Chromosome duplications and deletions and their mechanisms of origin

Duplications and deletions of the same gene loci or chromosome regions are known to produce different clinical manifestations and are significant factors in human morbidity and mortality. Extensive cytogenetic and molecular cytogenetic studies with cosmid and YAC probes in two patients with unique mosaicism for reciprocal duplication-deletion allowed us to further understand the origin of these abnormalities. The first patient's mosaic karyotype was 46,XX, inv dup(11) (q23q13)/46,XX,del(11)(q13q23). The second patient had a 46,XY,dup(7)(p11.2p13)/46,XY,del(7)(p11.2p13)/46,XY karyotype. Fluorescence in situ hybridization studies on the first patient placed the two breakpoints near the folate-sensitive fragile sites FRA11A and FRA11B. The presence of repeated sequences responsible for these fragile sites may have been involved in the patient's duplication-deletion. Our investigation leads us to conclude that, in addition to known mechanisms (such as unequal crossovers between homologs, unequal sister chromatid exchanges, excision of intrachromatid loops, and meiotic recombination within a single chromatid), duplication-deletion can also arise by the formation of an overlying loop followed by an uneven crossover at the level of the DNA strand.

Prenatal diagnosis of a mosaic extra structurally abnormal chromosome by spectral karyotyping

A de novo mosaic extra structurally abnormal chromosome (ESAC) was detected in 33 per cent of cultured amniotic fluid cells from a pregnant woman. Neither Q-banding nor fluorescence in situ hybridization (FISH) employing a DNA probe for nucleolar organizer region demonstrated the presence of satellites on the ESAC. Spectral karyotyping (SKY) was performed in this prenatal case and led to a quick and accurate determination of the ESAC as chromosome 14 in origin. The SKY finding was confirmed by conventional FISH analysis using a chromosome 14 specific painting probe. Subsequent hybridizations with a centromeric probe and a 14q subtelomeric probe were also performed to further characterize the ESAC. Absence of (TTAGGG)n sequence on the ESAC, determined postnatally, suggested it is a ring chromosome 14. Genetic counselling concerning these findings was provided to the parents who chose to continue the pregnancy. The male infant had no apparent abnormal phenotype at birth.

Detection of mosaicism in amniotic fluid cultures: a CYTO2000 collaborative study

PURPOSE

To evaluate the assumptions on which the American College of Medical Genetics (ACMG) Standards and Guidelines for detecting mosaicism in amniotic fluid cultures are based.

METHODS

Data from 653 cases of amniotic fluid mosaicism were collected from 26 laboratories. A chi-square goodness-of-fit test was used to compare the observed number of mosaic cases with the expected number based on binomial distribution theory.

RESULTS

Comparison of observed data from the in situ colony cases with the expected distribution of cases detected based on the binomial distribution did not reveal a significant difference (P = 0.525).

CONCLUSIONS

The empirical data fit the binomial distribution. Therefore, binomial theory can be used as an initial discussion point for determining whether ACMG Standards and Guidelines are adequate for detecting mosaicism.

Free beta hCG screening of hydropic and non-hydropic Turner syndrome pregnancies

Fourteen cases of Turner syndrome (45,X), two cases of mosaic Turner syndrome (45,X/47,XXX and 45,X/ 46,XX), and one case of Turner syndrome involving an isochromosome X [46,X,i(X)(q10)] were ascertained by prenatal maternal serum alpha-fetoprotein (MSAFP) and free beta human chorionic gonadotropin (hCG) screening or by ultrasound. Patient-specific risks for Down syndrome were calculated and used as the criteria to determine offering further testing. Eleven of the 17 cases had hydrops and presented with an increased Down syndrome risk based on MSAFP and free beta hCG screening. The median MOM level was 0.98 and 4.04 for MSAFP and free beta hCG, respectively. Three cases had hydrops but screened negative. The two cases of mosaic Turner syndrome were non-hydropic and screened positive. The 46,X,i(X)(q10) case was non-hydropic but had elevated MSAFP and free beta hCG levels. These data suggest that Turner syndrome pregnancies do not appear to screen positive due to hydrops alone, but screening may also be influenced by the inherent genetic imbalance in the fetus and placenta. Because the MSAFP levels in our series were within the normative range in all except one case with an elevated MSAFP, free beta hCG alone was the most effective screening marker for Turner syndrome pregnancies.

Conditional immortalization of human endometrial stromal cells with a temperature-sensitive simian virus 40

The study of immortalization and other alterations associated with neoplastic transformation of endometrial stromal cells is important to understanding the development of uterine sarcomas and mixed tumors. Because stromal cells are important regulators of associated epithelial cells, alterations in the regulation of stromal cell proliferation that influence epithelial cells may also contribute to the development of endometrial carcinomas. To study immortalization and associated phenotypic and genetic alterations of human endometrial stromal cells, cultures were transfected with a plasmid containing an ori-, temperature-sensitive mutant SV40, A209 (tsSV40). Morphologically transformed colonies were selected and propagated at the permissive temperature until they entered 'crisis'. In contrast to human fibroblasts, every clone tested was immortalization competent. The frequency of immortalization was approximately 1 x 10(-6). One uncloned and six cloned cell lines escaped from crisis and appear to be immortal. Two clones, M4 and B10T1, were selected for further study. Immortalization is conditional; proliferative arrest occurs at the restrictive temperature for large T antigen function. Furthermore, withdrawal of the large T antigen results in expression of the senescent phenotype of enlarged, flattened cells. Colony-forming efficiency at the restrictive temperature was undetectable. Immortalization is also associated with several genetic alterations. The DNA content of tsSV40 transfected cells was either diploid or tetraploid in the precrisis stage of proliferation, but became aneuploid upon immortalization. Several structural rearrangements of chromosomes were detected in the immortalized stromal cells which differ from those found in SV40 immortalized fibroblasts. Although their capacity for anchorage-independent proliferation (AIP) is variable, tsSV40-immortalized endometrial stromal cells have a higher capacity for AIP than their tsSV40-transfected progenitor cells in the period of proliferation prior to 'crisis'.

