Down syndrome congenital heart disease: a narrowed region and a candidate gene

PURPOSE

Down syndrome (DS) is a major cause of congenital heart disease (CHD) and the most frequent known cause of atrioventricular septal defects (AVSDs). Molecular studies of rare individuals with CHD and partial duplications of chromosome 21 established a candidate region that included D21S55 through the telomere. We now report human molecular and cardiac data that narrow the DS-CHD region, excluding two candidate regions, and propose DSCAM (Down syndrome cell adhesion molecule) as a candidate gene.

METHODS

A panel of 19 individuals with partial trisomy 21 was evaluated using quantitative Southern blot dosage analysis and fluorescence in situ hybridization (FISH) with subsets of 32 BACs spanning the region defined by D21S16 (21q11.2) through the telomere. These BACs span the molecular markers D21S55, ERG, ETS2, MX1/2, collagen XVIII and collagen VI A1/A2. Fourteen individuals are duplicated for the candidate region, of whom eight (57%) have the characteristic spectrum of DS-CHD.

RESULTS

Combining the results from these eight individuals suggests the candidate region for DS-CHD is demarcated by D21S3 (defined by ventricular septal defect), through PFKL (defined by tetralogy of Fallot).

CONCLUSIONS

These data suggest that the presence of three copies of gene(s) from the region is sufficient for the production of subsets of DS-CHD. This region does not include genes located near D21S55, previously proposed as a "DS critical region," or the genes encoding collagens VI and XVIII. Of the potential gene candidates in the narrowed DS-CHD region, DSCAM is notable in that it encodes a cell adhesion molecule, spans more than 840 kb of the candidate region, and is expressed in the heart during cardiac development. Given these properties, we propose DSCAM as a candidate for DS-CHD.

An excess of testicular germ cell tumors in Down's syndrome: three case reports and a review of the literature

BACKGROUND

The incidence of specific solid tumors in Down's syndrome (DS) is not well established. Testicular germ cell tumors (TGCT) might be increased in this population.

METHODS

The presence of TGCT among male subjects from the French department of Corrèze was recorded and literature on the subject reviewed.

RESULTS

A total of 120 living children and adults with DS and 17 pregnancies (12 births and 5 therapeutic abortions) were examined over an 8-year period (1987-1994). Three TGCT were diagnosed. A seminoma and an embryonal carcinoma were observed in two young adults and an intratubular germ cell neoplasm in a 22-week-old fetus. Because testicular tumors occur at an incidence rate of 4 cases per 100,000 person-years in the general population, these observations suggest a clearly increased risk of developing TGCT in the DS population. In addition, a review of the literature also shows an excess of TGCT in this population. Cryptorchidism alone, which is prevalent in individuals with DS, cannot explain this significantly increased incidence of TCGT. The authors hypothesize that an excess of luteinizing hormone and follicle-stimulating hormone gonadotropins and overexpression of the Ets-2 gene through gene dosage effect could predispose patients with DS to the development of TGCT.

CONCLUSIONS

Surveillance of the gonads of male patients with DS is recommended. A better understanding of the factors involved could also help to identify risk factors in the general population.

Syndrome of proximal interstitial deletion 4p15: report of three cases and review of the literature

We report on two boys and a girl with interstitial deletion in the short arm of chromosome 4 including the segment p15.2p15.33. All had normal growth with psychomotor retardation, multiple minor congenital anomalies, and a characteristic face distinct from that of the Wolf-Hirschhorn syndrome. One of the patients had congenitally enlarged penis. These patients resemble some of the previously reported patients with similar cytogenetic abnormalities and suggests the recognition of a specific clinical chromosome deletion syndrome.

