Absence of maternal contamination of chorionic villi used for fetal-gene analysis

Prenatal diagnosis of sickle cell disease by the technique of PCR

Sickle cell disease (SCD) is prevalent in Central India and causes major morbidity and mortality. There is a lack of prenatal diagnostic facility near population affected with SCD. This is the pilot study in our region with the aim to establish prenatal diagnostic facility for the couples carrying sickle cell gene in Central India, in order to help them take an informed decision regarding fetus affected with SCD and also to calculate sensitivity of polymerase chain reaction (PCR) technique in our set up with follow up high performance liquid chromatography (HPLC) of baby's blood sample. Fetal sampling was done by chorionic villous biopsy. Extracted DNA was subjected to amplification refractory mutation system (ARMS-PCR) to detect sickle cell mutation (GAG → GTG) in the sixth codon of β globin gene. Follow-up HPLC was done to detect baby's Hb pattern. Prenatal diagnosis of sickle cell anemia was offered in total 37 cases out of which one (2.7 %) fetal sample was inadequate. Total 26 (70.27 %) fetuses had AS Hb genotype, 3 (8.11 %) had AA Hb genotype and 3 (8.11 %) had SS Hb genotype while remaining 4 (10.81 %) were given AA/AS Hb genotype. All couples with SS fetuses opted for MTP. Follow up HPLC was performed in 24 cases, out of which 18 (75 %) were correlated and 6 (25 %) were mismatched. In present study sensitivity of ARMS-PCR was 75 %. ARMS-PCR is a simple technique to be established initially for providing rapid prenatal diagnosis to the couples with known sickle cell mutation. The sensitivity of ARMS-PCR can be increased by using suitable techniques to detect maternal cell DNA contamination.

Invasive prenatal diagnostic techniques

As aneuploid screening evolves to the first trimester a complimentary diagnostic procedure becomes increasingly important. Chorionic villus (CVS) sampling has emerged as the only safe invasive prenatal diagnostic procedure prior to the 14(th) week of gestation. Over 2 decades of experience have demonstrated the accuracy, efficacy and safety of CVS. The karyotype is identical to that of the fetus in over 98% of cases. In the remaining 1 to 2% confined placental mosaicism (CPM) occurs. Current knowledge of the etiology of CPM allows accurate clinical interpretation, the recognition of cases of fetal uniparental disomy, and identification of pregnancies at risk for poor perinatal outcome. Prospective comparative studies have demonstrated that with equally experienced operators, CVS and second trimester amniocentesis have similar procedure induced miscarriage rates. When CVS procedures are performed after 10 weeks gestation, no increased risk of fetal anomalies has been demonstrated.

A simple VNTR-PCR method for detecting maternal cell contamination in prenatal diagnosis

The effectiveness of variable number tandem repeats (VNTRs) was evaluated in the detection of maternal cell contamination. Nonradioactive PCRs were performed on 30 sets of prenatal tissue using VNTRs as primers. The combination of two VNTRs (YNZ22 and APOB) provided information on all 30 cases, distinguishing maternal-fetal genotype patterns and detecting maternal cell contamination in 5 of 30 prenatal cases. The amplification of these two VNTRs does not require radioactive or fluorescence labeling, and a small gel electrophoresis is sufficient to see the maternal-fetal genotype pattern. By this method, detection of maternal cell contamination in prenatal tissues can be obtained in 1 day, without the use of expensive instruments, thus providing DNA laboratories a very sensitive, rapid, and simple proof pretest on all prenatal tissues before performing the final genetic diagnostic testing.

Chorionic villus sampling

Chorionic villus sampling (CVS) has been used a successful and safe first-trimester prenatal diagnostic technique for over 12 years. Developed to avoid the medical and psychological complications of later prenatal diagnosis by amniocentesis, CVS rapidly has become a primary tool for the diagnosis of fetal cytogenetic, molecular, and biochemical disorders. In addition, its development has led to an improved understanding of several biological processes, including confined placental mosaicism and uniparental disomy.

