Prenatal detection of fra(X)(q27.3) in female identical twins: reliability of low level cytogenetic prenatal expression in females

Recently, we detected fra(X)(q27.3) in amniocyte cultures from female identical twins. The pregnant woman did not exhibit fra(X)(q27.3) in whole blood cultures but was the sister of 2 affected brothers. DNA marker analyses showed that she was a carrier of FRAXA. Amniotic fluid cultures (AFCs) from twins A and B exhibited the fragile X [fra(X)] chromosome, but the level of cytogenetic expression was very low in twin A's AFCs. DNA marker studies indicated both twins were carriers of FRAXA. Peripheral umbilical blood sample (PUBS) cultures exhibited fra(X)(q27.3) at a frequency of about 10% for both twins. DNA fingerprinting indicated that the twins were identical, confirming the clinical impression, with a very thin separating amniotic membrane. To our knowledge, this is the only report of prenatal fra(X)(q27.3) detection in female identical twins, and the second report of identical twin detection [Rocchi et al., 1985]. We have diagnosed prenatally fra(X)(q27.3) in 5 female fetuses using AFCs. The average fra(X) frequency was 4% for these positive female fetuses with a range of 0.5% to 8.5%. Follow-up whole blood studies confirmed our original results at an average fra(X) frequency of 25%.

IN CONCLUSION

1. Low frequencies, perhaps 1 or 2%, or a few positive cells in AFCs, are likely to increase in magnitude when confirmed in whole blood cultures either pre- or postnatally. 2. It appears likely that the risk is low for false positive results in AFCs when low frequencies of fra(X)(q27.3) are encountered.

Fra(X) prenatal diagnosis: are endoreduplicated and polyploid cells useful diagnostic criteria?

Cytogenetic and molecular protocols for prenatal ascertainment of the fragile X syndrome and the associated fragile site at Xq27.3 are relatively reliable. Any new diagnostic method which becomes available still elicits much interest. Kimchi-Sarfaty et al. [1991] reported an increase in frequency of endoreduplication and polyploidy in fra(X) lymphoblasts and amniocytes when cultured with methotrexate (MTX) or fluorodeoxyuridine. Recently we analyzed the endoreduplication/polyploidy system using amniotic fluid, chorionic villus, and fibroblasts from fra(X) positive abortus cell cultures and from control samples. We observed no increased expression of endoreduplicated or polyploid cells in fra(X) positive amniocytes after exposure to MTX. The data presented here clearly dispute the value of endoreduplication/polyploid scoring as a diagnostic aid in prenatal fra(X) analysis.

Dialyzed fetal bovine serum increases cytogenetic fragile X expression

Short-term whole blood cultures from 9 unrelated male individuals with the fragile X [fra(X)] syndrome were exposed to 5-fluorodeoxyuridine (FUdR). The fra(X) frequency was higher in 8 of 9 cases where the complete medium contained dialyzed fetal bovine serum (DFBS). In 3 of the cases, the fra(X) frequency nearly tripled (e.g., 12/100 to 33/100) while in 2 others, it nearly doubled (e.g., 15/100 to 29/100). When DFBS cultures from 2 other fra(X) individuals were exposed to increasing folic acid concentrations ranging from 2 to 4,000 x 10(-6) M, there was virtually no change in fra(X) expression. In 6 of 9 DFBS cultures, the mitotic index decreased, and it increased in 3. Therefore, although the fra(X) frequency increased, in most DFBS cultures the mitotic index decreased. Whether the reduction in mitotic index indicates an inverse correlation between reduced mitotic index and increased fra(X) expression, at least in cultures from some individuals, will be determined by additional studies.

IN CONCLUSION

(1) medium supplementation with dialyzed fetal bovine serum should be considered when using FUdR for fra(X) identification in order to avoid potentially false negative results; (2) there appears to be no direct correlation between increased mitotic index and increased fra(X) expression in whole blood cultures; (3) increased folic acid concentrations do not affect fra(X) expression when FUdR fra(X) induction is employed; therefore requesting people to refrain from taking vitamins, including folic acid, before fra(X) testing (a practice that still persists in some places) appears unnecessary.

