Unexpected results in the constitution of small supernumerary marker chromosomes.

Traditional approaches for the classification of Small Supernumerary Marker Chromosomes (sSMC), mostly based on FISH techniques, are time-consuming and not always sufficient to fully understand the true complexity of this class of rearrangements. We describe four supernumerary marker chromosomes that, after array-CGH, were interpreted rather differently in respect to the early classification made by conventional cytogenetics and FISH investigations, reporting two types of complex markers which DNA content was overlooked by conventional approaches: 1. the sSMC contains non-contiguous regions of the same chromosome and, 2. the sSMC, initially interpreted as a supernumerary del(15), turns out to be a derivative 15 to which the portion of another chromosome was attached. All are likely derived from partial trisomy rescue events, bringing further demonstration that germline chromosomal imbalances are submitted to intense reshuffling during the embryogenesis, leading to unexpected complexity and changing the present ideas on the composition of supernumerary marker chromosomes.

De novo balanced chromosome rearrangements in prenatal diagnosis.

OBJECTIVE: We surveyed the datasheets of 29 laboratories concerning prenatal diagnosis of de novo apparently balanced chromosome rearrangements to assess the involvement of specific chromosomes, the breakpoints distribution and the impact on the pregnancy outcome. METHOD: By means of a questionnaire, data on 269.371 analyses performed from 1983 to 2006 on amniotic fluid, chorionic villus and fetal blood samples were collected. RESULTS: A total of 246 balanced anomalies were detected at frequencies of 72% for reciprocal translocations, 18% for Robertsonian translocations, 7% for inversions and 3% for complex chromosome rearrangements. The total frequencies of balanced rearrangements were 0.09%, 0.08% and 0.05% on amniotic fluid, chorionic villus and fetal blood samples. CONCLUSION: A preferential involvement of chromosomes 22, 7, 21, 3, 9 and 11 and a less involvement of chromosomes X, 19, 12, 6 and 1 was observed. A nonrandom distribution of the breakpoints across chromosomes was noticed. Association in the location of recurrent breakpoints and fragile sites was observed for chromosomes 11, 7, 10 and 22, while it was not recorded for chromosome 3. The rate of pregnancy termination was about 20%, with frequencies decreasing from complex chromosomal rearrangements (33%), reciprocal translocations (24%) to inversions (11%) and Robertsonian translocations (3%).

FISH screening for subtelomeric rearrangements in 219 patients with idiopathic mental retardation and normal karyotype.

Subtelomeric rearrangements are a common cause of idiopathic mental retardation (MR) accounting for 6.3-10.2% of moderate to severe cases and less than 1% of mildly retarded patients. We report on a cohort of 219 patients with idiopathic MR and normal 400-550 band karyotype screened for subtelomeric rearrangements by multiprobe Fluorescence in situ hybridization (FISH) in three Italian Genetics Centers. Twelve positive cases (5.5%) were found. Six were de novo deletions (1p, 7p, 9p, 9q, 20p, 22q) and four unbalanced translocations [a der(6)t(6q; 18p) and a der(18)t(8p; 18q) both de novo, a der(12)t(12p; 17q)mat and a der(2)t(2q; 17q) of unknown origin]. The remaining two cases were apparently balanced reciprocal translocations [a t(4p; 18q) and a t(1p; 16p)] of undetermined origin whose role in the pathogenesis of the clinical phenotype is doubtful. Dysmorphic features were present in all unbalanced patients, whilst a family history of MR was present in only four of them. The proposition that subtelomeric rearrangements are a significant cause of idiopathic MR is supported by our survey. Collection of the clinical data of positive patients will help to delineate the phenotype associated with the various subtelomeric abnormalities, to tailor healthcare services to the needs of these patients and their families and to determine the appropriate use of the test.

Cytogenetic and molecular evaluation of 241 small supernumerary marker chromosomes: cooperative study of 19 Italian laboratories.

PURPOSE: We evaluated the experiences of 19 Italian laboratories concerning 241 small supernumerary marker chromosomes (sSMCs) with the aim of answering questions arising from their origin from any chromosome, their variable size and genetic content, and their impact on the carrier's phenotype. METHODS: Conventional protocols were used to set up the cultures and chromosome preparations. Both commercial and homemade probes were used for the fluorescent in situ hybridization analyses. RESULTS: A total of 113 of the 241 sSMCs were detected antenatally, and 128 were detected postnatally. There were 52 inherited and 172 de novo cases. Abnormal phenotype was present in 137 cases (57%), 38 of which were antenatally diagnosed. A mosaic condition was observed in 87 cases (36%). In terms of morphology, monocentric and dicentric bisatellited marker chromosomes were the most common, followed by monocentric rings and short-arm isochromosomes. The chromosomes generating the sSMCs were acrocentric in 132 cases (69%) and non-acrocentric chromosomes in 60 cases (31%); a neocentromere was hypothesized in three cases involving chromosomes 6, 8, and 15. CONCLUSION: The presented and published data still do not allow any definite conclusions to be drawn concerning karyotype-phenotype correlations. Only concerted efforts to characterize molecularly the sSMCs associated or not with a clinical phenotype can yield results suitable for addressing karyotype-phenotype correlations in support of genetic counseling.