Ming S. Lin

Direct duplication of 9p22→p24 in a child with duplication 9p syndrome

A de novo direct duplication of 9p22-->p24 was shown in a child with a duplication 9p phenotype by GTG banding and fluorescence in situ hybridization (FISH) using a chromosome-9 specific painting probe as well as 6 YAC DNA probes localized to the 9p13-9p23 region. The breakpoints in this patient and previously reported patients suggest that 9p22 may be the critical region for duplication 9p syndrome.

Sister chromatid exchanges and chromosome aberrations in fibroblasts from patients with retinoblastoma.

SummaryThe frequencies of sister chromatid exchanges (SCEs) and chromosome breaks were investigated in five diploid fibroblast strains derived from three patients with deletion 13[del(13)] retinoblastoma, one patient with a hereditary form of retinoblastoma, and one trisomy 13. The fibroblasts with del(13)(q14q22) showed slightly increased SCEs (at a P level of 5–10%), but the others, including del(13)(q12q14), the hereditary form of retinoblastoma, and trisomy 13, did not have increased SCEs as compared to normal controls. No increase in chromosome breaks was found in these fibroblasts. The results suggest that retinoblastoma is not associated with spontaneous increased chromosomal instability.

An in vitro and in vivo study of a BrdU-sensitive fragile site in the Chinese hamster

The frequencies of chromosome aberrations and development of the bromodeoxyuridine (BrdU)-sensitive fragile site were studied in vitro in Chinese hamster kidney and bone marrow cells and in vivo in Chinese hamster bone marrow cells. Chromosome aberrations in these cell systems were measured in response to different concentrations of BrdU, fluorodeoxyuridine, or both. The fragile site was found in both homologues of chromosome 1 at 1q22. Treatment with BrdU in vitro but not in vivo produces significant chromosome aberrations. About 50% of chromosome aberrations found after treatment in vitro were at the BrdU-sensitive fragile site compared with 12.5% after treatment in vivo. These results show that BrdU is much more potent in vitro than in vivo in inducing both chromosome aberrations and the expression of the BrdU-sensitive site.

Asynchronous DNA replication between 15q11.2q12 homologs: cytogenetic evidence for maternal imprinting and delayed replication

DNA replication kinetics of Prader-Willi/Angelman syndrome region of 15q11.2q12 was studied without synchronization in five human amniotic cell and five skin fibroblast strains with a marker 15 chromosome, i.e., 15p+ or der(15), as cytological marker to distinguish between the two homologs. BrdU-33258 Hoechst-Giemsa techniques were used to analyze and compare the late replication patterns in the 15q11.2q12 region between the homologs. Asynchronous replication between the homologs was observed in both amniocytes and fibroblasts. From cells of a marker 15 of known parental origin, the paternal 15q11.2q12 replicated earlier than that of the maternal 15 in 92%–95% of asynchronous metaphases. The remaining 5%–8% of asynchronous metaphases displayed maternal early/paternal late replication. This mosaic pattern of replication in the 15q11.2q12 region may be due to methylation mosaicism of genomic imprinting or a relative lack of self-control of replication. These results provide cytogenetic evidence of maternal imprinting and delayed replication in the 15q11.2q12 region.

De novo direct duplication of chromosome segment 22q11.2-q13.1

Lindsay et al. [1995] reported a case of de novo duplication of the segment 22q11-q12. Molecular cytogenetics studies showed that the segment includes the regions responsible for the {open_quotes}cat eye,{close_quotes} DiGeorge, and velo-cardio-facial syndrome, and extends distal to the breakpoint cluster region. The phenotype was milder than that of complete trisomy 22 and der(22)t(11;22) (q23;q11) syndrome and was similar in type and severity to that of {open_quotes}cat eye{close_quotes} syndrome (CES). They suggested that trisomy of gene(s) responsible for the CES might have a predominant phenotypic effect over other genes present in the region duplicated in their patient. 3 refs., 2 figs.