Fra(X)(q27.2), the common fragile site, observed in only one of 760 cases studied for the fragile X syndrome

Cell cultures from 760 whole blood, amniotic fluid, chorionic villus sample, and peripheral umbilical blood sample specimens were exposed to multiple fra(X)(q27.3) induction systems (none had aphidicolin). Fifty-three exhibited the rare fragile site, fra(X)(q27.3) or FRAXA, none of which demonstrated the common fragile site or FRAXD at band Xq27.2. Only one cell in one of the negative whole blood FUdR-treated cultures from a mentally retarded male showed FRAXD. Therefore, it appears that FRAXD occurs very rarely in cultures treated to induce FRAXA since only one positive cell was observed in over 88,000 analyzed. It appears that very low frequencies of fra(X)(q27) can be accounted for only in part by the presence of the common fragile site since only one of 9 cases, each with one fra(X)(q27) positive cell, exhibited FRAXD and the others were FRAXA. After confirmation of FRAXA with direct DNA testing in a large number of low frequency cases, it should be possible to rely on the detection of very low frequencies of fra(X)(q27.3), e.g., 1% with at least 2 positive cells.

Improved prenatal detection of fra(X)(q27.3): methods for prevention of false negatives in chorionic villus and amniotic fluid cell cultures

The reliable detection of fra(X)(q27.3) in prenatal samples is important for providing genetic counseling. We have identified 5 new cases of prenatal fragile X [fra(X)] detection in 3 chorionic villus sample (CVS) and 2 amniotic fluid (AF) cell cultures. In 4 of the 5 cases, either excess thymidine (THY) or a combination of THY and 5-fluorodeoxyuridine (FUdR) was clearly superior to FUdR alone as fra(X) inducers. Amniocytes from one case were cultured only in RPMI-1640 and later exposed to FUdR or THY separately. They showed only 2% fra(X) while parallel cultures initiated in Chang medium and incubated in RPMI for at least 7 days (recovery) before fra(X) induction exhibited strikingly increased fra(X) frequencies. Chang medium alone will not allow fra(X) induction in AF (Jenkins EC, Brown WT [1986]: "Genetic Disorders and the Fetus: Diagnosis, Prevention and Treatment." New York: Plenum Press, pp 185-204). Now, using CVS cells, we report that only 1% and 0% fra(X) were detected using FUdR or THY in cells cultured in RPMI for 4 days after removal from Chang medium. Cells with 7 days "recovery" in RPMI exhibited increases from 2 to 6%. Therefore, we have found that Chang medium is very helpful when the appropriate recovery time in another medium is allowed before fra(X) induction. Some false negative reports can be attributed to: induction in Chang medium alone; lack of sufficient recovery time after initiating cells in Chang before induction; and unavailability of the excess THY fra(X) induction system.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of autosomal fragile sites in specimens cultured for prenatal fragile X diagnosis

We reviewed the distribution of autosomal fragile sites (FS) and spontaneous chromosome breaks or gaps (CB) at chromosome locations other than those recognized as FS from 100 amniotic fluid samples (AF), 19 chorionic villus samples (CVS), and 5 percutaneous umbilical blood samples (PUBS) referred for fragile X [fra(X)] analysis. We present data on the degree of expression of autosomal fragility in AF, CVS, and PUBS samples, and the relationship between degree of expression and induction system. The most common observed FS were: 3p14, 9p32, and 6q26 in AF; 9q32, 3q27, and 8q22 in CVS; and 3p14, Xq22, and 16q23 in PUBS cases. Distribution of FS and CB, when compared by induction system, was not found to be identical. Our data also indicate that the presence of any particular FS cannot be used as an indicator for the effectiveness of the fra(X) induction system in prenatal samples.

