Dosage-sensitivity of imprinted genes expressed in the brain: 15q11-q13 and neuropsychiatric illness.

Imprinted genes, those genes subject to parent-of-origin-specific epigenetic marking resulting in monoallelic parent-specific expression, are sensitive to subtle changes in expression dosage. This has been illustrated in a number of experimental models and the fact that both decreased (or complete loss) and increased imprinted gene expression can lead to human diseases. In the present paper, we discuss the consequence of increased dosage of imprinted genes for brain function, focusing on the PWS (Prader-Willi syndrome) locus on human chromosome 15q11-q13 and how predicted increases in dosage of maternally expressed imprinted genes from this interval are associated with a higher risk of developing psychotic illness. The evidence for this comes from individuals with PWS itself and also non-syndromic cases of psychosis in carriers of a maternally derived copy number variant spanning this locus. Of the known imprinted genes in this region, the prime candidate is maternally expressed UBE3A, which encodes E6-AP (E6-associated protein) ubiquitin ligase and has an influence on a number of important neurotransmitter systems. Furthermore, these findings point to the fact that brain function is exquisitely sensitive to both decreases and increases in the expression of imprinted genes.

Episodic memory performance predicted by the 2bp deletion in exon 6 of the "alpha 7-like" nicotinic receptor subunit gene.

OBJECTIVE: There is evidence of linkage between chromosome 15q14 and the P50 auditory evoked response, a heritable neuropsychological marker associated with increased risk of schizophrenia. Chromosome 15q14 harbors the alpha-7 nicotinic receptor subunit gene (CHRNA7) and a hybrid gene of unknown function (CHRFAM7A). CHRNA7 is involved in memory formation, a core dysfunction in schizophrenia. The authors set out to determine if this locus is associated with memory dysfunction in schizophrenia. METHOD: A 2bp deletion in exon 6 of CHRFAM7A, which disrupts the hybrid gene and has previously been associated with P50 deficit, was genotyped in 251 individuals from the Maudsley Family Study of schizophrenia. Episodic memory function was assessed using the Wechsler Memory Scale. RESULTS: Significant associations were identified with delayed recall and percentage retained, with the presence of the deletion predicting worse performance. CONCLUSIONS: These observations indicate that episodic memory function is a schizophrenia endophenotype and implicate the CHRFAM7A/CHRNA7 locus in modulating its function.

Bends in human mitotic metaphase chromosomes revisited: 15q11-13 is the most frequent non-random autosomal bend in blood cultures.

We have investigated the preferential bending of some chromosome sites in blood cultures from normal and chromosomally abnormal subjects. A total of 2,262 centromeric and 2,718 non-centromeric bends were recorded, and 69 non-centromeric sites were found not to bend at random. 15q11-13 bending was found to be the most frequent non-random autosomal bend. Bends on chromosomes may be remnants of a folded chromosome state in the nucleus, and may facilitate the preferential involvement of some chromosomal bands in structural reorganizations such as the isoacentric fragments, or contribute to the high frequency of interstitial deletions and isodicentric inversion duplications involving the 15q11-13 region.

Components of the human spindle checkpoint control mechanism localize specifically to the active centromere on dicentric chromosomes.

The spindle checkpoint control mechanism functions to ensure faithful chromosome segregation by delaying cell division until all chromosomes are correctly oriented on the mitotic spindle. Initially identified in budding yeast, several mammalian spindle checkpoint-associated proteins have recently been identified and partially characterized. These proteins associate with all active human centromeres, including neocentromeres, in the early stages of mitosis prior to the commencement of anaphase. We have examined the status of proteins associated with the checkpoint protein complex (BUB1, BUBR1, BUB3, MAD2), the anaphase-promoting complex (Tsg24, p55CDC), and other proteins associated with mitotic checkpoint control (ERK1, 3F3/2 epitope, hZW10), on a human dicentric chromosome. Each of these proteins was found to specifically associate with only the active centromere, suggesting that only active centromeres participate in the spindle checkpoint. This finding complements previous studies on multicentric chromosomes demonstrating specific association of structural and motor-related centromere proteins with active centromeres, and suggests that centromere inactivation is accompanied by loss of all functionally important centromere proteins.

Structure and function of the human chromosome 15 imprinting center.

The Prader-Willi syndrome (PWS) and the Angelman syndrome (AS) are distinct neurogenetic disorders that are caused by a deficiency of paternal (PWS) or maternal (AS) contributions to chromosome 15. The affected genes are located in an imprinted chromosomal domain of 2 Mb, which is controlled by an imprinting center (IC). The IC has been mapped to a 100-kb region including the SNRPN gene and appears to have a bipartite structure. Mutations of the proximal part of the IC block the paternal-->maternal imprint switch during female gametogenesis, whereas mutations of the distal part of the IC block the maternal-->paternal imprint switch during, male gametogenesis. Imprinting involves differential DNA methylation, which appears to be instrumental in the regulation of gene activity and can be used for diagnostic purposes.

