We report on an 8-year-old girl with minor anomalies consistent with 18q- syndrome and mild developmental delay. Initially cytogenetics showed a terminal deletion of chromosome 21 with mosaicism for a small ring chromosome 21 as the only apparent karyotypic abnormality: mos 45,XX,-21/46,XX,+r(21) (48%/52%). Further studies including FISH and DNA analysis demonstrated a de novo unbalanced translocation of chromosomes 18 and 21 with the likely breakpoints in 18q23 and 21q21.1. Most of 21q was translocated to the distal long arm of one chromosome 18, and this derivative 18 appeared to lack 18q23-qter. The small ring chromosome 21 [r(21)], present in only 52% of the patient's blood lymphocytes, did not appear to be associated with the abnormal phenotype since all 13 chromosome 21 markers that were examined in genomic DNA were present in 2 copies, and the phenotype of the patient was consistent with the 18q- syndrome. The karyotype was reinterpreted as mos 45,XX,-18,-21,+der(18) t(18;21) (q23;q21.1)/46,XX,-18,-21,+der(18) t(18;21) (q23;q21.1), +r(21) (p13q21.1) (48%/52%). These results demonstrate the power of FISH in conjunction with DNA analysis for examination of chromosome rearrangements that may be misclassified by traditional cytogenetic studies alone.Close

Large terminal fragments of human chromosomes 2p, 6p, 8q, 12q, and 18q were cloned using yeast artificial chromosomes (YACs). RecA-assisted restriction endonuclease (RARE) cleavage analysis of genomic DNA samples from II unrelated individuals using YAC-derived probes confirmed the telomeric localizations of the half-YACs studied. The cloned fragments provide telomeric closure of maps for the respective chromosome arms and will supply the reagents needed for analyzing and sequencing these distal subtelomeric regions.Close

We describe a 17-month-old infant with clinical features of Down syndrome and a normal karyotype by standard chromosomal analysis, her two uncles aged 28 and 30 years, respectively, with reduced intelligence and unusual appearance but not apparent Down syndrome, and a severely retarded 6-year-old girl with dysmorphy and epilepsy from the same family. Cytogenetic studies of patients and normal intervening relatives had been carried out at different institutions with normal results. Fluorescence in situ hybridization using whole chromosome painting and unique-copy probes (cosmids) and high-resolution banding revealed a familial subtelomeric translocation of chromosomes 18 and 21, resulting in partial trisomy 21 in the infant and her two uncles, and partial monosomy 21 in the 6-year-old girl. Cytogenetic breakpoints were located in bands 18q23 and 21q22.1, respectively. The molecular breakpoint on chromosome 21 was located between D21S211 (proximal) and D21S1283 (distal) and thus maps within the Down syndrome critical region.Close

Recently much attention has been given to the detection of submicroscopic chromosome rearrangements in patients with idiopathic mental retardation. We have screened 27 subjects with mental retardation and dysmorphic features for such rearrangements using a genetic marker panel screening. The screening was a pilot project using markers from the subtelomeric regions of all 41 chromosome arms. The markers were informative for monosomy in both parents at 3661902 loci (40.6%, 95% confidence interval 37.0-44.2%) in the 22 families where DNA was available from both parents. In two of the 27 subjects, submicroscopic chromosomal aberrations were detected. The first patient had a 5-6 Mb deletion of chromosome 18q and the second patient had a 4 Mb deletion of chromosome 1p. The identification of two deletions in 27 cases gave an aberration frequency of 7.5% without adjustment for marker informativeness (95% confidence interval 1-24%) and an estimated frequency of 18% if marker informativeness for monosomy was taken into account. This frequency is higher than previous estimates of the number of subtelomeric chromosome abnormalities in children with idiopathic mental retardation (5-10%) although the confidence interval is overlapping. Our study suggests that in spite of the low informativeness of this pilot screening, submicroscopic chromosome aberrations may be a common cause of dysmorphic features and mental retardation.Close