Progress toward an internal control system for fragile-X induction by 5-fluorodeoxyuridine in whole-blood cultures

We have been attempting to develop a consistently reliable internal control to assure the effectiveness of the 5-fluorodeoxyuridine (FUdR) fragile-X [fra(X)] induction system. We carried out a systematic study of whole-blood specimens cultured from 56 individuals from two different laboratories. An analysis of nearly 9,000 cells demonstrated: (1) the importance of establishing baseline levels of fragile sites in each laboratory, and (2) that a combination of common fragile sites (different for each laboratory) could serve as a consistently reliable indicator of the effectiveness of the FUdR fra(X) induction system. It was suggested that a non-FUdR culture(s) should be incorporated into a laboratory's fra(X)-screening protocol, so that if there are any doubts about the effectiveness of the FUdR system a comparison to background or spontaneously occurring fragile sites can be made within the laboratory. Repeat cultures are recommended where no increase in common fragile-site frequency is observed in the FUdR induction system, and where fra(X) was strongly suspected but not found. In addition, the necessity of using more than one fra(X) induction system in whole-blood cultures was demonstrated, including the effectiveness of an FUdR/excess thymidine double-induction system. Finally, 2 cases of apparent mosaicism for Klinefelter syndrome in fra(X) individuals were observed.

Study of spindle microtubule reassembly in cells from Alzheimer and Down syndrome patients following exposure to colcemid

Numerous intraneuronal neurofibrillary tangles and senile (neuritic) plaques are the two characteristic lesions in Alzheimer disease (AD) and adult Down syndrome (DS). Evidence indicates that microtubule assembly is impaired in AD. We studied spindle microtubule repolymerization rates in EBV-transformed lymphoblasts from AD, DS, and control individuals after colcemid exposure. The distinctive arrangement of microtubules in spindle and its size make this structure an obvious choice for study. Recovery trends in the three patient groups differed significantly; in particular, the controls showed an earlier appearance of intact spindle microtubules than AD. Other researchers found similar results using AD fibroblasts. The results from the DS cells were inconsistent and difficult to interpret. It is unclear how the AD microtubules differ from controls, or whether a relationship exists between the altered microtubule repolymerization kinetics observed in this study and the presence of neurofibrillary tangles in AD patients. A difference in repolymerization rates can be the basis for a diagnostic test for AD if it can be verified.

Chromosomal abnormalities in amniotic fluid cell cultures: a comparison of apparent pseudomosaicism in Chang and RPMI-1640 media

The finding of chromosome mosaicism is one of the most difficult problems in fetal chromosome analysis. Whether the finding indicates true mosaicism or pseudomosaicism must be investigated. Studies detailing the incidence of true mosaicism and pseudomosaicism have been reported (Hsu & Perlis 1984, Bui et al. 1984, Worton & Stern 1984) but do not correlate pseudomosaicism with any particular type of culture media. Benn & Hsu (1985) compared cell growth and chromosome abnormalities in amniotic fluid cell cultures grown in Chang medium and RPMI-1640 medium and found no statistically significant difference in the rate of abnormalities in the two media. We have previously shown that Chang medium exhibited more abnormalities which were not verified in second and third cultures (Masia et al. 1986). In the current study we examined 212 cases grown in both Chang and RPMI-1640 media, and compared apparent single and multiple cell pseudomosaic abnormalities to medium type. The number of observed abnormalities was 22, occurring in 19 of the cases studied. Apparent pseudomosaic chromosome anomalies were observed in 18 Chang cultures and in 4 RPMI-1640 cultures. Statistical analysis found significant correlation between medium type and the degree of observed pseudomosaic cells. We conclude that the rate at which pseudomosaic cells are observed is partly a function of medium type, and in our laboratory Chang medium caused apparent pseudomosaicism at a greater level than RPMI-1640 medium.

