Making CENs of mammalian artificial chromosomes

Sequences associated with centromere competency in the human genome

Centromeres, the sites of spindle attachment during mitosis and meiosis, are located in specific positions in the human genome, normally coincident with diverse subsets of alpha satellite DNA. While there is strong evidence supporting the association of some subfamilies of alpha satellite with centromere function, the basis for establishing whether a given alpha satellite sequence is or is not designated a functional centromere is unknown, and attempts to understand the role of particular sequence features in establishing centromere identity have been limited by the near identity and repetitive nature of satellite sequences. Utilizing a broadly applicable experimental approach to test sequence competency for centromere specification, we have carried out a genomic and epigenetic functional analysis of endogenous human centromere sequences available in the current human genome assembly. The data support a model in which functionally competent sequences confer an opportunity for centromere specification, integrating genomic and epigenetic signals and promoting the concept of context-dependent centromere inheritance.

Genome medicine: gene therapy for the millennium, 30 September-3 October 2001, Rome, Italy

The recent surge of DNA sequence information resulting from the efforts of agencies interested in deciphering the human genetic code has facilitated technological developments that have been critical in the identification of genes associated with numerous disease pathologies. In addition, these efforts have opened the door to the opportunity to develop novel genetic therapies to treat a broad range of inherited disorders. Through a joint effort by the University of Vermont, the University of Rome, Tor Vergata, University of Rome, La Sapienza, and the CSS Mendel Institute, Rome, an international meeting, 'Genome Medicine: Gene Therapy for the Millennium' was organized. This meeting provided a forum for the discussion of scientific and clinical advances stimulated by the explosion of sequence information generated by the Human Genome Project and the implications these advances have for gene therapy. The meeting had six sessions that focused on the functional evaluation of specific genes via biochemical analysis and through animal models, the development of novel therapeutic strategies involving gene targeting, artificial chromsomes, DNA delivery systems and non-embryonic stem cells, and on the ethical and social implications of these advances.

Structure of human DNMT2, an enigmatic DNA methyltransferase homolog that displays denaturant-resistant binding to DNA

DNMT2 is a human protein that displays strong sequence similarities to DNA (cytosine-5)-methyltransferases (m(5)C MTases) of both prokaryotes and eukaryotes. DNMT2 contains all 10 sequence motifs that are conserved among m(5)C MTases, including the consensus S:-adenosyl-L-methionine-binding motifs and the active site ProCys dipeptide. DNMT2 has close homologs in plants, insects and Schizosaccharomyces pombe, but no related sequence can be found in the genomes of Saccharomyces cerevisiae or Caenorhabditis elegans. The crystal structure of a deletion mutant of DNMT2 complexed with S-adenosyl-L-homocysteine (AdoHcy) has been determined at 1.8 A resolution. The structure of the large domain that contains the sequence motifs involved in catalysis is remarkably similar to that of M.HHAI, a confirmed bacterial m(5)C MTase, and the smaller target recognition domains of DNMT2 and M.HHAI are also closely related in overall structure. The small domain of DNMT2 contains three short helices that are not present in M.HHAI. DNMT2 binds AdoHcy in the same conformation as confirmed m(5)C MTases and, while DNMT2 shares all sequence and structural features with m(5)C MTases, it has failed to demonstrate detectable transmethylase activity. We show here that homologs of DNMT2, which are present in some organisms that are not known to methylate their genomes, contain a specific target-recognizing sequence motif including an invariant CysPheThr tripeptide. DNMT2 binds DNA to form a denaturant-resistant complex in vitro. While the biological function of DNMT2 is not yet known, the strong binding to DNA suggests that DNMT2 may mark specific sequences in the genome by binding to DNA through the specific target-recognizing motif.

Mapping of a human centromere onto the DNA by topoisomerase II cleavage

We have mapped the positions of topoisomerase II binding sites at the centromere of the human Y chromosome using etoposide-mediated DNA cleavage. A single region of cleavage is seen at normal centromeres, spanning approximately 50 kb within the centromeric alphoid array, but this pattern is abolished at two inactive centromeres. It therefore provides a marker for the position of the active centromere. Although the underlying centromeric DNA structure is variable, the position of the centromere measured in this way is fixed relative to the Yp edge of the array, and has retained the same position for >100,000 years.

Molecular cytogenetic analysis of eight inversion duplications of human chromosome 13q that each contain a neocentromere

Neocentromeres are fully functional centromeres that have arisen in previously noncentromeric chromosomal locations on rearranged chromosomes. The formation of neocentromeres results in the mitotic stability of chromosomal fragments that do not contain endogenous centromeres and that would normally be lost. Here we describe a unique collection of eight independent patient-derived cell lines, each of which contains a neocentromere on a supernumerary inversion duplication of a portion of human chromosome 13q. Findings in these patients reveal insight into the clinical manifestations associated with polysomy for portions of chromosome 13q. The results of FISH and immunofluorescent analysis of the neocentromeres in these chromosomes confirm the lack of alpha-satellite DNA and the presence of CENtromere proteins (CENP)-C, -E, and hMAD2. The positions of the inversion breakpoints in these chromosomes have been placed onto the physical map of chromosome 13, by means of FISH mapping with cosmid probes. These cell lines define, within chromosome 13q, at least three distinct locations where neocentromeres have formed, with five independent neocentromeres in band 13q32, two in band 13q21, and one in band 13q31. The results of examination of the set of 40 neocentromere-containing chromosomes that have thus far been described, including the 8 neocentromere-containing chromosomes from chromosome 13q that are described in the present study, suggest that chromosome 13q has an increased propensity for neocentromere formation, relative to some other human chromosomes. These neocentromeres will provide the means for testing hypotheses about sequence requirements for human centromere formation.