Pierre Chartrand

Alloimmunization to Platelet antigen HPA-1a (PIA1) is strongly associated with both HLA-DRB3∗0101 and HLA-DQB1∗0201

Antibodies to the platelet HPA-1a antigen can elicit in the newborn a condition known as neonatal alloimmune thrombocytopenic purpura (NAITP). Previous studies based on RFLP analysis showed that 100% of HPA-1a-negative women who produced anti-HPA-1a antibodies (responders) were HLA-DRw52a (DRB3*0101). However, this specificity could also be found in some HPA-1a-negative women not producing anti-HPA-1a antibodies (nonresponders). We have analyzed in detail by PCR-SSOP the HLA-DR, -DQ, and -DP loci of 36 responders and 10 nonresponders. We found that while the allele DRB3*0101 was present in the vast majority of responders (91%), there were exceptions. Furthermore, the DQB1*0201 allele was found to be present in almost all responders (94%), but again was also found in nonresponders. The risk of alloimmunization to HPA-1a in an HPA-1b homozygous mother significantly increases with the presence of either allele, the odds ratio being 39.7 for DQB1*0201 and 24.9 for DRB3*0101. Sequencing of exon 2 of these two alleles from responders indicated no sequence difference when compared with the consensus sequences. This indicates that they do not represent variants when compared with the same alleles found in some nonresponders.

Analysis of immunoglobulin class, IgG subclass and titre of HPA‐1a antibodies in alloimmunized mothers giving birth to babies with or without neonatal alloimmune thrombocytopenia

We analysed the titre and isotype composition of antibodies produced by mothers giving birth to babies with or without neonatal alloimmune thrombocytopenic purpura (NAITP) and patients with post‐transfusion purpura (PTP). All these individuals produced an antibody specific for the HPA‐1a allotype present on the platelet glycoprotein IIb‐IIIa (GPIIb‐IIIa). Sera from mothers who gave birth to thrombocytopenic babies (group 1, n = 36), non‐thrombo‐cytopenic babies (group 2, n = 4) or from PTP patients (group 3, n = 3) were tested by an indirect‐ELISA. Results indicated no evident differences in the isotype composition or titre of the antibodies from the three groups of sera. The antibody titre ranged from 1: 120 to 1: 3500. Antibodies with the IgGl subclass were present in all sera. Most sera contained IgGl alone (24/43 sera tested) or in combination with IgG3 (10/43). IgG2 was never present and only three sera showed intermediate reactivity with anti‐IgG4 MAb. Few sera (nine sera from groups 1 and 2) were weakly positive when tested with the anti‐IgM antibodies. These results suggest that neither the titre nor the isotype composition can be used to predict the severity or the occurrence of thrombocytopenia in newborns.

A Double-Strand Break in a Chromosomal LINE Element Can Be Repaired by Gene Conversion with Various Endogenous LINE Elements in Mouse Cells

ABSTRACT A double-strand break (DSB) in the mammalian genome has been shown to be a very potent signal for the cell to activate repair processes. Two different types of repair have been identified in mammalian cells. Broken ends can be rejoined with or without loss or addition of DNA or, alternatively, a homologous template can be used to repair the break. For most genomic sequences the latter event would involve allelic sequences present on the sister chromatid or homologous chromosome. However, since more than 30% of our genome consists of repetitive sequences, these would have the option of using nonallelic sequences for homologous repair. This could have an impact on the evolution of these sequences and of the genome itself. We have designed an assay to look at the repair of DSBs in LINE-1 (L1) elements which number 105 copies distributed throughout the genome of all mammals. We introduced into the genome of mouse epithelial cells an L1 element with an I-SceI endonuclease site. We induced DSBs at the I-SceI site and determined their mechanism of repair. We found that in over 95% of cases, the DSBs were repaired by an end-joining process. However, in almost 1% of cases, we found strong evidence for repair involving gene conversion with various endogenous L1 elements, with some being used preferentially. In particular, the TF family and the L1Md-A2 subfamily, which are the most active in retrotransposition, appeared to be contributing the most in this process. The degree of homology did not seem to be a determining factor in the selection of the endogenous elements used for repair but may be based instead on accessibility. Considering their abundance and dispersion, gene conversion between repetitive elements may be occurring frequently enough to be playing a role in their evolution.