The sensitivity of allele-specific polymerase chain reaction can obviate concern of maternal contamination when fetal samples are genotyped for immune cytopenic disorders

OBJECTIVE

Fetuses at risk for immune cytopenic disorders can be identified by molecular genotyping assays. To better understand the impact of maternal contamination on genotyping results, the levels of contamination that are routinely encountered during prenatal testing of fetal samples and the sensitivity of allele-specific polymerase chain reaction in detecting paternal alloalleles were examined.

STUDY DESIGN

Reconstitution experiments were performed to define the sensitivity of allele-specific polymerase chain reaction assays. The sensitivities of allele-specific polymerase chain reactions and polymerase chain reaction-restriction fragment length polymorphism were compared for detection of the factor V Leiden mutation.

RESULTS

A quantitative analysis of variable-number tandem repeat loci revealed maternal contamination in 4 of 56 fetal samples. Contaminating deoxyribonucleic acid compromised genotyping results when it comprised between 94% and 99% of the total deoxyribonucleic acid. Allele-specific polymerase chain reaction was found to be the more sensitive technique (0.8% sensitivity vs 13% sensitivity).

CONCLUSION

These results illustrate that allele-specific polymerase chain reaction is well suited for reliable prenatal identification of fetuses at risk of immune cytopenic disorders.

Detection of transforming oncogenes in rat colon tumors induced by direct perfusion with N-methyl-N-nitrosourea

We assayed rat colon tumors induced by N-methyl-N-nitrosourea (MNU) for transforming oncogenes by the NIH 3T3 transfection and nude mouse tumorigenicity assays. Transfection of DNA from 3 of 3 adenomas and 3 of 5 carcinomas induced transformed foci on NIH 3T3 cells. DNA from 2 of 3 primary foci also possessed focus-forming activity, and rat-specific sequences were observed in secondary focus DNAs. Furthermore, NIH 3T3 cells transfected with DNA from a carcinoma and from a primary focus derived from it, both positive in the focus-forming assay, induced tumors in nude mice. We found no evidence for rat H-ras, K-ras, or N-ras sequences in the DNA of any of 16 primary foci derived from 6 rat tumors; thus, in contrast to other animal tumor models induced by MNU, activation of the ras genes does not appear to predominantly occur in MNU-induced rat colon tumors. We also did not observe, in any of these foci, sequences corresponding to the rat neu, raf, fms, met, or hst genes, thus indicating that none of these is the transforming oncogene in our model. These results suggest that an as yet unidentified transforming oncogene may be activated in rat colon tumors induced by MNU.

Oncogene alterations in rat colon tumors induced by N-methyl-N-nitrosourea

The authors assayed oncogene alterations in rat colon tumors induced by the direct-acting chemical carcinogen, N-methyl-N-nitrosourea (MNU). DNA isolated from 34 adenomas and eight carcinomas, as well as adjacent normal colon, of 11 rats was assayed by Southern blotting for restriction fragment length polymorphisms and gene amplifications and deletions in 13 oncogenes known to be involved in human or other animal tumors. In addition to finding apparent point mutations or other small alterations in the fos and abl genes in individual rat colon tumors, the authors observed what appear to be larger alterations (ie, rearrangements, or intragenic insertions or deletions) in the H-ras and myb loci in several tumors. In contrast, no changes in the K-ras, N-ras, myc, N-myc, neu, raf, fms, met, and hst genes were seen in any of these tumors. The frequency of myb gene alterations was higher in carcinomas than in adenomas, suggesting that these changes occurred relatively late during tumorigenesis and were not direct effects of the carcinogen. In addition, the finding of alterations in two or three oncogenes in several MNU-induced rat colon tumors suggests the possibility of more widespread genomic lesions in this model.

