Rubinstein–Taybi Syndrome: A Comprehensive Analysis of a Polish Cohort with Most Cases Due to Novel CREBBP and EP300 Variants

<b>Background:</b> Rubinstein-Taybi syndrome (RSTS) is characterized by intellectual disability, short stature, distinctive facial dysmorphism, broad thumbs/halluces, hearing loss, congenital heart or renal defects, and cryptorchidism in males. Pathogenic variants in <i>CREBBP</i> (~90% of cases) or <i>EP300</i> (~10%) underlie the disorder, with ~88% single nucleotide variants (SNVs) and ~12% copy number variants (CNVs) in <i>CREBBP</i>. <b>Materials and Methods:</b> We investigated 17 patients clinically diagnosed with RSTS at a tertiary hospital in Poland. Genetic confirmation was achieved by next-generation sequencing, multiplex ligation-dependent probe amplification (MLPA), array comparative genomic hybridization (aCGH), or Sanger sequencing. <b>Results:</b> Pathogenic variants were identified in <i>CREBBP</i> (13/17, 76%) and <i>EP300</i> (4/17, 24%). Variant types included frameshift indels (6/17, 35%), missense (4/17, 24%), nonsense (3/17, 18%), splice-site (2/17, 12%), and gross deletions (2/17, 12%). Notably, 13/17 (76%) were novel: ten in <i>CREBBP</i> (c.-49_12del, c.289C>T, c.1093_1096del, c.1094A>G, c.3178A>T, c.3401A>T, c.(3836+1_3837-1)_(4394+1_4395-1)del, c.4133+2T>G, c.4963dup, c.5028_5029dup) and three in <i>EP300</i> (c.1942C>T, c.3044_3045del, c.4713_4722del). Among the novel <i>CREBBP</i> variants, eight occurred de novo and two had unknown inheritance. Two novel <i>EP300</i> variants occurred de novo and one was of unknown origin. <b>Conclusions:</b> This first Polish RSTS cohort demonstrates a considerable proportion of gross deletions (12% overall; 15% in <i>CREBBP</i>) and an unexpectedly high rate of novel variants (76%), suggesting possible population-specific differences. These findings underscore the genetic heterogeneity of RSTS and highlight the importance of comprehensive molecular diagnostics and studies in underrepresented populations.