KBG syndrome (KBGS) is characterized by distinctive facial gestalt, short stature and variable clinical findings. With ageing, some features become more recognizable, allowing a differential diagnosis. We aimed to better characterize natural history of KBGS. In the context of a European collaborative study, we collected the largest cohort of KBGS patients (49). A combined array- based Comparative Genomic Hybridization and next generation sequencing (NGS) approach investigated both genomic Copy Number Variants and SNVs. Intellectual disability (ID) (82%) ranged from mild to moderate with severe ID identified in two patients. Epilepsy was present in 26.5%. Short stature was consistent over time, while occipitofrontal circumference (median value: −0.88 SD at birth) normalized over years. Cerebral anomalies, were identified in 56% of patients and thus represented the second most relevant clinical feature reinforcing clinical suspicion in the paediatric age when short stature and vertebral/dental anomalies are vague. Macrodontia, oligodontia and dental agenesis (53%) were almost as frequent as skeletal anomalies, such as brachydactyly, short fifth finger, fifth finger clinodactyly, pectus excavatum/carinatum, delayed bone age. In 28.5% of individuals, prenatal ultrasound anomalies were reported. Except for three splicing variants, leading to a premature termination, variants were almost all frameshift. Our results, broadening the spectrum of KBGS phenotype progression, provide useful tools to facilitate differential diagnosis and improve clinical management. We suggest to consider a wider range of dental anomalies before excluding diagnosis and to perform a careful odontoiatric/ear-nose-throat (ENT) evaluation in order to look for even submucosal palate cleft given the high percentage of palate abnormalities. NGS approaches, following evidence of antenatal ultrasound anomalies, should include ANKRD11.

Precise characterization of apparently balanced complex chromosomal rearrangements in non-affected individuals is crucial as they may result in reproductive failure, recurrent miscarriages or affected offspring. We present a family, where the non-affected father and daughter were found, using FISH and karyotyping, to be carriers of a three-way complex chromosomal rearrangement [t(6;7;10)(q16.2;q34;q26.1), de novo in the father]. The family suffered from two stillbirths, one miscarriage, and has a son with severe intellectual disability. In the present study, the family was revisited using whole-genome mate-pair sequencing. Interestingly, whole-genome mate-pair sequencing revealed a cryptic breakpoint on derivative (der) chromosome 6 rendering the rearrangement even more complex. FISH using a chromosome (chr) 6 custom-designed probe and a chr10 control probe confirmed that the interstitial chr6 segment, created by the two chr6 breakpoints, was translocated onto der(10). Breakpoints were successfully validated with Sanger sequencing, and small imbalances as well as microhomology were identified. Finally, the complex chromosomal rearrangement breakpoints disrupted the SIM1, GRIK2, CNTNAP2, and PTPRE genes without causing any phenotype development. In contrast to the majority of maternally transmitted complex chromosomal rearrangement cases, our study investigated a rare case where a complex chromosomal rearrangement, which most probably resulted from a Type IV hexavalent during the pachytene stage of meiosis I, was stably transmitted from a fertile father to his non-affected daughter. Whole-genome mate-pair sequencing proved highly successful in identifying cryptic complexity, which consequently provided further insight into the meiotic segregation of chromosomes and the increased reproductive risk in individuals carrying the specific complex chromosomal rearrangement. We propose that such complex rearrangements should be characterized in detail using a combination of conventional cytogenetic and NGS-based approaches to aid in better prenatal preimplantation genetic diagnosis and counseling in couples with reproductive problems.