Genotyping of macerated stillborn fetuses

It is generally impossible to collect blood or to culture tissue from a macerated stillborn fetus. Accurate genotyping of such a fetus may, however, be critical for the diagnosis of genetic diseases and appropriate genetic counseling. In the East Flanders Prospective Twin Study, placental tissue of twin and triplet sets, in some of which one or both members were stillborn and macerated, has been stored at -20 degrees C. Of all these fetuses, sex and zygosity could be determined accurately on the placental deoxyribonucleic acid. We tested the possibility of nongenetic changes in deoxyribonucleic acid that result from maceration or tissue degradation over time in storage on placental samples from monochorionic twins in which only one member was stillborn and macerated. The deoxyribonucleic acid variants in these monozygotic twins were identical whether or not either cotwin was macerated. Thus deoxyribonucleic acid variants can be determined accurately on the placental tissue of macerated fetuses, even after prolonged freezing.

Prenatal treatment in congenital adrenal hyperplasia due to 21-hydroxylase deficiency: up-date 88 of the French multicentric study

A multicentric study of prenatal treatment of congenital adrenal hyperplasia (CAH) resulting from 21-hydroxylase deficiency in 43 pregnancies at risk for CAH is presented. The mothers were given dexamethasone per os, 0.5 mg either 12-hourly or 8-hourly. From the analysis of the results obtained in the present study and review of the literature, it would appear that the first condition for successful prevention of female virilization in utero (a total of 6 cases) is to start treatment as early as possible, no later than the 7th week. The dose of dexamethasone should be related to maternal size: 20 micrograms/kg/day (in 2 or 3 fractioned) doses would seem to be both efficient and safe. Adrenal suppression of both maternal and fetal adrenal function should be controlled by appropriate hormonal determinations. Finally, the advantages of early prenatal diagnosis or no prenatal diagnosis are discussed.

Discordance in deoxyribonucleic acid analysis of fetus and trophoblast

Molecular analysis of deoxyribonucleic acid extracted from trophoblast has become an important component of modern prenatal diagnosis. Discrepancies between trophoblast and fetal tissue in both chromosomal and metabolic analysis have been reported. We studied 50 paired fetal and trophoblast deoxyribonucleic acid specimens with a highly polymorphic probe for repetitive "minisatellite" regions of deoxyribonucleic acid that may be areas of high recombination. Four pairs showed persistent variations in bands consistent with differences in fetal and trophoblast deoxyribonucleic acid. Discordance in trophoblast deoxyribonucleic acid as a reflection of fetal genotype may be secondary to mitotic recombination or sister chromatid exchange in the trophoblast, and the possibility of discordance must be considered in interpreting the results of prenatal diagnostic testing on trophoblast specimens.

Comparison of maternal and fetal chromosome heteromorphisms to monitor maternal cell contamination in chorionic villus samples

Maternal cell contamination (MCC) presents a potential problem in the analysis of chorionic villus sampling (CVS) preparations for early prenatal diagnosis by chromosomal, biochemical and molecular methods. Through the comparison of fluorescent chromosome variants from CVS and maternal cells, we found three out of 50 samples to have MCC. One of these was observed on a direct preparation. Routine chromosome heteromorphism analysis is suggested as a reliable method for monitoring MCC in CVS specimens.

Clinical applications of gene probes in human genetic disease, malignancy, and infectious disease

Recent developments in recombinant DNA technology have made possible the production of gene probes consisting of cloned gene segments, cloned segments of DNA linked to genes, and synthetic gene fragments. Several methods have been developed by which these probes may be used for the diagnosis of human disease. This technology has been outstandingly successful for prenatal diagnosis and carrier detection in many genetic diseases. These methods have also been successfully applied to the analysis of human malignancies, by providing for the determination of cell lineage and clonality in lymphoid neoplasms. Finally, these methods have shown potential for rapid and sensitive diagnosis of some infectious diseases.

Cytogenetic analysis of chorionic villi: a technical assessment

Eighty-five samples of chorionic villi from women undergoing prenatal diagnosis at 8 to 12 weeks' gestation were subjected to cytogenetic analysis. Samples were prepared by a direct technique that permits limited analysis within two hours and by a short-term culture technique that permits detailed structural analysis within one week. An adequate number of cell divisions for cytogenetic analysis was obtained from 96% of living fetuses. Using both the direct technique and short-term culture, satisfactory banded chromosomal preparations were made in 93% of cases. Eleven of 12 pregnancies (92%) shown by ultrasound to be dead shortly before sampling, had cytogenetic abnormalities. Further studies are needed to develop banding definition equivalent to that available on cultured amniocytes.

