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.

Evidence for genetic control of nondisjunction in man.

Data on factors associated with the occurrence of Down syndrome in a highly inbred population were evaluated to investigate the presence of a genetic control of nondisjunction in man. In Kuwait, close consanguinity occurs in 40% of marriages. In its main obstetric hospital, 20 trisomic Down babies out of 11,614 singleton births were delivered over a 12-month period. Chi-square analyses indicate the occurrence of Down syndrome to be linked to two independent factors: consanquinity of parents and maternal age. The relative risk is approximately four times greater for closely related than for nonrelated parents (P less than .005); a possible explanation for this is the existence of a gene that induces mitotic nondisjunction in the homozygous fertilized ovum. An alternative explanation is the existence of an autosomal recessive gene which results in meiotic nondisjunction in the homozygous parents. Consanguinity is usually perpetuated in certain families, or sections of the population, and parents in highly inbred families have a higher probability to be homozygotes for that gene.

Chromosome fragility and susceptibility of Bloom's syndrome fibroblasts to SV40 transformation.

A comparison of the frequencies of chromosomal aberrations and the rates of SV40 transformation was made using fibroblasts obtained from 2 patients with Bloom's syndrome (BS) and from a normal individual. BS cells were found to be more susceptible to chromosome damage, in confirmation of earlier reports, but surprisingly, BS cells were distinctly less prone to transformation.

Medical genetics for the otorhinolaryngologist.

Genetic disorders involving the ear, nose and throat can be looked at in essentially the same way that one would view other genetic problems. It is important to differentiate genetic disorders from those due to environmental influences. This may be difficult on clinical grounds, since similar clinical features may be produced by different environmental factors or by different genes or groups of genes. When the cause of the disorder can be established, the risks involved in a family having further offspring usually can be determined reasonably readily. In the uncharacterized situations, empiric risk figures are employed. Abnormalities of genetic origin fall into one of three main groupings: chromosomal abnormalities, single gene mutations or polygenic inheritance. These situations are discussed and examples are given.

Correlation between prognosis and bone marrow chromosomal patterns in children with acute nonlymphocytic leukemia: similarities and differences compared to adults.

The chromosomal complement of 28 children with the diagnosis of acute nonlymphocytic leukemia (ANLL) were examined. An abnormal cytogenetic pattern was found in 50% of these patients, which is similar to the results in adults with ANLL. Unlike the reports in adult patients, however, no specific chromosomal changes were found. This observation may imply that the etiology and mechanisms by which abnormal clones develop in ANLL could differ significantly between children and adults. Those patients with chromosomal abnormalities in their initial bone marrow sample had a median survival of 7.1 mo, whereas those with a normal diploid pattern in their bone marrow had a median survival of 20.5 mo (1-sided, p = 0.04). If all metaphases were abnormal, the median survival was only 3 mo.

Ocular findings in cytogenetic syndromes.

Several cytogenetic syndromes are reviewed, and the salient ocular and facial abnormalities that might lead to a diagnosis are pointed out. Examples are given of mongoloid slant to the palpebral fissures, not only in Down's syndrome, but also in monosomy 9p, where, in addition, the triangular skull is almost diagnostic. Antimongoloid slant is found in trisomy 9p, where the eyes also have enophthalmos of monosomy 9p. Hypertelorism is another common finding in these syndromes; in monosomy 5p it is almost always present, although it occurs in other conditions as well, including trisomy 12p. The ring 22 syndrome has a distinguishing finding called "doe's eyes" because of the shape of the palpebral fissures. Trisomy 13 has numerous ocular findings as well as skull and facial involvements.

Regional mapping of the gene for human UDPGal 4-epimerase on chromosome 1 in mouse-human hybrids.

Somatic cell hybrids between mouse and human cells containing two different reciprocal translocations involving human chromosome 1, 46,X,t(1;X)(q12;q26) and 47,XX,+21,t(1;17)(p32;p13), were studied for the expression of human uridine diphosphate galactose 4-epimerase (UDPGal 4-epimerase, E.C. 5.1.3.2) by starch-gel electrophoresis. Analysis of the hybrid clones for the expression of the enzyme and the presence of the translocation chromosome 1 has permitted the assignment of the gene for human UDPGal 4-epimerase to the pter yields p32 region of chromosome 1.

Detection of lateral asymmetry in the C band of human chromosomes by BrdU-DAPI fluorescence.

A lateral asymmetry in the C-band region of human chromosomes is revealed by 4',6-diamidino-2-phenylindole (DAPI) fluorescence in amniotic cells grown for one replication cycle in the presence of 5-bromodeoxyuridine (BrdU). Short-arm segments and satellites of acrocentric chromosomes fluoresce as brightly with quinacrine as the distal part of the long arm of the Y chromosome, and also show a lateral asymmetry. This asymmetry appears to be correlated with highly repetitious DNA, including satellite fractions responsible for C-band staining, and may reflect an unequal distribution of thymine residues between the two polynucleotide chains of the DNA in the C-band region. The C-band region of chromosome 9 fluoresces brightly in both sister chromatids and appears to be symmetrical. This may be the result of multiple inversions in the C-band heterochromatic region.

