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

Autosomal fragile sites, unlike their X-linked counterparts, are not known to be associated with disease. However, one case report has highlighted a possible relationship between the inheritance of a rare folate-sensitive fragile site in band 11q23.3 (FRA11B) and the chromosome 11q23-->qter deletion in Jacobsen (11q-) syndrome. The mother and brother of the reported Jacobsen syndrome child are FRA11B carriers, suggesting that in vivo breakage at the fragile site during early development could have given rise to the chromosome deletion. We have tested this hypothesis by high resolution physical mapping of FRA11B and of the deletion chromosome breakpoint in the Jacobsen syndrome patient. A detailed restriction map of 600 kb of human chromosome band 11q23.3 has been assembled which covers the PBGD, CBL2 and THY1 genes. FISH experiments with YACs and cosmids from this region have localised FRA11B to an interval of approximately 100 kb containing the 5' end of the CBL2 gene, which includes a CCG trinucleotide repeat. This class of repeat is expanded in the four cloned examples of fragile site and therefore the CBL2 repeat is a candidate for the location of FRA11B. Further, it is shown that the chromosomal deletion breakpoint of the Jacobsen syndrome child maps within the same interval as the fragile site. The breakpoint has apparently been repaired and stabilised by the de novo addition of a telomere. These data are consistent with a role for an inherited fragile site in the aetiology of a chromosome deletion syndrome.Close

Close

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

Close

Four children and one spontaneously aborted fetus from 2 separate families have a similar pattern of malformation secondary to duplication of distal 15q. In both families, the abnormal chromosomes were derived from balanced reciprocal translocations carried by the mothers. Clinical features common to the 4 liveborn children include appropriate birth weight, length, and head circumference for gestational age; similar craniofacial anomalies, including sloping forehead, bulbous nose, prominent nasal bridge and septum, midline crease in the lower lip, and micrognathia; arachnodactyly; joint contractures involving hands and feet; cardiac defects; and genital anomalies. The 2 infants with duplication 15q22.1—-qter and deletion 13q32.3—-qter died in the immediate neonatal period. The abortus, who shared the same chromosome constitution, had an omphalocele and a cephalic defect in neural tube closure. The 2 children with duplication 15q22—-qter and deletion 11q25—-qter survived but have severe psychomotor retardation and postnatal onset growth deficiency, at 48 and 30 months, respectively. The findings in these 5 cases plus review of the literature permit further delineation of a recognizable pattern of malformation secondary to duplication of distal 15q.Close

A female infant with severe growth-weight retardation and with a ring chromosome 11, associated with trisomy X in 15% of metaphases, has been reported. A literature review of cases of r(11) shows that the clinical features of these patients, although showing different frequencies, are similar to those of the del(11q) syndrome. It has been suggested that the variability of the mental retardation in r(11) patients is attributable to the unstability of the ring and to the different break points in these two chromosomal rearrangements. The origin of the r(11) was also addressed by studying fragile sites of the parents at 11p15 and 11q25.Close

Jacobsen syndrome is caused by segmental aneusomy for the distal end of the long arm of chromosome 11. Typical features include mild to moderate psychomotor retardation, trigonocephaly, facial dysmorphism, cardiac defects, and thrombocytopenia, though none of these features are invariably present. To define the critical regions responsible for these abnormalities, we studied 17 individuals with de novo terminal deletions of 11q. The patients were characterized in a loss-of-heterozygosity analysis using polymorphic dinucleotide repeats. The breakpoints in the complete two-generation families were localized with an average resolution of 3.9 cM. Eight patients with the largest deletions extending from 11q23.3 to 11qter have breakpoints, between D11S924 and D11S1341. This cytogenetic region accounts for the majority of 11q- patients and may be related to the FRA11B fragile site in 11q23.3. One patient with a small terminal deletion distal to D11S1351 had facial dysmorphism, cardiac defects, and thrombocytopenia, suggesting that the genes responsible for these features may lie distal to D11S1351. Twelve of 15 patients with deletion breakpoints as far distal as D11S1345 had trigonocephaly, while patients with deletions distal to D11S912 did not, suggesting that, if hemizygosity for a single gene is responsible for this dysmorphic feature, the gene may lie distal to D11S1345 and proximal to D11S912.Close