Chorionic villus sampling

Chorionic villus sampling in the first trimester of pregnancy has the potential to become a major tool in the prenatal diagnosis and therapy of genetic disorders. Villus samples can be used for cytogenetic and biochemical studies as well as DNA analysis. However, little is known about the effects of chorionic villus sampling on a continuing pregnancy, or the long-term effects on the subsequently delivered infants. Despite these limitations, chorionic villus sampling appears to be a major breakthrough in prenatal diagnosis.

Comparative study of 15 lysosomal enzymes in chorionic villi and cultured amniotic fluid cells. Early prenatal diagnosis in seven pregnancies at risk for lysosomal storage diseases

A large number of chorionic villi samples obtained from women undergoing elective first trimester termination of pregnancy was analysed by enzyme assays similar to those applied to cultured amniotic cells. The levels of 15 lysosomal enzymes were compared to those observed in tissue cultures of amniotic cells obtained through amniocentesis at 16-18 weeks of pregnancy and the results were discussed in order to assess the usefulness of trophoblast biopsy for first trimester diagnosis of hereditary lysosomal diseases. The data suggest the applicability of this source of fetal cells for prenatal diagnosis of fifteen respective genetically determined enzyme deficiencies with the probable exception of alpha-L-iduronidase deficiency. Enzyme determinations were performed on chorionic villi samples of two pregnancies at risk for Tay-Sachs disease, three pregnancies for GM1 gangliosidosis type 1, one for mucopolysaccharidosis type VI and one for Wolman's disease.

Prenatal diagnostic techniques. Chorionic villus sampling

Chorionic villus sampling (CVS) is a new prenatal diagnostic technique which is performed in the first trimester of pregnancy. Traditional methods of prenatal diagnosis, including amniocentesis and fetoscopy, must be performed in the midtrimester. In concert with the development of DNA methods of fetal cell analysis, first trimester fetal diagnosis utilizing CVS offers many advantages over traditional mid-trimester techniques. In addition, CVS may potentially allow therapeutic intervention to prevent or ameliorate some congenital defects.

Chorionic villus sampling for first-trimester prenatal diagnosis: Northwestern University program

We present our initial experience in developing a chorionic villus sampling program at Northwestern University. In phase 1, we performed chorionic villus sampling in 58 patients prior to elective first-trimester abortion, assessing the reliability and reproducibility of obtaining adequate villus samples and performing cytogenetic analysis by means of both the direct and culture methods. Specimens were categorized according to quality: class I, multiple identifiable villi (n = 20); class II, few villi or villi mixed with decidua (n = 15); class III, no villi (n = 23). There was a positive trend between operator experience, amount of villi obtained, and quality of cytogenetic preparations. In March, 1984, we received Institutional Review Board approval to perform chorionic villus sampling in continuing pregnancies (phase 2). Among the first 20 cases we found two abnormalities (47,XY, + 13; 45,X). The remaining 18 pregnancies were continuing. Recommendations are made for developing a chorionic villus sampling program.

Molecular approach to analyzing the human 5p deletion syndrome, cri du chat

DNA unique or low-copy fragments were isolated from a genomic DNA library specific for the short (p) arm of human chromosome 5. These chromosome 5p-specific DNA fragments were used to analyze, by Southern blot experiments, somatic cell hybrids that retained either a normal chromosome 5 homolog or a homolog with a partial deletion of 5p, which was derived from either of two persons with the common human deletion syndrome, cri du chat or 5p- syndrome. In these studies, two classes of DNA fragments were identified, those located outside the region deleted in the persons with cri du chat and those located within the deleted region. This latter class of DNA probes will help to define, at the molecular level, a region of 5p that is critical in producing the phenotype associated with the cri du chat syndrome.

