Trisomy 15 is frequently observed as a minor clone in patients with Anemia/MDS/NHL and as a major clone in patients with AML

Trisomy 15 as the sole karyotypic aberration is an uncommon clonal cytogenetic aberration in hematological malignancies, making its significance unclear. Previous studies have reported relations of trisomy 15 with low-grade myelodysplasia or a benign age-related phenomenon associated with loss of the Y chromosome. To define the significance of trisomy 15, we conducted a retrospective study of all examples of trisomy 15 accessed in our laboratories. Trisomy 15 was observed as a clonal abnormality (> or =2 cells) in 17 cases and nonclonal (single cell) in 9 cases. The majority of cases (14/17 clonal cases) had a minor clone (5-35% of metaphase cells) of trisomy 15. The minority of cases (3/17) had a major clone (80-95% of metaphase cells) of trisomy 15. Two of these 3 cases were diagnosed as having acute myelocytic leukemia. Fluorescence in-situ hybridization (FISH) with the use of a chromosome 15-specific alpha-satellite probe was performed on 3 of 17 clonal cases and on 3 of 9 nonclonal cases. FISH results revealed the presence of a minor clone (from 3 to 5 of 700 interphase cells) in 5 of them, 2 of which had trisomy 15 in 20% of metaphase cells. These results may indicate that the 20% of trisomy 15 are very likely an overrepresentation of a very minor clone that could be transitory. In summary, the analysis of our cytogenetic and FISH results revealed the presence of two types of trisomy 15 clones: a minor clone that could be transitory or indolent and a major clone that could be of a neoplastic nature.

Detection of mosaicism in amniotic fluid cultures: a CYTO2000 collaborative study

PURPOSE

To evaluate the assumptions on which the American College of Medical Genetics (ACMG) Standards and Guidelines for detecting mosaicism in amniotic fluid cultures are based.

METHODS

Data from 653 cases of amniotic fluid mosaicism were collected from 26 laboratories. A chi-square goodness-of-fit test was used to compare the observed number of mosaic cases with the expected number based on binomial distribution theory.

RESULTS

Comparison of observed data from the in situ colony cases with the expected distribution of cases detected based on the binomial distribution did not reveal a significant difference (P = 0.525).

CONCLUSIONS

The empirical data fit the binomial distribution. Therefore, binomial theory can be used as an initial discussion point for determining whether ACMG Standards and Guidelines are adequate for detecting mosaicism.

Reactivation of XIST in normal fibroblasts and a somatic cell hybrid: abnormal localization of XIST RNA in hybrid cells

The XIST gene, expressed only from the inactive X chromosome, is a critical component of X inactivation. Although apparently unnecessary for maintenance of inactivation, XIST expression is thought to be sufficient for inactivation of genes in cis even when XIST is located abnormally on another chromosome. This repression appears to involve the association of XIST RNA with the chromosome from which it is expressed. Reactivated genes on the inactive X chromosome, however, maintain expression in several somatic cell hybrid lines with stable expression of XIST. We describe here another example of an XIST-expressing human-hamster hybrid that lacks X-linked gene repression in which the human XIST gene present on an active X chromosome was reactivated by treatment with 5-aza-2'-deoxycytidine. These data raise the possibility that human XIST RNA does not function properly in human-rodent somatic cell hybrids. As part of our approach to address this question, we reactivated the XIST gene in normal male fibroblasts and then compared their patterns of XIST RNA localization by subcellular fractionation and in situ hybridization with those of hybrid cells. Although XIST RNA is nuclear in all cell types, we found that the in situ signals are much more diffuse in hybrids than in human cells. These data suggest that hybrids lack components needed for XIST localization and, presumably, XIST-mediated gene repression.