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Human chromosomes terminate with specialized telomeric structures including the simple tandem repeat (TTAGGG)n and additional complex subtelomeric repeats. Unique sequence DNA for each telomere is located 100-300 kilobases (kb) from the end of most chromosomes. A high concentration of genes and a number of candidate genes for recognizable syndromes are known to be present in telomeric regions. The human telomeric regions represent a major diagnostic challenge in clinical cytogenetics, because most of the terminal bands are G negative, and cryptic deletions and translocations in the telomeric regions are therefore difficult to detect by conventional cytogenetic methods. In fact, several submicroscopic chromosomal abnormalities in patients with undiagnosed mental retardation or multiple congenital anomalies have been identified by other molecular methods such as DNA polymorphism analysis. To improve the sensitivity for deletion detection and to determine whether such cryptic rearrangements represent a significant source of human pathology that has not been previously appreciated, it would be valuable to have specific FISH probes for all human telomeres. We report here the isolation and characterization of a complete set of specific FISH probes representing each human telomere. As most of these clones are at a known distance of within 100-300 kb from the end of the chromosome arm, this provides a 10-fold improvement in deletion detection sensitivity compared with high-resolution cytogenetics (2-3 Mb resolution). While testing these probes, we serendipitously identified a family with multiple members carrying a cryptic 1q;11p rearrangement in the balanced or unbalanced state.Close

The purpose of the present study was to use the FISH method to establish the origin of chromosome aberrations currently unidentifiable by routine banding procedures. It was done in 13 cases with structurally rearranged chromosomes, seven of them with non-satellited marker chromosomes; in two of the latter an isochromosome 18p was identified which was consistent with a clinical picture of a tetrasomy 18p. FISH with chromosome-specific painting probes showed a deletion 18q in a girl with a cytogenetically balanced t(8;18). Two patients with deletions and two with 18 ring chromosomes were studied using a telomeric probe: both deletions had telomeric integrity and telomeric material was not present in the 18 rings. In a patient with an abnormal chromosome 18, the FISH analysis confirmed a pericentric inversion. We conclude from these results that FISH can provide a rapid and unequivocal cytogenetic diagnosis, which may improve genetic counseling.Close

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Telomere-specific clones are a valuable resource for the characterization of chromosomal rearrangements. We previously reported a first-generation set of human telomere probes consisting of 34 genomic clones, which were a known distance from the end of the chromosome ( approximately 300 kb), and 7 clones corresponding to the most distal markers on the integrated genetic/physical map (1p, 5p, 6p, 9p, 12p, 15q, and 20q). Subsequently, this resource has been optimized and completed: the size of the genomic clones has been expanded to a target size of 100-200 kb, which is optimal for use in genome-scanning methodologies, and additional probes for the remaining seven telomeres have been identified. For each clone we give an associated mapped sequence-tagged site and provide distances from the telomere estimated using a combination of fiberFISH, interphase FISH, sequence analysis, and radiation-hybrid mapping. This updated set of telomeric clones is an invaluable resource for clinical diagnosis and represents an important contribution to genetic and physical mapping efforts aimed at telomeric regions.Close

We report on a patient with a deletion of 18q23. At both 2 and 4 years of age, she displayed few of the facial features or other clinical features associated with the 18q- syndrome. Fluorescent in situ hybridisation and microsatellite marker and RFLP analysis were performed to characterise the extent of the deletion, and a terminal deletion of 18q23 was confirmed. The deleted region includes the gene for myelin basic protein, suggesting that hemizygosity of this gene does not invariably lead to mental and developmental delay. The clinical presentation of this patient suggests that either she is not deleted for the genes involved in the 18q- clinical phenotype or this patient represents one end of the spectrum of the clinical variability seen with 18q terminal deletions.Close

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Large terminal fragments of human chromosomes 2p, 6p, 8q, 12q, and 18q were cloned using yeast artificial chromosomes (YACs). RecA-assisted restriction endonuclease (RARE) cleavage analysis of genomic DNA samples from II unrelated individuals using YAC-derived probes confirmed the telomeric localizations of the half-YACs studied. The cloned fragments provide telomeric closure of maps for the respective chromosome arms and will supply the reagents needed for analyzing and sequencing these distal subtelomeric regions.Close