Genetic screening of prospective parents and of workers: some scientific and social issues

Genetic screening programs are based on assumptions and values that reflect the history of racial and social eugenics in the United States and Europe. They stigmatize individuals by shifting the focus from social, economic, and political decisions that affect the health of prospective parents, newborns, and workers to "bad genes," that is, intrapersonal factors that are given the status of "causes" of disease. Prenatal screening, at best, can help the relatively few individuals who know that their future children are at risk for a particular inherited disease or disability; it has little positive value for the average person. Workplace genetic screening has not been shown to reduce occupational disease, but it has led to employment discrimination and has drawn attention away from controlling exposures to toxic chemicals in the workplace.

Chorionic villi sampling: clinical experience, immediate complications, and patient attitudes

To develop chorionic villi sampling as a procedure for prenatal diagnosis, a pilot study was undertaken to perfect the obstetric and laboratory techniques, to evaluate our success with the procedure in continuing pregnancies, and to assess the attitudes of potential users of the procedure. Women about to have elective first-trimester abortions for nongenetic reasons were enrolled in the first phase of the study. Of the patients with a positive pregnancy test, 12.4% were found to have a nonviable pregnancy on ultrasound examination. Samples adequate for cytogenetic analysis were obtained in 130 of the 155 remaining cases, and the success rate was 93% in the 100 most recent cases. Direct cytogenetic analysis was undertaken in those cases successfully sampled, and karyotypes could be prepared in 97%. Immediate complications occurred in 5% of the pregnancies. Eight women at risk of bearing a child with a genetic defect had diagnostic chorionic villi sampling. Cytogenetic analysis was performed successfully on all of them. One had an induced abortion following the procedure because of the fetal diagnosis (a male with a 50% risk of Duchenne's muscular dystrophy). The other pregnancies are continuing uneventfully at 22 to 35 weeks' gestation. Finally, from preliminary analysis of our survey of potential users it appears that women 35 years old or over would prefer chorionic villi sampling to amniocentesis if the risks of the sampling were known to be low.

Recombinant DNA approach to X-linked mental retardation

Currently, approximately 115 X-linked diseases have been documented. Many of these defects are neurological in nature and about 25% result in a broad spectrum of mental impairments which can be distinguished by biochemical or clinical means. It is not known how many of the retarded population carry an X-linked defect due to lack of a distinct marker. By constructing a molecular linkage map of the X chromosome using DNA polymorphisms, it will be possible to identify these individuals, classify their disorder by the chromosomal region in which their polymorphism is detected, offer genetic counseling and prenatal diagnosis to their family members and try to determine which gene the polymorphism is linked to in order to identify the defect and devise therapy.

Genetic screening: marvel or menace?

Genetic screening is a systematic search in the population for persons of certain genotypes. The usual purpose is to detect persons who themselves or whose offspring are at risk for genetic diseases or genetically determined susceptibilities to environmental agents. Is genetic screening a marvel about to free us from the scourge of genetic disease or a menace about to invade our privacy and determine who may reproduce? There are three different types of genetic screening. Newborn screening identifies serious genetic disease at birth, permitting prompt treatment to prevent mental and physical retardation. Fetal screening and prenatal diagnosis identify genetic disease in the fetus permitting selective termination of pregnancy and the opportunity to have children free of defects detectable in utero. Carrier screening identifies individuals heterozygous for a gene for a serious recessive disease who may be at risk for affected offspring. The challenge to society is to provide (by way of cost-effective programs) expert services, including genetic counseling and follow-up, to all who may benefit, to ensure confidentiality and freedom of choice, and to avoid misunderstanding and stigmatization. It is recommended that the objective of screening programs should be to maximize the options available to families at risk rather than to reduce the incidence of genetic diseases. Whenever possible, the providers of these services should be the providers of primary health care. Urgently needed are a greater awareness of avoidable genetic diseases on the part of primary care providers and efforts to familiarize the public with the basic concepts of human genetics through the public school system.