A Rare 6q11+ Heteromorphism: Cytogenetic Analysis and in situ Hybridization

A rare variant of 6q11+ heteromorphism was found in a fetus and the mother during amniocentesis. The G- and Q-banding and DA/DAPI stain were negative. The C-banding was positive and the C-banded segment was 3-fold longer in the variant than in its homologue. Neither of the C-banded regions of chromosomes 6 decondensed when exposed to distamycin A or 5-azacytidine. A DNA replication study indicated that the C-banded variant was late replicating. The lateral asymmetry observed in the 6q11 variant after one replication cycle in 5-bromodeoxyuridine may result from an unequal interstrand distribution of thymidine in the repetitive DNA. Fluorescent in situ hybridization using a chromosome-6-specific alpha-satellite probe (D6Z1) demonstrated hybridization signals on the centromere of chromosome 6. The 6q11 variant showed a signal which was 3-fold larger than its homologue. These results indicate that the 6q11 variant is an amplification of a chromosome-6-specific alpha repeat, and the size of the hybridization signal correlates with the size of the laterally asymmetric and C-banded region.

Comparison of expression of the fragile site at Xq27 in T and B lymphocytes.

SummaryWe compared the fragile X (fraX) expression in T and B lymphocytes from four hemizygous males with fraX. Blood cultures were stimulated with a T cell mitogen (phytohemagglutinin: PHA) and with a B cell mitogen (pokeweed mitogen: PWM). A significant decrease in fraX expression was observed in cultures stimulated with PWM when compared to PHA-stimulated ones.

Lack of specificity of DA/DAPI fluorescence.

Cytogenetic studies showed 47,XY, + mar in a developmentally retarded child with some features of Prader‐Willi syndrome, and 46,XX in his mother. The marker chromosome showed a single subterminal primary constriction, bisatellites, and two C‐bands. DA/DAPI staining showed two intense bands in the marker chromosome, which most likely was derived from chromosome 15.

Ultraviolet light and mitomycin C induced sister-chromatid exchanges in fibroblasts from patients with retinoblastoma

Ultraviolet light and mitomycin C (MMC) induced sister-chromatid exchanges (SCEs) were investigated in 6 diploid fibroblast strains derived from 3 patients with deletion 13 and retinoblastoma, one patient with a hereditary form of retinoblastoma, one patient with trisomy 13, and one normal control. Two fibroblast strains with del(13)(q14q22) showed a significant increase in SCEs compared to the control after UV and MMC treatments. In contrast, cell strains with del(13)(q12q14) and trisomy 13 did not show increased SCEs. The frequency of SCEs in fibroblasts from a patient with autosomal dominant retinoblastomas (no deletions) was significantly increased by UV, but not by MMC. The results suggest that cell strains with different deletions of chromosome 13 have different SCE responses to UV and MMC inductions. The cells with del(13)(q14q22) may have a DNA-repair defect.

The sequence of DNA replication in an iso-dicentric X-chromosome in peripheral blood lymphocytes and skin fibroblasts from the same individual

SummaryA comparison of the sequence of DNA replication in an isodicentric (idic) X chromosome was made between peripheral blood lymphocytes and skin fibroblasts from a 33-year-old female with primary amenorrhea, somatic stigmata of Turner syndrome, and normal stature and intelligence. The patient had a karyotype 45,X/46,X,idic(X)(q27.1) in lymphocytes and 46,X,idic(X)(q27.1) in skin fibroblasts. Both centromeric regions of the idic X showed C-staining but only one primary constriction. BrdU-33258 Hoechst-Giemsa techniques were used to analyze regional DNA replication patterns. The idic X chromosome was always late replicating in lymphocytes and skin fibroblasts, except that about 1–2% of cells completed replication simultaneously in both normal and idic X chromosomes. Fifty-six percent of the asymmetric patterns in lymphocytes showed an equal proportion of early and late functional and non-functional centromere halves. In skin fibroblasts, 60.8% of cells were asymmetric: the functional half tended to replicate later than the non-functional half. Some differences were observed between these two cell types. As examples, band q23 was late replicating in lymphocytes, but early replicating in fibroblasts; q25 was intermediate to late replicating in lymphocytes, but one of the last bands to complete replication in fibroblasts. Thus, different cell typed influenced the replication kinetics in the idic(X). Furthermore, several variants of the replication sequence were found in both cell types. The findings support the hypothesis that the control of DNA replication in the inactive X chromosome is multifocal, and suggest that the active idic X chromosome replication may reflect a relative lack of self-control or heterogeneity of cell population.