Dividing cells from persons with Bloom's syndrome, an autosomal recessive disorder of growth, exhibit increased numbers of chromatid breaks and rearrangements. A highly characteristic feature of the chromosome instability in this syndrome is the tendency for exchanges to occur between chromatids of homologous chromosomes at homologous sites. In the present experiments, a cytogenetic technique by which the sister chromatids of a metaphase chromosome are stained differentially has been used to demonstrate a striking and possibly specific, but hitherto unrecognized, increase in the frequency with which sister chromatids also exchange segments. The cells were grown in bromodeoxyuridine and stained with 33258 Hoechst and Giemsa. Whereas phytohemagglutinin-stimulated lymphocytes from normal controls had a mean of 6.9 sister chromatid exchanges per metaphase (range 1-14), those from persons with Bloom's syndrome had a mean of 89.0 (range 45-162). Normal frequencies of sister chromatid exchanges were found in cells heterozygous for the Bloom's syndrome gene, and also in cells either homozygous or heterozygous for the genes of the Louis-Bar (ataxia telangiectasia) syndrome and Fanconi's anemia, two other rare disorders characterized by chromosome instability.In a differentially stained chromatid interchange configuration discovered during the study, it was possible to determine the new distribution of both sister and nonsister-but-homologous chromatids that had resulted from numerous exchanges. By following shifts in the pattern of staining from chromatid to chromatid, visual evidence was obtained that the quadriradial configurations long recognized as characteristic of Bloom's syndrome represent exchanges between homologous chromosomes, apparently at homologous points.We postulate that the increase in the frequency of exchanges between nonsister-but-homologous chromatids and those between sister chromatids in Bloom's syndrome represents aspects of one and the same disturbance. A study of this phenomenon in relation to the clinical features of Bloom's syndrome may be helpful eventually in understanding the biological significance of chromatid exchange in somatic cells.Bloom's syndrome is a rare genetic disorder of man characterized clinically by growth retardation, a sun-sensitive eruption of the face, a disturbance of immune function, and a predisposition to cancer (1, 2). In addition, cultured blood lymphocytes and dermal fibroblasts from affected homozygotes (the genotype of which may be described as bI/bi)

Cytogenetic data are presented for 11,473 chorionic villus sampling (CVS) procedures from nine centres in the U.S. NICHD collaborative study. A successful cytogenetic diagnosis was obtained in 99.7 per cent of cases, with data obtained from the direct method only (26 per cent), culture method only (42 per cent), or a combination of both (32 per cent). A total of 1.1 per cent of patients had a second CVS or amniocentesis procedure for reasons related to the cytogenetic diagnostic procedure, including laboratory failures (27 cases), maternal cell contamination (4 cases), or mosaic or ambiguous cytogenetic results (98 cases). There were no diagnostic errors involving trisomies for chromosomes 21, 18, and 13. For sex chromosome aneuploidies, one patient terminated her pregnancy on the basis of non-mosaic 47,XXX in the direct method prior to the availability of results from cultured cells. Subsequent analysis of the CVS cultures and fetal tissues showed only normal female cells. Other false-positive predictions involving non-mosaic aneuploidies (n = 13) were observed in the direct or culture method, but these cases involved rare aneuploidies: four cases of tetraploidy, two cases of trisomy 7, and one case each of trisomies 3, 8, 11, 15, 16, 20, and 22. This indicates that rare aneuploidies observed in the direct or culture method should be subjected to follow-up by amniocentesis. Two cases of unbalanced structural abnormalities detected in the direct method were not confirmed in cultured CVS or amniotic fluid. In addition, one structural rearrangement was misinterpreted as unbalanced from the direct method, leading to pregnancy termination prior to results from cultured cells showing a balanced, inherited translocation. False-negative results (n = 8) were observed only in the direct method, including one non-mosaic fetal abnormality (trisomy 18) detected by the culture method and seven cases of fetal mosaicism (all detected by the culture method). Mosaicism was observed in 0.8 per cent of all cases, while pseudomosaicism (including single trisomic cells) was observed in 1.6 per cent of cases. Mosaicism was observed with equal frequency in the direct and culture methods, but was confirmed as fetal mosaicism more often in cases from the culture method (24 per cent) than in cases from the direct method (10 per cent). The overall rate of maternal cell contamination was 1.8 per cent for the culture method, but there was only one case of incorrect sex prediction due to complete maternal cell contamination which resulted in the birth of a normal male.(ABSTRACT TRUNCATED AT 400 WORDS)