We have evaluated a patient with Jacobsen syndrome. The patient presented with growth retardation, hypotonia, trigonocephaly, telecanthus, downward slanting palpebral fissures, bilateral inferior colobomas (of the iris, choroid, and retina), hydrocephalus, central nervous system (CNS) abnormalities, and an endocardial cushion defect, features commonly seen in Jacobsen syndrome. Endocrine evaluation showed growth hormone deficiency and central hypothyroidism. Chromosome analysis showed a 46,XX,del(11)(q23q25) de novo karyotype. Cytogenetically, the deletion appeared to include most of bands 11q23 and q24 and a portion of q25. Using chromosome specific paint probe, a combination of chromosome 11 centromere, telomere, and region specific cosmid probes from 11q14.Close

We present a case of new phenotypic findings not previously reported associated with a partial deletion of chromosome 11 with a break point at 23q - (46,XY,del(11)(q23). Partial deletion of chromosome 11q was first described by Jacobsen et al(4). Forty-eight patients have been reported during the last 30 years, with variable break points between 11q11 and 11qter. New phenotypic findings in our patient with the associated 11q deletion are imperforate anus, bilateral cataracts, and hypoplastic, multilobed lungs.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 45,X karyotype was found in a boy with dysmorphic features, hypoglycaemia and pancytopenia. DNA analysis showed the presence of the Y-chromosomal DNA sequences SRY, ZFY, DYZ4, DYZ3 and DYS1. Using fluorescent in situ hybridization, we located DYZ4 and DYZ3 on chromosome 11qter and concluded that a de novo translocation (Y;11) (q11.2;q24) with a deletion of 11q24—-qter and a deletion of Yq11.2—-Yqter were present; Jacobsen syndrome and azoospermia are associated with these deletions. Signs of Jacobsen syndrome were observed in the patient.Close

Close

A YAC library enriched for telomere clones was constructed and screened for the human telomere-specific repeat sequence (TTAGGG). Altogether 196 TYAC library clones were studied: 189 new TYAC clones were isolated, 149 STSs were developed for 132 different TY-ACs, and 39 P1 clones were identified using 19 STSs from 16 of the TYACs. A combination of mapping methods including fluorescence in situ hybridization, somatic cell hybrid panels, clamped homogeneous electric fields, meiotic linkage, and BLASTN sequence analysis was utilized to characterize the resource. Forty-five of the TYACs map to 31 specific telomere regions. Twenty-four linkage markers were developed and mapped within 14 proterminal regions (12 telomeres and 2 terminal bands). The polymorphic markers include 12 microsatellites for 10 telomeres (1q, 2p, 6q, 7q, 10p, 10q, 13q, 14q, 18p, 22q) and the terminal bands of 11q and 12p. Twelve RFLP markers were identified and meiotically mapped to the telomeres of 2q, 7q, 8p, and 14q. Chromosome-specific STSs for 27 telomeres were identified from the 196 TYACs. More than 30,000 nucleotides derived from the TYAC vector-insert junction regions or from regions flanking TYAC microsatellites were compared to reported sequences using BLASTN. In addition to identifying homology with previously reported telomere sequences and human repeat elements, gene sequences and a number of ESTs were found to be highly homologous to the TYAC sequences. These genes include human coagulation factor V (F5), Weel protein tyrosine kinase (WEE1), neurotropic protein tyrosine kinase type 2 (NTRE2), glutathione S-transferase (GST1), and beta tubulin (TUBB). The TYAC/P1 resource, derivative STSs, and polymorphisms constitute an enabling resource to further studies of telomere structure and function and a means for physical and genetic map integration and closure.Close