Constitutive fragile sites in fra(X) individuals

Recently, it was proposed that the constitutive fragile site at 3p14 be used as an "internal control" to indicate the effectiveness of the FUdR fragile site induction system. We have tested this hypothesis by determining the frequency of constitutive fragile sites at 1p31, 3p14, and 16q23 in cultures from 42 known fra(X) individuals. At least 50 cells were analyzed from each case. Seventy-four percent (31/42), 95% (40/42) and 90% (38/42) of the fra(X) individuals exhibited frequencies of less than 4% at constitutive fragile sites 3p14, 1p31 and 16q23, respectively. Of the 42 individuals tested, 12 or 28.6% showed no fragility at any of the 3 sites studied. On the other hand, at least one constitutive fragile site was observed in 50 cells studied from over 70% of the 42 people studied. It is suggested that "positive controls" continue to be used, while at the same time recording all fragile sites to identify a combination of constitutive fragile sites that may serve as an internal control indicator, and that DNA marker studies be used to complement cytogenetic testing.

Recent experience in prenatal fra(X) detection

At least 35 cases of prenatal fra(X) diagnosis have been confirmed and reported. Amniotic fluid, fetal blood and chorion -ic villus samples have exhibited fra(X) (q27.3) in cultures from 26 males and 9 females. Here we have detected fra(X) in female and male amniotic fluid specimens, AF1/fra(X),X and AF2/fra(X),Y, respectively, and a male CVS/fra(X),Y using both FUdR and excess thymidine (THY) to demonstrate the marker chromosome. Both FUdR and THY detected fra(X) and usually FUdR was superior to THY with the exception of placental cultures. It was important to examine more than one culture per protocol since no fra(X) was observed in one AF2 FUdR culture while another exhibited 19.2% expression. Similarly, confirmation studies in lung fibroblast cultures for AF2 exhibited 4.3% fra(X) in one lab while another found negative results. A similar observation in whole blood cultures was also made recently by us. In addition, we have recently experienced our first false negative fra(X),X prenatal diagnosis. We have observed another case where only one cell in 300 exhibited fra(X) where the male fetus was 50% at-risk and was referred to us after the 20th week of gestation by sonography. On the basis of our experience we recommend the following: 1) the excess THY fra(X) induction system is effective but not superior to FUdR; 2) at least two duplicate cultures per induction system should be analyzed for the marker chromosome to avoid the possibility of false-negative diagnosis; 3) where fra(X) is not demonstrated or is present in very low frequencies in CVS and/or amniotic fluid cultures, complementary DNA marker studies and/or fetal blood cultures must be made available; 4) gestational age dating by ultrasonography is recommended as early as possible.

Fragile X expression in short-term whole blood cultures is affected by cell density

The effect of cell density on expression of the fragile site at (X)(q27.3) in short-term whole-blood cultures from patients with fragile X [fra(X)] or Martin-Bell syndrome was studied. A significant increase in fra(X) frequency was observed in 7 of 8 samples when cell density was decreased. Higher fra(X) frequency was not always noted at below-routine density, but in some cases fra(X) expression was depressed at above-routine density. We conclude that decay of the FUdR effect explains the fact that fra(X) expression is affected by culture density. It is significant that a relationship exists between the two; it suggests that in order to maximize fra(X) expression in cases with low-percentage fra(X) with standard methods, cell density may have to be adjusted. It is possible that in individuals who are normally nonexpressing, such as some obligate female carriers and nonpenetrant males, fra(X) expression may be sensitive to cell density effects.