Chromosome analysis of first trimester chorionic villus biopsies prepared by a maceration technique

A technique for the culture of chorionic villi for cytogenetic studies is described which is simple, reliable, rapid, and suitable for routine laboratory use. This method eliminates the need for multi-stage and time-consuming procedures inherent in previous techniques which have been developed for this tissue. Cultures suitable for analysis were obtained in an average of sixteen days and the risk of maternal cell contamination was evaluated. Results of cultures initiated from 50 chorion biopsies obtained by transcervical blind aspiration at a weekly termination clinic are presented, and the potential use of this technique as a routine procedure is discussed.

DNA and enzyme studies on chorionic villi for use in antenatal diagnosis

A study has been undertaken to determine the efficiency of current methods in providing an adequate amount of chorionic villus DNA for antenatal diagnosis using recombinant DNA techniques or enzyme assay. Chorionic biopsies were obtained from 40 women undergoing elective first trimester termination of pregnancy (8-12 weeks) under general anaesthesia. The villus tissue was isolated from maternal tissue under a dissection microscope and the presence of any remaining contamination was ascertained by conventional histology and immuno-cytochemical examinations. A high level of success was achieved in obtaining a pure fetal sample. In the first 20 samples the DNA yield obtained using the method of Williamson et al [1] was found to be 0.5 +/- 0.5 micrograms/mg wet weight of villus tissue (mean +/- 1 SD). In the subsequent 20 biopsies using a modified procedure, the yield was significantly improved to 1.0 +/- 0.65 (p less than 0.002). A normal range for the enzyme iduronate sulphatase, which is deficient in Hunter's syndrome (mucopolysaccharidosis II), is also reported. It is suggested that as little as 20 mg of chorionic villi may be used to provide sufficient material for a reliable study using recombinant DNA or biochemical methods.

Molecular genetics, recombinant DNA techniques, and genetic neurological disease

The molecular defects responsible for Huntington's disease, the spinocerebellar degenerations, myotonic muscular dystrophy, neurofibromatosis, and tuberous sclerosis, among other major dominant inherited diseases of the nervous system, will be identified using the new techniques of molecular genetics. With synthesized nucleic acid segments complementary to portions of the patient's DNA, known as complementary DNA probes, it will be possible to identify and isolate the mutant gene responsible for a particular disease. These events are referred to as gene cloning. In addition, complex genetic regulatory mechanisms involved in cell differentiation during neuroembryogenesis will be elucidated with the application of these strategies. It is important for the clinician to become familiar with the precision and potential of these new methodologies, because they will soon influence significantly the practice of neurology.

Chorion villi sampling (CVS) for prenatal diagnosis of genetic disorders: first results and future research

Chorion villi sampling (CVS) in the first trimester of pregnancy has become available recently as an alternative method to second trimester amniocentesis for prenatal diagnosis of genetic defects. Currently there are six different tissue sampling techniques being investigated in different centers around the world, but there are very few trials in ongoing pregnancies. From chorionic villi material cytogenetic and biochemical studies as well as DNA analyses can be performed. Different methods of chromosome analysis are being investigated at the University of California San Francisco and elsewhere to determine the most efficient and reliable techniques. Larger studies will be needed to establish the efficacy and the safety of the chorionic villi sampling procedure for the mother and the developing fetus. Although CVS is preferable to amniocentesis for psychological and medical reasons (earlier elective termination of a fetus with a genetic disorder), many questions remain to be answered in carefully controlled studies.

Diagnostic application of first trimester trophoblast sampling in 100 pregnancies

The results of the diagnostic application of first trimester trophoblast sampling in 100 pregnancies are reported in detail. Further improvement of the method for routine, direct chromosome analysis resulted in a technique which proved to be fast, simple, and efficient. We found that short-term incubation of villi permits the application of many experimental methods, such as visualization of sister chromatid exchanges and bromodeoxyuridine (BrdU) incorporation. Fetal karyotyping was successful in each of the 96 pregnancies in which fetal material was obtained from a total of 98 fetuses. There were 42 males and 56 females, and an abnormal chromosome constitution was found in 12 cases. Two trisomic fetuses were found among the eight pregnancies at risk for Duchenne muscular dystrophy, and this indicates that fetal sexing (which is achieved with our method in two hours) should not be performed without chromosome visualization. The results indicate a risk of 8% of an abnormal fetus for mothers aged 35 years or more, while the risk of failure of sampling and of spontaneous abortion after villi sampling were 4 and 6%, respectively. Enzyme determinations were performed in three pregnancies at risk for gangliosidosis GM1, Niemann-Pick disease, and Hurler syndrome. In this last case inconsistency between the results of the assay of iduronidase on chorionic villi and amniotic fluid cells was found. This unexplained error indicates the need for extensive characterisation in chorionic villi of the series of enzymes involved in metabolic diseases.