Canadian multicenter randomized clinical trial of chorion villus sampling and amniocentesis. chromosome mosaicism in CVS and amniocentesis samples

Data on 1040 chorionic villus and 969 amniotic fluid samples were collected from women studied in the Canadian Multicentre Randomized Clinical Trial of Chorion Villus Sampling and Amniocentesis. Cytogenetic results were obtained from 98.0 per cent of chorionic villus samples and from 99.9 per cent of amniotic fluid samples. Level I mosaicism (a single cell with an abnormal karyotype) occurred frequently in both chorionic villus and amniotic fluid samples and appeared to have no clinical significance. Level II mosaicism occurred in 0.9 per cent of CVS mesenchyme and 1.5 per cent of amniotic fluid cultures and in general was not perceived to be of sufficient concern to warrant cytogenetic follow-up studies. Level III mosaicism was reported in 18 CVS cases (15 cytotrophoblast, 1 mesenchyme, and 2 with both cell methods) and in one amniotic fluid case. In all cases but one (fetus with trisomy 18), level III mosaicism was confined to the placenta. Maternal cell contamination occurring with a frequency of 6.4 per cent in the mesenchyme analyses was a concern. This study supports the final report of the Canadian Multicentre Randomized Clinical Trial of Chorion Villus Sampling and Amniocentesis. Cytogenetic analysis of chorionic villus samples appears to be an acceptable alternative to the analysis of amniotic fluid samples. However, because of mosaicism and maternal cell contamination concerns, the examination of both cytotrophoblast preparations and mesenchyme cultures from chorionic villus samples is recommended.

Triploidy arising from a first meiotic non-disjunction in a mother carrying a reciprocal translocation

We report on a 22 year old mother, a carrier of a 6;14 balanced reciprocal translocation, who aborted a triploid conceptus carrying a similar translocation. We showed the maternal origin of this triploidy, after non-disjunction at meiosis I. The phenotypic expression as a non-molar pregnancy, the contribution of the maternal translocation, and possible aetiological factors of triploidy are discussed.

MCA/MR syndrome in a female infant with tetraploidy mosaicism: review of the human polyploid phenotype

We report on a 3-month-old girl with unusual facial appearance, short neck with low posterior hairline, wide chest, valvular pulmonic stenosis, abnormal fingernails, and diploid-tetraploid mosaicism (46,XX/92,XXXX in 7.2% of peripheral leucocytes and in 29% of skin fibroblasts). Comparison with 11 previously reported cases with mosaic or complete tetraploidy does not establish an easily recognizable syndrome. However, a malformation pattern is apparent when tetraploidy patients are compared with 14 cases of triploid mosaicism and 44 previously reported cases of nonmosaic triploidy. A history of sex hormone exposure was present in 5 of 11 pregnancies resulting in tetraploidy; this exposure may correlate with the occurrence of tetraploidy in polycystic ovary syndrome and in tumors of the female reproductive tract. The mechanism of dysmorphogenesis involved in polyploidy is considered, including hypotheses of altered nuclear/cytoplasmic ratio, of trophoblastic alteration, of delayed cell division, or of altered autosome/active X chromosome ratio.

A chromosome marker for the early detection of mouse embryos carrying the neural tube defect mutation splotch

A major problem in the study of neural tube defects caused by the splotch (Sp) gene in the mouse has been the identification of gene carriers or potentially affected embryos at an early stage of development, since the gene's effects become visible only late in gestation or after birth. To aid in the identification of Sp carriers, we have developed a technique using a Robertsonian translocation as a marker for this gene. The accuracy of identification is reduced by crossing-over between the Sp locus and the centromere but, because of crossover suppression in the particular cross used, there was only 23.2% recombination compared with the known map distance of 36%. Paternal age had no effect on the frequency of recombination, but individual males differed significantly in the degree of crossover suppression.

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.

Chromosomal changes in secondary leukemias of childhood and young adulthood

The increasing success of antineoplastic therapy has resulted in a growing number of long-term survivors. These people are at risk for complications of the therapy itself. Among these induced acute nonlymphoid leukemia (ANLL) has been both common and often lethal. We reviewed 72 recently reported patients under 30 years of age at the time of initial diagnosis who developed a secondary, karyotypically defined leukemia. Fifty-eight patients contracted ANLL a mean of 4 1/2 years from the initial diagnosis. In 25 patients, this was preceded by a preleukemic phase characterized by a hypercellular bone marrow with abnormal precursors, often accompanied by peripheral pancytopenia, that lasted a mean of 6 months. Three additional patients died in this preleukemic phase. In all 61, the most common chromosomal abnormalities were numerical errors. Twenty-four patients had a hypodiploid karyotype, most often in those in whom the primary diagnosis was lymphoma (22 of 43). The most common chromosomes missing in whole or in part were number 7 (18 patients), number 5 (8 patients), number 17 (5 patients), and number 21 (4 patients). The anomalies were frequently multiple and complex. Monosomy 7 figured particularly strongly and may be similar to a karyotypically identical myeloproliferative disorder characterized by micromegakaryocytes, giant platelets, and abnormal granulocyte function arising de novo in children. These findings are similar to those in older patients with ANLL induced by environmental carcinogens or antineoplastic therapy. They are different from the karyotypic changes seen in de novo ANLL in children and young adults, suggesting a different etiology. Also, they reinforce the need to find less leukemogenic treatment programs.

