Genitopatellar syndrome (GPS) and Say-Barber-Biesecker-Young-Simpson Syndrome (SBBYSS) are clinically distinct neurodevelopmental disorders caused by monoallelic pathogenic variants in KAT6B. In some cases, GPS and SBBYSS features can overlap, determining an intermediate phenotype. In the present study, we describe seven patients, four with a clinical diagnosis of SBBYSS and three presenting with an intermediate phenotype. All patients carried de novo pathogenic variants in KAT6B that were identified by exome sequencing. Five variants were novel. We provide both molecular and clinical findings, highlighting the previously undescribed association with two additional features: partial penoscrotal transposition and hypopigmented macules. We performed a review of the literature, listing the clinical features of 152 patients described in 33 papers, with a molecularly confirmed diagnosis of KAT6B-related disorders, reporting the frequency of each clinical feature detected in patients diagnosed with SBBYSS and GPS. The present work provides new insights into the phenotype associated with "KAT6B-related disorders", expanding the spectrum of features that can lead to a clinical suspicion of these conditions, also guiding the molecular investigations.

Say-Barber-Biesecker-Young-Simpson (SBBYSS) variant of Ohdo syndrome is a rare, autosomal dominant and clinically heterogenous disorder, caused by pathogenic variants in the KAT6B gene located on chromosome 10q22.2. KAT6B encodes a highly conserved histone acetyltransferase belonging to the MYST family. Currently, diseases caused by pathogenic variants in KAT6B (KAT6B-related disorders) comprise two allelic entities: SBBYSS variant of Ohdo syndrome and genitopatellar syndrome (GPS). Increase in the number of cases with overlapping GPS/SBBYSS phenotype which makes it necessary to redefine this group of phenotypes as KAT6B-related disorders or KAT6B spectrum disorders. Individuals with SBBYSS usually present with facial abnormalities, hypotonia, joint laxity, feeding problems, and long thumbs/great toes. This syndrome also typically involves skeletal problems including patellar hypoplasia/agenesis. Here we report six SBBYS syndrome patients with the same dysmorphic features but a different course of the disease. One known and five novel KATB6 pathogenic variants were identified by molecular diagnostics using Next Generation Sequencing (NGS). We present a detailed phenotypic analysis of six individuals with KAT6B-related disorders, in whom a heterozygous pathogenic variant in KAT6B gene was found. In all of our patients facial dysmorphism as well as developmental and speech delay were present. Additionally, all but one patients presented with hypotonia, ocular abnormalities and long thumbs. Most of our probands showed blepharophimosis and skeletal (mainly knee) defects. Contrary to previously reported severe patellar defects (hypoplasia/agenesis) anomalies presented by our patients were less severe (dysplasia, habitual dislocation, subluxation) referring to KAT6B-related disorders. While most of the anomalies found in our patients comply with SBBYSS criteria, phenotypic differences in our probands support a broader spectrum of the disease phenotype. To establish the range of this spectrum, a detailed analysis of clinical variability among patients with SBBYSS requires further investigation.

The phenotypic variability associated with pathogenic variants in Lysine Acetyltransferase 6B (KAT6B, a.k.a. MORF, MYST4) results in several interrelated syndromes including Say-Barber-Biesecker-Young-Simpson Syndrome and Genitopatellar Syndrome. Here we present 20 new cases representing 10 novel KAT6B variants. These patients exhibit a range of clinical phenotypes including intellectual disability, mobility and language difficulties, craniofacial dysmorphology, and skeletal anomalies. Given the range of features previously described for KAT6B-related syndromes, we have identified additional phenotypes including concern for keratoconus, sensitivity to light or noise, recurring infections, and fractures in greater numbers than previously reported. We surveyed clinicians to qualitatively assess the ways families engage with genetic counselors upon diagnosis. We found that 56% (10/18) of individuals receive diagnoses before the age of 2 years (median age = 1.96 years), making it challenging to address future complications with limited accessible information and vast phenotypic severity. We used CRISPR to introduce truncating variants into the KAT6B gene in model cell lines and performed chromatin accessibility and transcriptome sequencing to identify key dysregulated pathways. This study expands the clinical spectrum and addresses the challenges to management and genetic counseling for patients with KAT6B-related disorders.

Say-Barber-Biesecker-Young-Simpson syndrome (SBBYSS) and Genitopatellar syndrome (GTPTS) are very rare conditions caused by KAT6B truncating variants. Because of both syndromes often share common features the associated phenotypes are usually grouped under the term "KAT6B-related disorders." However, particular signs of each syndrome have been reported and their appearance seems to be dependent on where the KAT6B variant is located. Thus, whereas truncating variants associated with SBBYSS have their highest density in the distal part of exon 18, those resulting in GTPTS are distributed between the end of exon 17 and beginning of exon 18. Here, we reported two de novo heterozygous KAT6B truncating variants. The first variant (c.5802delA; p.A1935Pfs*16), identified in a boy with SSBYSS phenotype, resulting in the most distal KAT6B truncating variant reported up-to-date in the scientific literature. The second variant (c.3152delG; p.S1051Tfs*63), located in a region hitherto defined as specific of SBBYSS, seems to cause an overlapping SBBYSS/GTPTS phenotype. The clinical and genetic characterization of these patients could contribute to the understanding of the KAT6B-related disorders.

KAT6B sequence variants have been identified in both patients with the Say-Barber-Biesecker-Young-Simpson syndrome (SBBYSS) and in the genitopatellar syndrome (GPS). In SBBYSS, they were reported to affect mostly exons 16-18 of KAT6B, and the predicted mechanism of pathogenesis was haploinsufficiency or a partial loss of protein function. Truncating variants in KAT6B leading to GPS appear to cluster within the proximal portion of exon 18, associated with a dominant-negative effect of the mutated protein, most likely. Although SBBYSS and GPS have been initially considered allelic disorders with distinctive genetic and clinical features, there is evidence that they represent two ends of a spectrum of conditions referable as KAT6B-related disorders. We detected a de novo truncating variant within exon 7 of KAT6B in a 8-year-old female who presented with mild intellectual disability, facial dysmorphisms highly consistent with SBBYSS, and skeletal anomalies including exostosis, that are usually considered component manifestations of GPS. Following the clinical diagnosis driven by the striking facial phenotype, we analyzed the KAT6B gene by NGS techniques. The present report highlights the pivotal role of clinical genetics in avoiding clear-cut genotype-phenotype categories in syndromic forms of intellectual disability. In addition, it further supports the evidence that a continuum exists within the clinical spectrum of KAT6B-associated disorders.