Rapid achievement of PML-RAR alpha polymerase chain reaction (PCR)-negativity by combined treatment with all-trans-retinoic acid and chemotherapy in acute promyelocytic leukemia: a pilot study.

The reverse transcriptase-polymerase chain reaction (RT-PCR) for the fusion transcript of PML-RAR alpha can be used to detect minimal residual disease (MRD) in acute promyelocytic leukemia (APL). We have applied a semi-quantitative two-step PCR assay (sensitivity: step 1 = 1 in 10(3) cells; step 2 = 1 in 10(6) cells) to monitor the dynamics of MRD after combined therapy with all-trans-retinoic acid (ATRA) and chemotherapy (CT) in 5 patients in whom complete clinical remission (CR) was achieved. The patients received an induction treatment with ATRA for 47, 40, 38, 14 or 10 days. In three patients ATRA was followed by CT. Two patients with hyperleukocytosis at diagnosis or after ATRA received an overlapping CT starting from day 3 or 7. Four of the five patients became two-step PCR-negative in their bone marrow within 43 to 82 days after onset of therapy. Two-step PCR-negatively was achieved with ATRA plus one course of CT in these four patients who are still in continuous complete remission after 19, 18, 7 and 5 months. One of these patients did not even receive consolidation CT because of congestive heart failure. The fifth patient remained second-step PCR-positive and relapsed after 5 months. Our results indicate that the combined regimen can rapidly reduce MRD below a detection limit of 1 in 10(6) cells within 1-3 months and that these results can even by achieved by a short course of ATRA together with only one cycle of CT.

Somatic pairing of centromeres and short arms of chromosome 15 in the hematopoietic and lymphoid system.

Normal human bone marrow and peripheral blood leukocytes as well as malignant cells from a variety of leukemias and lymphomas, demonstrate somatic pairing of centromeres and p arms of chromosome 15 during interphase. This phenomenon, effected by sequences on the p arm and requiring the intranuclear transport of spatial domains for at least one of the homologs, was not seen in amniotic fluid cells, uterine cervical tissue or in tissue fibroblasts. These studies contribute to the recent evidence of somatic pairing of homologous chromosomes in man and provide support for mobile chromosomal domains in interphase. It appears that sequences on the p arm of chromosome 15, possibly the nucleolar organizing genes, are uniquely important in the maturation of benign and malignant cells of hemato-lymphopoietic origin.

Delineation of translocation t(15; 17) in acute promyelocytic leukemia by chromosomal in situ suppression hybridization.

In this study we demonstrate the feasibility of chromosomal in situ suppression (CISS) hybridization to detect the translocation t(15; 17) in metaphase spreads of patients with acute promyelocytic leukemia. Using DNA libraries from sorted human chromosomes 15 and 17 the translocation t(15; 17) can be unequivocally identified even if the spread and the morphology of the chromosomes are poor. The sensitivity of CISS hybridization is compared with the sensitivity of conventional G-banded karyotypes.

Membranoproliferative glomerulonephritis in a child with Prader-Willi syndrome.

We describe a girl with Prader-Willi syndrome and membranoproliferative glomerulonephritis. She had a deletion at 15q11-13. The deletion may have made the child susceptible to renal disease.

Establishment and characterization of a thymic carcinoma cell line (Ty-82) carrying t(15;19)(q15;p13) chromosome abnormality.

A new human cell line, designated Ty-82, was established from the pleural effusion of a 22-year-old woman with undifferentiated thymic carcinoma. This cell line consisted of primitive cells that were positive for alpha-naphthyl butyrate esterase and acid phosphatase. The cells were shown to express epithelial membrane antigen, but were completely negative for cytokeratin, carcinoembryonic antigen, glial fibrillary acidic protein, desmin, S-100 protein, lysozyme, Leu-7, HLA-DR (Ia), leukocyte common antigen, Ki-I antigen, T-cell antigens, B-cell antigens, myelomonocyte antigens, and Epstein-Barr-virus nuclear antigen. Electron microscopy showed that the cells were highly anaplastic, with no sign of cellular differentiation to any lineages. The Ty-82 cell line was found to have a karyotype of 46,XX,t(15;19)(q15;p13), being identical to that of the patient's tumor cells. Four of 5 nude mice inoculated sub-cutaneously with Ty-82 cells developed tumors which displayed a histological picture similar to the original tumor. Thymic carcinoma is a recently recognized entity, and its cellular and clinical behavior are poorly understood. The newly established thymic carcinoma cell line would provide a useful tool for the better understanding of this rare disease.

Lack of involvement of the G-CSF gene by chromosomal translocation t(15;17) in acute promyelocytic leukemia.

Using a human G-CSF cDNA as a probe, we analyzed the t(15;17) breakpoint by Southern blot analysis with conventional and/or pulsed-field gel electrophoresis in 12 patients with acute promyelocytic leukemia. The results did not show the rearrangement, deletion, or restriction fragment length polymorphism within the gene and in the surrounding sequences.