During a prospective prenatal study of numerical abnormalities of chromosomes 13, 18, 21, X and Y using locus-specific probes, we incidentally found a case with only one signal for chromosome 18 per cell in a chorionic villus sampling (CVS) associated with an otherwise apparently normal G-banded karyotype. This led us to discover a cryptic t(11;18) segregating in a four-generation family. The CVS was performed because of mental retardation in the brother to the father of the fetus. A subtelomeric chromosome 18 probe revealed one signal on 18qter and one on 11qter of the father. Thus the father had a balanced reciprocal t(11;18) in spite of the apparently normal G-banded karyotype. Using the same probes, we found an unbalanced translocation 46,XX,-18,+der (18)t(11;18)-(q25;q23)pat in the fetus. Further investigation of the family showed the translocation in balanced and unbalanced form in four generations in mentally normal and retarded individuals, respectively. The study emphasizes the need for a follow-up with molecular cytogenetic techniques in dysmorphic and retarded children.Close

A two-year-old girl trisomic for the segment 1q25 leads to qter and partially monosomic for band 18q23 as a consequence of a de novo t(1;18)(q25;q23) is reported. Most of the proposita's clinical findings have been described in the 1qter trisomy and some others in the 18q monosomy. This observation is interpreted as additional evidence of epi- , iso-, and hypostatic interactions at the chromosomal level.Close

Most patients who present with the 18q- syndrome have an apparent terminal deletion of the long arm of chromosome 18. For precise phenotypic mapping of this syndrome, it is important to determine whether the deletions are terminal deletions or interstitial deletions. A human telomeric YAC clone has been identified that hybridizes specifically to the telomeric end of 18q. This clone was characterized and used to analyze seven patients with 18q deletions. By FISH and Southern blotting analysis, all patients were found to lack this chromosomal region on their deleted chromosome, demonstrating that the patients do not have cryptic interstitial deletions.Close

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The 18q- syndrome is a deletion syndrome that is characterized by mental retardation, hearing loss, midfacial hypoplasia, growth deficiency, and limb anomalies. Most patients with this syndrome have deletions from 18q21-qter. We report on three patients with deletions of 18q23. A mother and daughter with identical deletions of 18q23 have many of the typical features of the 18q- syndrome, including midfacial hypoplasia and hearing loss. In contrast, the third patient has few of the symptoms of the 18q- syndrome. A contig of the 18q23 region was generated to aid in the mapping of the breakpoints. FISH was used to map both breakpoints to the same YAC clone. Furthermore, somatic-cell hybrids from the daughter and the third patient were isolated. The mapping results of sequence-tagged sites relative to the two breakpoints were identical, suggesting that the two deletion breakpoints map very close to one another. The analyses of these patients demonstrate that the critical region for the 18q- syndrome maps to 18q23 but that a deletion of 18q23 does not always lead to the clinical features associated with the syndrome. These patients demonstrate the wide phenotypic variability associated with deletions of 18q.Close

A cryptic translocation t(5;18)(qter;qter) was detected in a large family, using a FISH-based approach combining subtelomeric probes to allow the subtelomeric regions of most chromosome ends to be analysed for deletions and balanced or unbalanced translocations. Unbalanced karyotypes (duplication 5qter/deficiency 18qter) resulted in a previously undescribed association of moderate to severe mental retardation, microcephaly, pre- and postnatal growth retardation, distinct facial dysmorphism, narrow auditory canals, genital hypoplasia, left heart hypoplasia in one patient and severe behaviour difficulties in another. Some of the features observed in affected individuals are characteristic of known syndromes involving either 18q (growth deficiency, nystagmus, narrow auditory canals, genital hypoplasia, behaviour problems in 18q deletion syndrome) or 5q (umbilical and inguinal hernias, congenital heart defects in distal 5q trisomy).Close