Curved helix segments can uniquely orient the topology of supertwisted DNA

To show that DNA containing sharp sequence-directed curvature can preferentially establish ends of supertwisted domains, a highly curved DNA from Crithidia fasciculata was cloned into two sites separated by 28% in pBR325. When this construct (pJGC2) was examined by electron microscopy, 63% of the supercoiled molecules were branched with three or more arms, and the remaining molecules appeared as linear interwound rods. The distance between the tips of two of the arms for the branched molecules measured within 2% of 28% of the DNA contour for 32% of the pJGC2 molecules, as contrasted with only 6.6% for the poorly branched pBR325 DNA. When one of the curved segments in pJGC2 was replaced by a highly curved fragment from SV40, similar results were obtained.

Cationic metals promote sequence-directed DNA bending

A DNA segment of approximately 200 base pairs (bp) from Crithidia fasciculata kinetoplast minicircles was previously shown by electron microscopy (EM) to bend into a small circle due to its unique nucleotide sequence containing repeated blocks of 4-6 A's. When this segment was flanked by 207 bp of plasmid DNA on one side and 460 bp on the other, the resulting 890-bp DNA was found to appear either relatively straight or extremely bent as visualized by EM. The bend was located one-third the distance from one end. The fraction of molecules with the most extreme bend increased from approximately 2% to 50-60% following incubation of the DNA with increasing concentrations of Zn2+, Co2+, Ba2+, and Mn2+. These observations suggest that sequence-directed bending in DNA is an inducible and not a static phenomenon. Possible roles of transitions between the bent and straight conformations in the control of gene expression are discussed.

Characterization of hCG regulation in cultured human amniotic fluid cells: II. Mechanisms for stimulation

We showed previously that sodium butyrate stimulated human chorionic gonadotropin (hCG) measured by radioimmunoassay of medium from human second trimester amniotic fluid cell cultures, termed AF cells. We now find that stimulation of hCG in the presence of sodium butyrate takes as long as 20 h. When AF cells are preincubated with sodium butyrate, hCG levels increase in direct relation to length of the preincubation period. These findings suggest that elevation of hCG is not due merely to a release of hormone from the cells. Addition of cycloheximide or Actinomycin D inhibited protein synthesis and RNA synthesis, respectively, and prevented the stimulation of hCG by sodium butyrate. These results lend support for a mechanism of regulation involving protein and RNA synthesis, the increase in hCG levels being due to new synthesis of the hormone. Other agents reported to influence hCG production by different types of cell cultures include dibutyryl cyclic AMP, epidermal growth factor (EGF), methotrexate, and hydroxyurea. Dibutyryl cyclic AMP and EGF have no effect on hCG production in our AF cells: methotrexate causes a minimal increase, hydroxyurea causes a further increase, but sodium butyrate has the strongest stimulatory effect. We conclude that amniotic fluid cells in culture are susceptible to environmental agents capable of modulating synthesis of hCG by mechanisms involving synthesis of RNA and protein.

The characterization of hCG regulation in cultured human amniotic fluid cells

The media from primary cultures and subcultures of second trimester human amniotic fluid (AF) cells were assayed by radioimmunoassay to quantitate production of human chorionic gonadotropin (hCG). Primary AF cultures produce more hCG per cell than do the corresponding subcultures. Sodium butyrate (2mM) stimulates AF subcultures to produce 5-13 times more hCG per cell or per mg of cellular protein than do untreated subcultures. This stimulatory effect of sodium butyrate is dose dependent between 0 and 5 mM. Addition of sodium butyrate 24 hours after subculture, while stimulating production of hCG during the subsequent 3 days, also results in fewer cells and less protein per culture. This effect on cell growth is also dose-dependent. Previous investigators have proposed that the stimulation of hCG by sodium butyrate in other types of cell cultures is due to an effect of that agent on culture growth. Therefore, in these studies AF cells are allowed to grow to confluency before sodium butyrate was added. Production of hCG was stimulated by sodium butyrate about four-fold during the next 5 days although no significant changes were observed either in number of cells or amount of cellular protein per culture. These results suggest that stimulation of hCG by sodium butyrate is not dependent on its effect on growth of the cultures.

Multinucleate cells in cultures of human amniotic fluid form by fusion

The major class of cells subcultured from human amniotic fluid retains properties of trophoblast. The objective of this study was to determine whether the multinucleate cells appearing in these cultures form by fusion, as is true for syncytiotrophoblast. Culture methodology involved subdivision of one flask into two, one of which was labeled with 3H-thymidine and the other with 14C-thymidine. Aliquots of each labeled culture were mixed together and allowed to grow. Autoradiographic study revealed binucleate cells containing one nucleus labeled with 3H and the other with 14C. This result indicates that fusion is one means by which binucleate cells form. The distribution of labeled and unlabeled nuclei in 217 binucleate cells obtained in two experiments was then determined. The distribution conformed to that predicted from the assumption that fusion was random and the only way for binucleate cells to form. We conclude that the binucleate cells appearing in cultures of human amniotic fluid form by fusion, as do the multinucleate cells in trophoblast. Our results suggest further that fusion is the sole means by which binucleate cells arise.