Further evidence for genetic heterogeneity in the fragile X syndrome

The X-linked fragile X [fra(X)] syndrome, associated with a fragile site at Xq27.3, is the most common Mendelian inherited form of mental deficiency. Approximately 1 in 1060 males and 1 in 677 females carry the fra(X) chromosome. However, diagnosis of carrier status can be difficult since about 20% of males and 44% of females are nonpenetrant for mental impairment and/or expression of fra(X). We analyzed DNA from 327 individuals in 23 families segregating fra(X) for linkage to three flanking polymorphic probes: 52A, F9, and ST14. This allowed probable nonpenetrant, transmitting males and carrier females to be identified. A combined linkage analysis was conducted using these families and published probe information on F9 in 27 other families, 52A in six families, and ST14 in five families. The two-point recombination fraction for 52A-F9 was 0.13 (90% confidence interval, 0.10-0.16), for F9-fra(X) was 0.21 (0.17-0.24), and for fra(X)-ST14 was 0.12 (0.07-0.17). Tight linkage between F9 and fra(X) was observed in some families; in others loose linkage was seen suggesting genetic linkage heterogeneity. Risk analysis of carrier status using flanking DNA probes showed that probable nonpenetrant transmitting males were included in families showing both tight and loose linkage. Thus, in contrast to our previous conclusions, it appears that the presence or absence of nonpenetrant, transmitting males in a family is not an indicator of heterogeneity. To determine if heterogeneity was present, we employed the admixture test. Evidence for linkage heterogeneity between F9 and fra(X) was found, significant at P less than 0.0005. Nonsignificant heterogeneity was seen for 52A-F9 linkage. No heterogeneity was found for fra(X)-ST14. The frequency of fra(X) expression was significantly lower in families with tight F9-fra(X) linkage than in families with loose linkage. Cognition appeared to relate to linkage type: affected males in tight linkage families had higher IQs than those in loose linkage families. These findings of genetic heterogeneity can account in part for the high prevalence and apparent high new mutation rate of fra(X). They will affect genetic counseling using RFLPs. An understanding of the basis for genetic heterogeneity in fra(X) will help to clarify the nature of the unusual pattern of inheritance seen in this syndrome.

Localization of a human gene homologous to the PrP gene on the p arm of chromosome 20 and detection of PrP-related antigens in normal human brain

Infectious fractions prepared from scrapie-infected hamster brains contain a protein, PrP 27-30, which shares antigenic determinants with polypeptides found in similarly prepared fractions from patients with Creutzfeldt-Jakob disease. cDNA sequences encoding the hamster PrP 27-30 identified homologous sequences in the human genome as well as in normal human brain mRNA preparations. Antibodies raised against the mouse PrP's identified antigenically related peptides in both normal hamster and human brain as well as in scrapie-infected hamster brain and CJD-affected human brain. By using in situ hybridization we localized the homologous human genomic sequences on the short arm of chromosome 20. Our results indicate that the reportedly unique proteins detected in human CJD preparations derive from normal human gene products.

Patterns of BrdU incorporation in homogeneously staining regions and double minutes

The replication chronology of two structural chromosome abnormalities linked to the amplification phenomenon of DNA sequences was investigated. Three cell lines containing homogeneously staining region (HSR) chromosomes (IMR-32, MK42, and COLO-320) and one line with double minutes (DM) (SW-613) were examined. Using a bromodeoxyuridine-Hoechst 33258-Giemsa method, the HSR in the three cell lines were shown to be composed of subunits that replicated their DNA throughout all portions of the S-phase of the cell cycle. The double minute chromosomes were observed to replicate randomly throughout the entire S-phase, with no pattern evident. These results are consistent with the suggestion that DNA from HSR and DM are structurally and functionally related. Moreover, this observation that these amplified regions replicate their DNA throughout the entire S-phase favors the idea that, during amplification processes, both early and late replicating sequences are included. The apparent discordance between staining characteristics and replication behavior exhibited by some HSR and DM are also discussed.

Fragile X expression increased by low cell-culture density

The effect of cell density on expression of fra(X) was studied. A lymphoblast cell line from one fra(X) individual and whole blood from another individual was tested at various cell densities using RPMI-1640 with FUdR (0.1 microM) 24 hrs before harvest. Densities from 0.25 X 10(5) to 2 X 10(6) cells/ml were tested. Chromosomes were G-banded and analyzed for fra(X) frequency. Increased density caused fra(X) frequency to decline in lymphoblasts and whole blood. In the established line low density fra(X) frequency was 51.2% and decreased to 6.5% at the high density. In blood fra(X) frequency was 34.7% at low density and decreased to 18% fra(X) in high density. We suspect that decay of the FUdR effect may explain the results. Our results suggest that to maximize fra(X) frequency, cultures should be inititated at low density. This may be important in analysis of low-percentage fra(X) patients, nonexpressing female carriers, and obligate nonpenetrant males.