Inactivation and reactivation of sex-linked steroid sulfatase gene in murine cell culture

The murine X-linked steroid sulfatase gene (Sts) normally escapes X inactivation. However, we have observed that most long-term murine cell cultures are deficient in STS activity even though only the L cells are known to be derived from an STS- mouse strain. To investigate this phenomenon, we developed a selective system whereby STS+ cells could be selected from STS- populations. The system is based on making cells dependent on cholesterol-sulfate as the sole source of cholesterol, allowing only STS+ cells to grow. Two STS- cell lines, after treatment with either 5-azacytidine (5AC) or ethyl methane sulfonate (EMS), yielded STS+ revertants, suggesting that their STS- phenotype was due to hypermethylation. To study the evolution of STS- cell lines, we established XO and XX primary lines from STS+ strains; the XX cell line remained STS+ after more than 200 cell doublings whereas the XO became STS- after about 100 doublings. Treatment of this STS- XO cell line with 5AC produced clones with restored STS activity. All the revertants showed a growth disadvantage compared to their STS- counterparts. It would appear that aberrant methylation is the basis for much of the STS deficiency observed in established murine lines and that its propagation is due to the growth advantage of STS- over STS+ cells.

High rate of recombination and double crossovers in the mouse pseudoautosomal region during male meiosis

The recombination rate in meiosis between the mouse X and Y chromosomes was analyzed. Mice heterozygous at two pseudoautosomal alleles, the steroid sulfatase gene and the Mov-15 provirus marker, were crossed. The provirus in the Mov-15 transgenic mouse strain had been previously shown to be carried in the pseudoautosomal region of the sex chromosomes. Recombination frequencies were shown to be 7-fold higher in this region in male meiosis than in female meiosis. Three-point crosses indicated the occurrence in male meiosis of double recombination events in the pseudoautosomal region, with little or no crossover interference, which is in marked contrast to observations made on the similar region of the human sex chromosomes. This result is contrary to a previous model, which predicted a single crossover event in male meiotic pairing of mammalian sex chromosomes.

Linkage of the steroid sulfatase gene to the sex-reversed mutation in the mouse

Dosage studies and the inheritance pattern of the gene for steroid sulfatase (Sts) in the mouse have previously provided indirect evidence for a functional Y-linked allele which recombines obligatorily with its X-linked allele in male meiosis. In this study, we have investigated the linkage relationship of Sts and the sex-reversed mutation (Sxr), a gene which is known to reside in the pairing region of the Y chromosome. The results clearly demonstrate that Sxr and Sts are linked in a region of obligatory recombination and Sts maps proximal to Sxr with most recombinants occurring proximal to the two genes.

Analysis of spreading of inactivation in eight X autosome translocations utilizing the high resolution RBG technique

Eight X autosome translocations were studied with replication banding to localize spreading of late replication into the autosomal segments. Partial spreading into the autosomal segment was seen in four translocations and no spreading of late replication was seen in four translocations. In those translocations with partial spreading of late replication into the autosomal segment, late replication did not always spread continuously from the X chromosome breakpoint throughout the autosome. Instead, it appeared to skip some bands and affect others. The data on the pattern of replication, taken to indicate also a spread of inactivation into these autosomal segments, correlated well with the clinical data in most cases and suggest that spreading of late replication is often incomplete and may be discontinuous.

Brief clinical report: prune belly syndrome in an anencephalic male

We describe a postmature anencephalic infant with atrophy of the abdominal musculature (prune belly syndrome). Other associations of these conditions are noted.

Pericentric X inversion in dizygotic twins who differ in X chromosome inactivation and menstrual cycle function

A 21-year-old female dizygotic twin was referred for cytogenetic evaluation because of mild mental retardation. Significant history, clinical, and physical findings included irregular menses, mildly coarse facies, and microcornea. Chromosome analysis revealed a pericentric inversion of the X chromosome, 46, X, inv (X)(p11;q22). Her twin who is phenotypically normal was also found to carry the same inversion. The twins differ significantly in X chromosome inactivation and menstrual cycle function.