Variation in lateral asymmetry of human chromosome 1.

Variations in lateral asymmetry of human chromosome 1 were studied in 17 amniotic cell samples and eight blood samples by the 5-bromodeoxyuridine (BrdU) quenching of 4'-6-diamidino-2-phenylindole (DAPI) fluorescence. The size and the relative proportion of the bright fluorescent spots on each chromatid in the heterochromatic region of chromosome 1 (1qh) are variable from different amniotic (or blood) samples after one cycle of BrdU incorporation. However, the particular pattern for a given chromosome 1 is consistent within the individual sample. Size variations were classified into three groups, and variations in the pattern (proportion) of bright fluorescence on each chromatid in the 1qh region were classified into four groups. A preliminary estimate of the type and frequency of lateral asymmetry variations was obtained. These results suggest a high frequency of variability of heterochromatin in the population. The BrdU-DAPI fluorescence technique was found to be very useful for characterizing variations in the 1qh region; variations in organization of heterochromatin DNA with the 1qh region can be detected, and a simple system of nomenclature is proposed for naming the variations in this region.

Ambiguous genitalia in XX male children: report of two infants.

Two infants with ambiguous genitalia were recognized to have the XX male syndrome. Although most XX males have normal penile development, a review of the reported cases showed that eight of the 14 affected children, diagnosed before age 15 years, had penile abnormalities, most commonly hypospadias and/or chordee. This syndrome should be considered in children with incomplete genital differentiation. The available indirect evidence suggests that deficient testosterone production by the fetal testes accounts for the genital ambiguity. Although no explanation has been established for the presence of testes in the apparent absence of the Y chromosome, studies of the X-linked Xg blood group in XX males demonstrate a high frequency of unusual inheritance patterns. This implies that the abnormality in the transmission of maleness in affected families may also be X-related rather than autosomal.

Optical Studies of the interaction of 4'-6'-diamidino-2-phenylindole with DNA and metaphase chromosomes.

The optical absorption and fluorescence characteristics of 4'-6-diamidino-2-phenylindole (DAPI) with DNA and chromosomes were studied. There is a decrease in extinction coefficient and chift in the absorption spectra to a higher wavelength when the dye binds to DNA. The fluorescence of DAPI is enhanced by both A-T and G-C base-pairs. The enhancement by A-T rich is significantly greater than by G-C rich DNA. The chromosomes and the constrictions of human chromosomes 1 and 16; these regions are known to contain A-T rich DNA and show dull fluorescence when treated with quinacrine. This dye may be useful for identifying A-T rich region in chromosomes. The fluorescence of DAPI bound to polynucleotides or chromosomes is partially quenched by the introduction of BrdU. This suppression of dye fluorescence allows optical detection of sister chromatid exchanges and chromosome region containing DNA with an unequal distribution of thymidine between polynucleotide chains after BrdU incorporation.

Differential fluorescence of sister chromatids with 4'-6-diamidino-2-phenylindole.

Differential fluorescence of sister chromatids and sister chromatid exchanges (SCE) in chromosomes from human lymphocytes grown two replication cycles in medium containing 5-bromodeoxyuridine can be detected by fluorescence microscopy after staining with 4'-6-diamidino-2-phenylindole (DAPI). The DAPI fluorescence appears to be more stable than that of the dye 33258 Hoechst and may provide a more sensitive method for the detection of SCE.

Detection of sister chromatid exchanges by 4'-6-diamidino-2-phenylindole fluorescence.

This paper describes a 4'-6-diamidino-2-phenylindole (DAPI) fluorescent technique for differentiation of sister chromatids and for the study of sister chromatid exchanges (SCE) in mouse chromosomes. The advantages of the DAPI fluorescent technique are also described. Differences in the occurrence of SCE between the centromeric heterochromatin (C-banded) and the chromosomal arm chromatin were studied in mouse cells (RAG) with or without mitomycin C treatment. Single strand exchanges between the DNA double helices in the sister chromatids were not detected. SCE and chromosome breakage appeared to occur more frequently in the centromeric region than in the chromosomal arm. This might play an important role in chromosome evolution in mice.

The 9p- syndrome.

Six patients (4 females and 2 males) with terminal deletion of the short arm of chromosome 9 distal to band p22 are described. The disorder constitutes a clinically identifiable syndrome consisting of mental retardation, sociable personality, trigonocephaly, mongoloid eyes, wide flat nasal bridge, anteverted nostrils, long upper lip, short neck, long digits mostly secondary to long middle phalanges, and predominance of whorls on fingers. The findings suggest that the clinical features are antithetical to the trisomy 9p syndrome. The deleted chromosome segment is relatively small and could be easily overlooked. It is hoped that this delineation of clinical features seen in 9,p- patients may help in focusing attention on the small deletion.

Trisomy 19 q.

Two sibs with trisomy for the long arm of chromosome 19 are reported. The common features included flat facial profile with microcephaly, hypertelorism, ptosis, prominence of the glabella, small nose with anteverted nostrils and a characteristic fish-shaped mouth. In addition congenital heart disease, physical retardation and seizures were seen in both sibs. That tristomy 19q can be suspected clinically is emphasized.