46,XY differences of sex development (DSD) are conditions with extreme phenotypic and genetic heterogeneity. Therefore, their diagnosis remains a major challenge for both clinicians and geneticists. In this study, we aimed to identify the underlying genetic causes of DSD in a series of 3 Moroccan patients with syndromic 46,XY DSD recruited in the BRO Biobank. Methods: Karyotyping analysis was performed on peripheral blood samples using standard R banding techniques. SRY gene was analyzed using PCR amplification followed by Sanger sequencing. Whole exome sequencing (WES) was performed after unsuccessful conventional genetic analyses. Candidate variants were evaluated by segregation analysis and molecular modeling. WES identified three pathogenic variants in genes encoding various components of the epigenetic machinery: in patient 1, a novel heterozygous frameshift variant c.4072dup (p.Glu1358GlyfsTer29) in the KAT6B gene associated with two clinically distinct syndromes (genitopatellar syndrome and Say-Barber-Biesecker-Young-Simpson syndrome) was detected; in patient 2, we identified a previously reported de novo heterozygous nonsense variant c.12943C>T (p.Gln4315Ter) in KMT2D responsible for Kabuki syndrome; in patient 3, WES revealed a novel heterozygous missense variant c.4056C>G (p.Phe1352Leu) in CHD7 responsible for CHARGE syndrome. We discuss the genotype-phenotype correlation in these syndromic 46,XY DSD and discuss the relevance of the epigenetic genes in sexual development. Our findings highlight the utility of WES in discriminating clinically overlapping syndromic 46,XY DSD to provide an accurate diagnosis, thus allowing better follow-up and appropriate patient management. In addition, our study enriched the mutational spectrum of syndromic 46,XY DSD and confirmed the genotype-phenotype correlations.

KAT6B mutations are responsible for Say-Barber-Biesecker-Young-Simpson syndrome or Genitopatellar syndrome, with most mutations occurring in its exon 18. A pregnancy with normal early antenatal examination revealed the presence of hypospadias in the fetus but with no abnormal amniotic fluid volume in ultrasonography at 29th+ 4d weeks' gestation. After amniocentesis, the trios' whole exome sequencing was performed and a novel frameshift mutation (KAT6B: exon10: c.2153_2159del, p. R718Lfs*3) was identified for her fetus, then verified by the parents as a de novo mutation. Following this couple's decision to induce labor, the appearance of the fetus had hypospadias but with a normal face and was able to be palpated for the patella. KAT6B mutations often occur with a variety of symptoms. To our acknowledgment, this is the first report of a novel de novo KAT6B mutation causing only hypospadias for the fetus, which further expands the spectrum of KAT6B variants and the genotype-phenotype relationship for this disease.

Loss of the gene encoding the histone acetyltransferase KAT6B (MYST4/MORF/QKF) causes developmental brain abnormalities as well as behavioral and cognitive defects in mice. In humans, heterozygous variants in the KAT6B gene cause two cognitive disorders, Say-Barber-Biesecker-Young-Simpson syndrome (SBBYSS; OMIM:603736) and genitopatellar syndrome (GTPTS; OMIM:606170). Although the effects of KAT6B homozygous and heterozygous mutations have been documented in humans and mice, KAT6B gain-of-function effects have not been reported. Here, we show that overexpression of the Kat6b gene in mice caused aggression, anxiety, and spontaneous epilepsy. Kat6b overexpression led to an increase in histone H3 lysine 9 acetylation and upregulation of genes driving nervous system development and neuronal differentiation. Kat6b overexpression additionally promoted neural stem cell proliferation and favored neuronal over astrocyte differentiation in vivo and in vitro. Our results suggest that, in addition to loss-of-function alleles, gain-of-function KAT6B alleles may be detrimental for brain development.

Ohdo syndrome Say-Barver-Biesecker-Young-Simpson (SBBYS) variant is a rare autosomal dominant disorder characterized mainly by intellectual disability, developmental delays, contractures of the knees and hips contractures, thyroid dysfunction, and dysmorphic appearance. The Ohdo syndrome SBBYS variant is caused by heterozygous loss of function mutation in the KAT6B gene. The peripheral blood mononuclear cells from a patient carrying heterozygous frameshift mutation of the KAT6B gene were reprogrammed using the CytoTune-iPS2.0 Sendai Reprogramming Kit. The mutation in the KAT6B gene causes the abnormal protein variant. The established human induced pluripotent cell line allow proper in vitro disease modelling of Ohdo syndrome SBBYS variant.

Pathogenic variants in KAT6B (Lysine acetyltransferase 6B) are associated with two clinically overlapping autosomal dominant disorders Say-Barber-Biesecker-Young-Simpson syndrome (SBBYSS) (OMIM 603736), and Genitopatellar syndrome (GPS) (OMIM 606170). More recently, the clinical spectrum of KAT6B disorders has expanded and KAT6B disorders have been suggested to consist of a spectrum of disorders with intermediate and overlapping clinical manifestations. Pathogenic variants in KAT6B mainly occur de novo, with only 3 reports of inherited variants. Here, we describe clinical and molecular findings in a three-generation Danish family with a segregating, previously unreported, pathogenic KAT6B variant. The variant is associated with a phenotype not otherwise specified (neither SBBYSS nor GPS) and with variable expressivity, adding further evidence that KAT6B disorders should be seen as a broad clinical spectrum. Furthermore, we highlight the existence of intra-familial variability and that pathogenic variants in KAT6B can be inherited from mildly affected parents.