, dysplasia, or morphologic variants. ILO is likely genetically heterogeneous. Further study is required to determine more of the underlying genetic etiologies and potential associations with currently unrecognized patterns of malformation.

From Whole Gene Deletion to Point Mutations of EP300-Positive Rubinstein-Taybi Patients: New Insights into the Mutational Spectrum and Peculiar Clinical Hallmarks.

Rubinstein-Taybi syndrome (RSTS) is a rare congenital neurodevelopmental disorder characterized by growth deficiency, skeletal abnormalities, dysmorphic features, and intellectual disability. Causative mutations in CREBBP and EP300 genes have been identified in ∼55% and ∼8% of affected individuals. To date, only 28 EP300 alterations in 29 RSTS clinically described patients have been reported. EP300 analysis of 22 CREBBP-negative RSTS patients from our cohort led us to identify six novel mutations: a 376-kb deletion depleting EP300 gene; an exons 17-19 deletion (c.(3141+1_3142-1)_(3590+1_3591-1)del/p.(Ile1047Serfs*30)); two stop mutations, (c.3829A>T/p.(Lys1277*) and c.4585C>T/p.(Arg1529*)); a splicing mutation (c.1878-12A>G/p.(Ala627Glnfs*11)), and a duplication (c.4640dupA/p.(Asn1547Lysfs*3)). All EP300-mutated individuals show a mild RSTS phenotype and peculiar findings including maternal gestosis, skin manifestation, especially nevi or keloids, back malformations, and a behavior predisposing to anxiety. Furthermore, the patient carrying the complete EP300 deletion does not show a markedly severe clinical picture, even if a more composite phenotype was noticed. By characterizing six novel EP300-mutated patients, this study provides further insights into the EP300-specific clinical presentation and expands the mutational repertoire including the first case of a whole gene deletion. These new data will enhance EP300-mutated cases identification highlighting distinctive features and will improve the clinical practice allowing a better genotype-phenotype correlation.