The prenatal detection of the fragile X chromosome: review of recent experience

The fragile X chromosome has been identified in specimens from 17 male and 10 female fetuses in 11 laboratories throughout the world, obtained from at least 79 fetuses at increased risk for the fra(X) syndrome. Of these, 19 were confirmed, 6 were pending, 1 was negative and 1 could not be confirmed. Twenty-five of the 79 cases were studied in our laboratory (Institute for Basic Research [IBR]) and resulted in fra(X) demonstration in specimens from 3 male and 5 female fetuses. All 3 males and 2 of the 5 females have been confirmed. When amniocytes from the two confirmed female fetuses were exposed to FUdR after culturing in Chang medium, fra(X) frequencies were virtually negative indicating that Chang medium should not be used in fragile X studies at least when FUdR is used to induce fragility. Finally, amniocytes from a fra(X) male fetus studied in 3 different laboratories exhibited strikingly different frequencies. To date, we have experienced no false-positives or negatives, but the latter case was controversial. It is recommended that laboratories undertaking fra(X) prenatal detection use a combination of at least two different proven induction systems as well as complementary DNA marker studies to prevent false negative diagnosis.

Fragile X chromosome frequency is consistent temporally and within replicate cultures

Fragile X frequencies differ widely between different fra(X)-positive individuals. The basis for this variation is unknown, but may reflect genetic differences or unknown environmental factors. To determine more fully the individual and familial variation in fra(X) frequencies, we studied 15 individuals. The present study showed that fra(X) frequency, with few exceptions, was consistent for the same individual both over time and within replicate cultures. This confirms previous observations by others on the consistency of fra(X) expression by others, and indicates the extent of expected variability within replicate cultures. Consideration of this variability should enable improved fra(X) identification.

Frequency of tri- or multiradial configurations in fragile X identification

Using the FUdR system for fragile X induction, we have observed no triradial or bisatellited configurations at fra (X) (q27.3) in over 5,000 fra(X) chromosomes examined from over 150 fra(X) individuals. Based on our observations, and those of Turner and Jacobs (1983) and Daniel et al (1984), we hypothesize that triradial configurations may not occur at Xq27 with FUdR induction. To test this hypothesis we cultured whole blood simultaneously in parallel folate-deficient and FUdR fra(X) induction systems, and systematically examined fra(X) chromosomes for triradials. Neither autosomes nor X chromosomes exhibited any apparent triradial figures in the FUdR system, while 1.4% of the fra(X) chromosomes in TC 199 exhibited a triradial. Also we observed one autosomal triradial at 4q35. We conclude that triradial configurations occur in low frequencies in the folate deficient system and seldom if ever in the FUdR system.

Analysis of the fragile-X chromosome: localization and detection of the fragile site in high resolution preparations

Fragile X chromosome preparations were analyzed at levels of up to 850 bands per haploid set. We were able to consistently sublocalize the Xqter fragile site to band Xq27.3 using high-resolution methods. Chromosome length versus the frequency of fragile X expression was also compared. The fragile site appeared at a higher percentage in more condensed chromosome preparations. The importance of this finding is that high-resolution chromosome preparations do not optimize fragile-X detection.

Double minutes and other chromosomal aberrations in two malignant cell lines of the German cockroach Blattella germanica

Structural anomalies of mitotic chromosomes from two tumorigenic cell lines of the German cockroach (Blattella germanica) is described. Aberrations, such as unpairable marker chromosomes, double minutes, di- and tricentrics, ring chromosomes, tri- and quadriradials, and chromosome and chromatid gaps and breaks, were observed in varying proportions. This study reports that the double minute chromosomes (DMs) are associated with insect tumor cells, similar to the findings in both murine and human tumor cells.