Sensitivity of chromosomal mosaicism detection by different tissue culture methods

Preparation, Culture, and Analysis of Amniotic Fluid Samples

Amniotic fluid obtained via amniocentesis provides a source of fetal material used in prenatal diagnosis. The fluid may be used directly for biochemical analyses, fluorescence in situ hybridization (FISH), and isolation of DNA for molecular studies, including chromosomal microarray analysis (CMA). The fluid is typically cultured as a source of metaphase cells for chromosome analysis and to provide additional material for biochemical and DNA-based testing. This unit describes an in situ method for the preparation, culture, and harvest of amniotic fluid samples for metaphase chromosome analysis. Cells are grown, harvested for metaphase spreads, and analyzed on glass coverslips. The unit also describes methods to obtain cells for additional studies (such as molecular genetic analyses) by growing cells in flasks either following passaging of cells from a glass coverslip culture or by directly establishing a flask culture from the amniotic fluid specimen. When cells are grown in flasks, they must be removed from the flask with trypsin before they can be used in studies. Lastly, this unit describes a method for isolating DNA for CMA from uncultured amniotic fluid and cultured cells. © 2018 by John Wiley & Sons, Inc.

Preparation, culture, and analysis of amniotic fluid samples

Amniotic fluid obtained via amniocentesis provides a source of fetal material used in prenatal diagnosis. The fluid is used directly for biochemical analyses and as a source of fetal cells for biochemical assays, DNA testing, and chromosome studies. This unit describes an in situ method for the preparation, culture, and harvest of amniotic fluid samples for chromosome analysis. Cells are grown, harvested for metaphase spreads, and analyzed on glass coverslips. The unit also describes methods to obtain cells for additional studies (such as molecular genetic analyses) by growing cells in flasks either following passaging of cells from a glass coverslip culture or by directly establishing a flask culture from the amniotic fluid specimen. When cells are grown in flasks, they must be removed from the flask with trypsin before they can be used in studies.

Incidence and outcome of chromosomal mosaicism found at the time of chorionic villus sampling

OBJECTIVE

Chromosomal mosaicism has been reported in about 1% to 3% of chorionic villus sampling specimens. This report provides incidence and outcome information that should be useful in counseling patients found to have mosaicism on chorionic villus sampling.

STUDY DESIGN

A retrospective analysis of 11,200 consecutive patients undergoing chorionic villus sampling at the University of California, San Francisco, during the period from Jan. 1, 1984, to June 1, 1996, was undertaken.

RESULTS

A total of 140 cases of mosaicism were identified for an incidence of 1.3%. Follow-up information was available for 130 cases, 26 of which (20%) were confirmed in fetal tissue. Confirmation rates for specific types of mosaicism were as follows: autosomal trisomy 7.6%, sex chromosome 25%, structural abnormality 27.3%, and marker chromosome 77.8%. Neonatal outcome was normal in all cases for which pregnancy continued.

CONCLUSION

The data indicate that in most cases of chromosomal mosaicism found by chorionic villus sampling the mosaicism is unlikely to be clinically significant in the fetus.

Probability tables for exclusion of mosaicism in prenatal diagnosis

The decision concerning the number of metaphases that need to be analysed to detect mosaicism of a certain degree depends mainly, for the same confidence levels, on the culture method used (in situ or flask methods). Several probability tables, designed for either the in situ or the flask method, have been reported and can be used to assist laboratories in making the decisions referred to above. However, there are instances where part of the analysis is done using the in situ and flask methods. In such situations, the previously published tables are of limited use. We have generated a new table that can be used in such situations, as well as in cases where only the flask method is used.

Is the 15-in situ clone protocol necessary to detect amniotic fluid mosaicism?

OBJECTIVE

Our purpose was to evaluate the 15-clone analysis for detecting amniotic fluid mosaicism by the in situ method.

STUDY DESIGN

A 10-year review was performed of all amniotic fluid mosaicism cases at two institutions using the in situ method exclusively, with sequential clonal analysis to determine the first and second clone in which the abnormal cell line occurred.

RESULTS

Of the 28,497 amniotic fluid samples, 73 met criteria for amniotic fluid mosaicism by in situ method (0.26%). There were 54 cases (0.19%) with potential clinical significance (23 autosome and 31 sex chromosome mosaicism); 49 of the 54 cases (89%) were detected in the first six clones, including 22 of 23 involving autosomes and 27 of 31 involving sex chromosomes. In one of the six cases detected after clone 6 (46,XX/47,XX,+21) the mosaic cell line was present in 20% of the clones analyzed and was followed by a voluntary termination of the pregnancy. In the other five cases amniotic fluid mosaicism was present in < 20% of the clones; these included one case of 46,XX/47,XX+mar (15% amniotic fluid mosaicism, voluntary termination of pregnancy), two cases of 45,X/46,XY (10% to 12% amniotic fluid mosaicism, both normal at birth), and two cases of 45,X/46,XX (8% amniotic fluid mosaicism, lost to follow-up; 12% amniotic fluid mosaicism, voluntary termination of pregnancy). By limiting the analysis to six clones, approximately 20% of analysis time could be saved per case, but one autosomal amniotic fluid mosaicism case per 10,000 samples could potentially be missed.

CONCLUSION

Reducing the number of clones analyzed by in situ method could result in increased efficiency, decreased costs, and minimal loss of sensitivity.

