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Introduction: In approximately 6% of balanced chromosomal rearrangements carriers, intellectual disability, dysmorphic features and congenital anomalies can be found. The abnormal phenotype might be the result of genomic imbalance or aberrant expression caused by direct breakage of a dosage sensitive gene., The Aim of This Study: To estimate the frequency and implication of the submicroscopic chromosomal aberrations on the abnormal phenotypes present in patients with balanced chromosomal rearrangements. Also an attempt was made to define the type of genetic defect and gene identification responsible for the intellectual disability and additional clinical features., Material and Methods: 22 patients with intellectual disability, congenital anomalies and dysmorphic features were analysed. Molecular karyotyping was performed in all patients using FISH with region-specific BAC clones, high resolution comparative genomic hybridization (HR-CGH) or array CGH (aCGH). A targeted or whole genome microarrays were applied., Results: In 5 of 22 carriers 6 microdeletions and one duplication were found (7/22, 31.8%). Only two microdeletions were mapped at the chromosomal breakpoints. Three rearrangements had more complex structure than conventional methods demonstrated. In the chromosomal breakpoints of 21 patients the 24 genes, which functions suggest the relationship between abnormal gene expression and patients' intellectual disability, were mapped., Conclusions: We showed that in a considerable group of patients with balanced chromosomal rearrangements and abnormal phenotype the cryptic aberrations, unidentified by conventional methods, are present. These results confirmed the legitimacy of detailed analysis of the chromosomal breakpoints as well as the whole genome screening with the use of new cytogenetic methods.

Introduction: In about 6% of individuals with intellectual disability, dysmorphic features and congenital anomalies, an abnormal, apparently balanced karyotype is found. These abnormalities may result from abnormal expression of genes at the breakpoints, presence of a submicroscopic deletion, or other unbalanced chromosome aberrations. In such cases, the detailed analysis of breakpoints of balanced chromosome rearrangements may help with identification of genes responsible for patient's clinical features., Aim of Work: Was the explanation of causes of abnormal phenotype in the carriers with abnormal but balanced karyotype., Material and Methods: Cytogenetic-molecular analysis performed in nine patients with mental retardation, dysmorphic features and congenital anomalies. Studies with subtelomeric probes, high resolution comparative genomic hybridization (HR-CGH) and fluorescence in situ hybridization (FISH) with region-specific BAC clones were performed., Results: Seventeen chromosome breakpoint regions were narrowed to 200-400 kb. In one case, an 0.5-Mb submicroscopic deletion associated with more complex rearrangement has been found. Mapping of the breakpoints and information obtained from the UCSC Human Genome Browser data base enabled identification of 46 genes in these regions. Twelve genes, that may have been disrupted as a result of the patients' chromosomal rearrangement, were found. At four different breakpoints the identified genes (NRCAM, NPTX1, NMT1, MAPT, HDAC5 and MEF2C) may be due to a position effect., Conclusions: The results confirm earlier suggestions concerning reasons of abnormal phenotype in the patients with balanced chromosome rearrangements and present the value of detailed analysis of the genome in such cases.

Introduction: Until recently, great variety of marker chromosomes and difficulties with their identification have presented a problem for cytogenetic and clinical interpretation of the karyotype. At present, molecular cytogenetic methods of chromosome analysis enable precise characterization of such abnormalities providing knowledge necessary for estimation of their genetic risk., Aim: The aim of the study was molecular cytogenetic characterization of marker chromosomes recognized in three patients, an analysis of clinical features in relation to the abnormality and estimation of genetic risk of identified markers., Patients and Methods: Karyotypes of three phenotypically abnormal patients were estimated in lymphocytes from peripheral blood by G banding analysis. Marker chromosomes were identified by fluorescence in situ hybridization (FISH), multiplex FISH, multicolor band and high resolution comparative genomic hybridization methods., Results: Marker chromosomes were identified as inv dup(22)(pter->q11.2::q11.2->pter), der(8)(:p22->q11.2:), der(2l)(:pter->q21.3:) and der(19)(:p11->q13.1). All of them contained euchromatic sequences. First marker, an inverted duplication of chromosome 22q11.2 corresponding to tetrasomy of this chromosome region was recognized in a child with partial cat eye syndrome. Two further markers derived from chromosomes 8 and 21 were found in a child with mosaic karyotype and clinical features of trisomy 8p. In the third case additional chromosome material was derived from chromosome 19 and it was found in a patient with mild mental retardation and clinical features of ovary dysgenesis. Genetic risk of identified marker chromosomes except for mar(19) was estimated as high., Conclusions: Our results provide further evidence for diagnostic value of molecular cytogenetic methods. They also confirmed the general opinion of the high risk of phenotypic abnormalities in the carriers of marker chromosomes containing euchromatic sequences.