Familial apparently balanced translocations (ABTs) are usually not associated with a phenotype; however, rarely, ABTs segregate with discordant phenotypes in family members carrying identical rearrangements. The current study was a follow-up investigation of four familial ABTs, where whole exome sequencing (WES) was implemented as a diagnostic tool to identify the underlying genetic aetiology of the patients’ phenotypes. Data were analysed using an in-house bioinformatics pipeline alongside VarSome Clinical. WES findings were validated with Sanger sequencing, while the impact of splicing and missense variants was assessed by reverse-transcription PCR and in silico tools, respectively. Novel candidate variants were identified in three families. In family 1, it was shown that the de novo pathogenic STXBP1 variant (NM_003165.6:c.1110+2T>G) affected splicing and segregated with the patient’s phenotype. In family 2, a likely pathogenic TUBA1A variant (NM_006009.4:c.875C>T, NP_006000.2:p.(Thr292Ile)) could explain the patient’s symptoms. In family 3, an SCN1A variant of uncertain significance (NM_006920.6:c.5060A>G, NP_008851.3:p.(Glu1687Gly)) required additional evidence to sufficiently support causality. This first report of WES application in familial ABT carriers with discordant phenotypes supported our previous findings describing such rearrangements as coincidental. Thus, WES can be recommended as a complementary test to find the monogenic cause of aberrant phenotypes in familial ABT carriers.

Background/Objectives: Congenital anomalies and neurodevelopmental disorders frequently co-occur and exhibit substantial genetic and phenotypic heterogeneity, posing a persistent diagnostic challenge. Exome sequencing has become an important first- or second-tier diagnostic tool for these conditions, yet diagnostic yields vary considerably depending on phenotype, ancestry, sequencing strategy, and interpretation, with over half of referrals remaining without a definitive genetic diagnosis. Methods: We analyzed data from 692 patients referred to the Department of Cytogenetics and Genomics at the Cyprus Institute of Neurology and Genetics between January 2021 and December 2025 for clinical exome sequencing or whole-exome sequencing as part of the diagnostic work-up for congenital disorders and/or syndromic or non-syndromic neurodevelopmental disorders. Results: A total of 134 distinct variants were identified, corresponding to an overall diagnostic yield of 17.9% out of which 52 (38.8%) were novel, and 50 variants (37.3%) were de novo, as expected from the high proportion of severe neurodevelopmental presentations. Missense variants were the most prevalent within our cohort, while chromatin and transcriptional regulator genes constituted the largest functional gene category, followed by variants in collagen-encoding genes. Conclusions: This study provides the first systematic, mutational-level characterization of a Cypriot Mendelian disease cohort, establishing a local baseline diagnostic yield and revealing a high proportion of novel variants that reflect the underrepresentation of Eastern Mediterranean populations in global databases. These findings underscore the value of submitting population-specific variants to public repositories and of phenotype-driven reanalysis targeting recurrent gene families, supporting more efficient diagnostics and future precision medicine initiatives in Cyprus.

Background/Objectives: X-linked intellectual disability (XLID) is a highly heterogeneous disorder accounting for ~10% of all males with ID. Next-generation sequencing (NGS) has revolutionized the discovery of causal XLID genes and variants; however, many cases remain unresolved. We present a four-generation syndromic XLID family with multiple males exhibiting variable degree of ID, focal dystonia and epilepsy. Methods: Extensive cytogenetic and targeted genetic testing was initially performed, followed by whole-exome sequencing (WES) and short-read whole-genome sequencing (WGS). Apart from the routine NGS analysis pipelines, sequencing data was revisited by focusing on poorly covered/mapped regions on chromosome X (chrX), to potentially reveal unidentified clinically relevant variants. Candidate variant validation and family segregation analysis were performed with Sanger sequencing. Results: All initial diagnostic testing was negative. Subsequently, 300 previously reported “dark” chrX coding DNA sequences, overlapping 97 genes, were cross-checked against 29 chrX genes highly associated (p < 0.05) with ID and focal dystonia, according to Phenomizer. Manual inspection of the existing NGS data in two low-coverage regions, chrX:25013469-25013696 and chrX:111744737-111744820 (hg38), revealed a recurrent pathogenic ARX variant NM_139058.3:c.441_464dup p.(Ala148_Ala155dup) (ARXdup24) associated with non-syndromic or syndromic XLID, including Partington syndrome. Sanger sequencing confirmed ARXdup24 in all affected males, with carrier status in their unaffected mothers, and absence in other unaffected relatives. Conclusions: After several years of diagnostic odyssey, the pathogenic ARXdup24 variant was unmasked, supporting a genotype–phenotype correlation in the first Partington syndrome family in Cyprus. This study highlights that re-examining underrepresented genomic regions and using phenotype-driven tools can provide critical diagnostic insights in unresolved XLID cases.