Clinical use of DNA markers linked to the gene for Duchenne muscular dystrophy

Seventy families with Duchenne muscular dystrophy (DMD) known to the Institute of Child Health fall into three categories with respect to potential linkage analysis with the X chromosome DNA markers RC8 and L1.28 that bridge the DMD gene. Families in which there is at least one obligatory female heterozygote (n = 13). Here 'prediction' and 'exclusion' of DMD gene transmission may be possible, the accuracy being dependent on the closeness of the linkage of the DNA marker(s) to the DMD gene; an illustrative case is reported. Families in which there is a single affected boy, who also has one or more healthy brothers (n = 26). Given an informative restriction fragment length polymorphism (RFLP), the probability that the boy represents a new mutation can be reassessed; it is also possible to 'exclude' the DMD gene in a sister. Families with a single affected boy with no brother (n = 30). Here 'exclusion' of the DMD gene in a sister may be possible. Only in one family was there no possibility of useful linkage analysis. The linkage analysis required is described, and the need to check DMD families for informative RFLPs is stressed.

Close linkage of fragile X-mental retardation syndrome to haemophilia B and transmission through a normal male

The fragile X-mental retardation syndrome is defined by a moderate to severe mental retardation associated with a cytogenetic marker, a fragile site localized on the long arm of the X chromosome at band Xq 27. This syndrome has recently been recognized as one of the major causes of genetically determined mental retardation, and as one of the most important X-linked diseases with respect to its frequency (analogous to that of Duchenne muscular dystrophy or of haemophilia A) and severity. In the absence of treatment, genetic screening for this disease would seem particularly important. Prenatal diagnosis is now feasible although difficult and detection of heterozygous carriers is only possible in approximately 50% of cases. The recent demonstration of genetic linkage between the glucose 6-phosphate dehydrogenase (G6PD)-colour blindness cluster (at Xq28) and the fragile X locus has suggested that the fragile site is indeed the site of the mutation. We show here that the fragile X and haemophilia B loci are closely linked, using as genetic marker a polymorphism of the coagulation factor IX gene. Our study of a large family has demonstrated transmission through a phenotypically normal male, a feature previously described in retrospective analysis of a few other fragile X pedigrees. Restriction polymorphisms associated with the factor IX gene should be useful for analysing this peculiar aspect of the genetics of the fragile X syndrome, and for genetic screening of the disease.

Prenatal diagnosis of sickle-cell anemia in the first trimester of pregnancy

To investigate the usefulness of chorionic biopsy for prenatal diagnosis of sickle-cell anemia by restriction-endonuclease analysis of fetal DNA, we studied 30 pregnancies before elective abortion. When the reproducibility of the technique for obtaining adequate DNA samples was established, we successfully applied the test to five pregnancies at risk for sickle-cell anemia. In two cases, sickle-cell disease of the fetus led to a decision to terminate the pregnancy. In three other cases, a normal or AS genotype was demonstrated. One normal infant has been born, and one other pregnancy is continuing normally. In one case in which fetal death was observed three weeks after sampling, placental abnormalities found on histologic examination were compatible with a chromosomal aberration. Our study shows that chorionic biopsy is feasible for the prenatal diagnosis of sickle-cell disease before the 10th gestational week. If subsequent experience demonstrates this technique to be safe enough for mother and fetus, the ability to test in early pregnancy may make prenatal diagnosis acceptable to more couples at risk for serious genetic disorders.