Exclusion of chromosomal mosaicism in amniotic fluid cultures: determination of number of colonies needed for accurate analysis

Most laboratories use the in situ or flask culture method and a two-stage approach to mosaicism detection. Determination of the optimum number of metaphases to be counted depends largely on whether or not the colonies that grow from the cells in the amniotic fluid can be considered independent. Previous statistical analysis of data from mixed male and female amniocyte cultures indicated that for mosaicism detection these colonies can be considered independent (Cheung et al., 1990). This analysis was repeated with a set of mosaic cases from two independent prenatal diagnosis programmes. The same degree of colony independence was found with this set of data. This result was used to calculate the level of mosaicism that is excluded at a particular confidence level when set numbers of colonies are analysed at each stage. The tables generated apply directly to the in situ method and with modification they can be used with the flask method. The conclusions are (1) analysis of cells from multiple colonies enhances the likelihood of excluding true mosaicism; (2) analysis of more than one metaphase per colony offers little advantage in excluding mosaicism; and (3) the two-stage approach is the most efficient. These conclusions should be used together with the expected clinical outcome of the actual cytogenetic abnormality, as discussed by Hsu et al. (1992).

Cytogenetic diagnoses after chorionic villus sampling are less reliable in very-high-or very-low-risk pregnancies

An increasing number of cytogenetic prenatal diagnoses are performed on chorionic villus samplings. The accuracy of this method is influenced by chromosomal mosaicism, mostly confined to direct preparation methods. Especially those investigators who have experienced false-negative and false-positive findings propagate the combined use of direct and culture methods. Yet large collaborative studies have shown that in approximately two-thirds of diagnostic cases only one procedure is applied. Moreover, the accuracy of a cytogenetic investigation depends not only on the ontogenetic origin of the tissues investigated, but also on interacting factors such as the 'a priori risk' and the 'predictive value of a cytogenetic finding'. On this basis a differentiated prenatal diagnostic procedure is discussed, including either sole short-term culture (STC), combined STC and long-term culture (LTC), primary amniocentesis (AC), or primary percutaneous umbilical blood sampling (PUBS). The predictive value of the cytogenetic diagnosis from CVS varies significantly dependent on the a priori risk of a chromosome aberration and, in the case of an abnormal karyotype, on the specific chromosome involved. A non-mosaic and 'non-lethal' trisomy detected in STC is highly representative of the embryo/fetus, but there are exceptions of limited predictive value, e.g., trisomy 18. Guided by the strategy of an optional follow-up by LTC, AC, or PUBS in 1317 successive CV samplings, we are not aware of a false-negative diagnosis, but probably had one false-positive diagnosis: 47,XXY after STC; 46,XY after LTC. When referring to the rate of fetuses with an unbalanced karyotype expected in the different indication groups, a relative increase of false-positive findings in the very-low-risk group (maternal age < or = 35 years of age) and of false-negative findings in the very-high-risk group (abnormal ultrasonographic findings) of pregnant women when only performing CVS becomes obvious. Because of this dilemma, AC or--especially in the latter group--PUBS might be primarily offered to these indication groups instead of CVS.

Proposed guidelines for diagnosis of chromosome mosaicism in amniocytes based on data derived from chromosome mosaicism and pseudomosaicism studies

Currently, accepted protocol which has been developed at the Prenatal Diagnosis Laboratory of New York City (PDL) requires that when a chromosome abnormality is found in one or more cells in one flask, another 20-40 cells must be examined from one or two additional flasks. Chromosome mosaicism is diagnosed only when an identical abnormality is detected in cells from two or more flasks. In a recent PDL series of 12,000 cases studied according to this protocol, we diagnosed 801 cases (6.68 per cent) of single-cell pseudomosaicism (SCPM), 126 cases (1.05 per cent) of multiple-cell pseudomosaicism (MCPM), and 24 cases (0.2 per cent) of true mosaicism. Pseudomosaicism (PM) involving a structural abnormality was a frequent finding (2/3 of SCPM and 3/5 of MCPM), with an unbalanced structural abnormality in 55 per cent of SCPM and 24 per cent of MCPM. We also reviewed all true mosaic cases (a total of 50) diagnosed in the first 22,000 PDL cases. Of these 50 cases, 23 were sex chromosome mosaics and 27 had autosomal mosaicism; 48 cases had numerical abnormalities and two had structural abnormalities. Twenty-five cases of mosaicism were diagnosed in the first 20 cells from two flasks, i.e., without additional work-up, whereas the other 25 cases required extensive work-up to establish a diagnosis (12 needed additional cell counts from the initial two culture flasks; 13 required harvesting a third flask for cell analysis). Our data plus review of other available data led us to conclude that rigorous efforts to diagnose true mosaicism have little impact in many instances, and therefore are not cost-effective. On the basis of all available data, a work-up for potential mosaicism involving a sex chromosome aneuploidy or structural abnormality should have less priority than a work-up for a common viable autosomal trisomy. We recommend revised guidelines for dealing with (1) a numerical versus a structural abnormality and (2) an autosomal versus a sex chromosome numerical aneuploidy. Emphasis should be placed on autosomes known to be associated with phenotypic abnormalities. These new guidelines, which cover both flask and in situ methods, should result in more effective prenatal cytogenetic diagnosis and reduced patient anxiety.