Folic acid blinded trial in identical twins with fragile X syndrome

Monozygous twin 14-year-old mentally retarded boys with the fragile X syndrome were treated either with 10 mg folic acid by mouth daily or with a placebo for three test periods of 3-month duration each in a blind study. For each twin, tests of cognitive functioning, reading, spelling, and math skills, and linguistic and perceptual skills were compared. Although there was considerable variation in performance on these tests during the two baseline periods, there were no observable beneficial effects of therapy. The routine use of folic acid in patients with established mental retardation and the fragile X syndrome is not indicated.

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.

Fetal mortality at the time of chorionic villi sampling

To estimate the background fetal loss rates among women who might be candidates for chorionic villi sampling (CVS) for prenatal diagnosis, we examined the frequency of spontaneous abortion and of non-viable fetuses in two groups of women thought to be pregnant at 8-12 weeks' gestation. Among 1519 women over 35 years given an appointment for amniocentesis 1978-1981, 9.8% had a spontaneous abortion prior to 16 weeks' gestation. For those under observation before week 12, the loss rate by 16 weeks was 15.3%. Among all 190 candidates for elective termination of pregnancy between 6 and 12 weeks' gestation, 12.6% were found to have a non-viable fetus at the scheduled date of abortion. The frequency of non-viability was 14% among those seen before week 12. The data suggest that the background loss rate between the time of CVS and the time of amniocentesis is approximately 1-2% and is unlikely to be higher than 9%. Until randomized clinical trials of the procedure are completed we will not know how much, if at all, the loss rate associated with CVS is increased above this background. Nevertheless, knowledge of these background risk estimates may be useful in counseling women considering participating in trials of CVS.

Genetics of palate development

We are beginning to see a tantalizing picture by looking into the "genetic window" on the teratogenesis of cortisone-induced cleft palate and on normal palate development. We must continue to open this window. So far, all strains of mice that have been treated with cortisone do react with induced cleft palate and, from their dose-response behaviors, they appear to react in the same way. It is tempting to conclude that dosage tolerance is the only genetic variation in the reaction. We do not attach any significance to this because we must emphasize the fact that these different strains represent an insignificant sample of the hundreds of genetically different, recognized strains of the mouse. We have emphasized previously that systematic strain surveys must become an active part of teratological genetics in order to obtain an approximation of how much genetic variation exists in a trait in the species (e.g., Biddle, 1981). Now, we need to go beyond strain surveys. Crosses between strains must become part of this activity. Dominance appears to be a property of the homeostatic model of palate development as far as the cortisone probe is concerned, but it may not be a universal property. The significance of dominance and the architecture of gene action can only be interpreted by exploring more strain pairs in the mouse. A study of two or three or four strains is simply inadequate. From the crosses between a limited number of strains, major genetic factors in cortisone reactivity have been identified. These major genetic factors may regulate the homeostatic model of palate development, but we do not know this for a fact. The identification of major genetic factors has moved us to a new horizon; we can move now from a purely descriptive biology to a study of the functional biology of the process of palate development. This move will require not only the molecular probes with which to walk along the specific chromosomes to the major genetic factors but also a library of fully characterized strains of the mouse in which to use these probes. The library of strains that have been genetically characterized for the cleft palate reaction is missing.

Genetic models of carcinogenesis

The major genetic models of carcinogenesis are critically reviewed to determine their validity and relevance for clinical and experimental oncologists. Of major concern are the "two-hit" theory of Knudson and the host resistance system of Matsunaga. These models may be used to explain the pathobiologic peculiarities of human neoplasms, particularly those occurring in early life. It is proposed that certain benign and regressive tumors encountered in early life are expressions of the activity of the host resistance system.