In addition to increased DNA-strand exchange, a cytogenetic feature of cells lacking the RecQ-like BLM helicase is a tendency for telomeres to associate. We also report additional cellular and biochemical evidence for the role of BLM in telomere maintenance. BLM co-localizes and complexes with the telomere repeat protein TRF2 in cells that employ the recombination-mediated mechanism of telomere lengthening known as ALT (alternative lengthening of telomeres). BLM co-localizes with TRF2 in foci actively synthesizing DNA during late S and G2/M; co-localization increases in late S and G2/M when ALT is thought to occur. Additionally, TRF1 and TRF2 interact directly with BLM and regulate BLM unwinding activity in vitro. Whereas TRF2 stimulates BLM unwinding of telomeric and non-telomeric substrates, TRF1 inhibits BLM unwinding of telomeric substrates only. Finally, TRF2 stimulates BLM unwinding with equimolar concentrations of TRF1, but not when TRF1 is added in molar excess. These data suggest a function for BLM in recombination-mediated telomere lengthening and support a model for the coordinated regulation of BLM activity at telomeres by TRF1 and TRF2.

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The evaluation of circulating cell-free DNA by massively parallel shotgun or targeted sequencing to determine the risk of fetal aneuploidy has been rapid and extensive. Recent published studies have demonstrated the incremental value of the use of cell-free DNA for noninvasive prenatal testing (NIPT). 1 Various methods and technologies have been used for NIPT, with impressive results. In one study, NIPT demonstrated 100% sensitivity for both trisomy 21 and trisomy 18, with a specificity of ≥99.7% for both. 1 Data recently presented by two independent groups in 2013 2 and 2014 3 prompted us to review the concordance of results among cases with positive or negative NIPT results referred to Quest Diagnostics for confirmation with cytogenetic studies. MATERIALS AND METHODSWe evaluated the results from 109 consecutive specimens prenatally and/or postnatally studied by standard karyotyping, fluorescence in situ hybridization analysis (AneuVysion; Abbott Molecular/Vysis, Abbott Park, IL), and/or oligo-single-nucleotide polymorphism microarrays (CytoScanHD; Affymetrix, Santa Clara, CA) after NIPT. The NIPT providers were listed in 42 cases and included Panorama (Natera, San Carlos, CA; 20 cases), Harmony (Ariosa Diagnostics, San Jose, CA; 13 cases), MaterniT21 (Sequenom, San Diego, CA; 8 cases), and Verifi (Illumina, Redwood City, CA; 1 case). The most common initial NIPT-positive result was trisomy 21 (41 cases), followed by trisomy 18 (25 cases), trisomy 13 (16 cases), sex chromosome aneuploidy (16 cases), trisomy 16 (3 cases), monosomy 21 (2 cases), and 1 case each of triploidy and microdeletion of 22q11.2. Four samples negative for NIPT but positive for ultrasound findings were included. RESULTSCytogenetic results were positive for trisomy 21 in 38 of the 41 NIPT-positive cases (true-positive rate: 93%) and for trisomy 18 in 16 of the 25 NIPT-positive cases (true-positive rate: 64%) ( Table 1). The true-positive rate was only 44% (7/16 cases) for trisomy 13 and 38% (6/16 cases) for sex chromosome aneuploidy. A total of six cases with positive NIPT results for either monosomy 21, trisomy 16, triploidy, or 22q11.2 microdeletion had normal cytogenetic findings. Only one case had very-lowlevel mosaicism (~5-10%) for trisomy 16. Confined placental mosaicism was confirmed in two cases (2/105, 2%): one with 3% mosaic for trisomy 18 and another with a mosaic segmental uniparental disomy for 11p15.5-p11.2. A false-negative result for NIPT was identified in nonmosaic trisomies 9 and 21, a marker chromosome, and a mosaic sex chromosome aneuploidy.The findings from our laboratory and those presented by the above-mentioned two groups (n = 80 and n = 46) show that Purpose: Recent published studies have demonstrated the incremental value of the use of cell-free DNA for noninvasive prenatal testing with 100% sensitivity for trisomies 21 and 18 and a specificity of ≥99.7% for both. Data presented by two independent groups suggesting positive results by noninvasive prenatal testing were not confirmed by cytogenetic studies. Methods:C...