KIN17, a mouse nuclear protein, binds to bent DNA fragments that are found at illegitimate recombination junctions in mammalian cells

Illegitimate recombination is the dominant mechanism of recombination in mammalian somatic cells. It is responsible for most genome rearrangements such as translocations, deletions and integrations. Little is known as yet about the mechanism of illegitimate recombination and the enzymes involved. Recently, it has been shown that intrinsically bent DNA, also known as curved DNA, is present at chromosomal sites of illegitimate recombination events. Here we report that KIN17, a new mouse nuclear protein, binds to the curved DNA fragments found at illegitimate recombination sites.

Elimination of heteroduplex artifacts when sequencing HLA genes amplified by polymerase chain reaction (PCR).

The polymerase chain reaction (PCR) can be used for DNA sequence analysis. When a unique sequence is amplified, the products generated are homogenous and can be sequenced directly. However, when multiple genes or alleles are amplified at the same time, such as in the case of HLA genes, the products generated are a heterogenous mixture that has to be cloned into bacteria before sequencing. In the latter case, variant sequences can arise during the PCR procedure as a result of in vitro recombination or of heteroduplex formation. In vitro recombination events result from end pairing of incompletely amplified sequences; synthesis is then completed and the resulting molecules amplified. A number of authors (Jansen and Ledley 1990; Lawlor et al. 1991; Meyerhans et al. 1990; Reynaud et al. 1991) have reported this phenomenon. However, the proportion of recombinant molecules to the total products of amplification is usually low (1-5%; Jansen and Ledley 1990; Meyerhans et al. 1990) and recombinants are relatively easily identified. Furthermore, the shorter the DNA template is, the lower is the risk of being incompletely synthesized by the Taq polymerase and of generating recombinants. Heteroduplex molecules are generated when fully synthesized complementary strands originating from distinct genes or alleles hybridize together. Jansen and Ledley (1990), when analyzing cDNA clones of the human methylmalonyl CoA mutase gene, estimated that 50 % of the final amplified products can be heteroduplexes. Actually the formation of heteroduplexes between PCR amplified homologous sequences has been used for the identification of heterozygotes, the detection of mutations

Integration of a vector containing a repetitive LINE-1 element in the human genome.

Mammalian cells contain numerous nonallelic repeated sequences, such as multicopy genes, gene families, and repeated elements. One common feature of nonallelic repeated sequences is that they are homeologous (not perfectly identical). Our laboratory has been studying recombination between homeologous sequences by using LINE-1 (L1) elements as substrates. We showed previously that an exogenous L1 element could readily acquire endogenous L1 sequences by nonreciprocal homologous recombination. In the study presented here, we have investigated the propensity of exogenous L1 elements to be involved in a reciprocal process, namely, crossing-overs. This would result in the integration of the exogenous L1 element into an endogenous L1 element. Of over 400 distinct integration events analyzed, only 2% involved homologous recombination between exogenous and endogenous L1 elements. These homologous recombination events were imprecise, with the integrated vector being flanked by one homologous and one illegitimate junction. This type of structure is not consistent with classical crossing-overs that would result in two homologous junctions but rather is consistent with one-sided homologous recombination followed by illegitimate integration. Contrary to what has been found for reciprocal homologous integration, the degree of homology between the exogenous and endogenous L1 elements did not seem to play an important role in the choice of recombination partners. These results suggest that although exogenous and endogenous L1 elements are capable of homologous recombination, this seldom leads to crossing-overs. This observation could have implications for the stability of mammalian genomes.

Characterization of nonconservative homologous junctions in mammalian cells.

Homologous recombination in mammalian cells between extrachromosomal molecules, as well as between episomes and chromosomes, can be mediated by a nonconservative mechanism. It has been proposed that the key steps in this process are the generation (by double-strand cleavage) of overlapping homologous ends, the creation of complementary single-strand ends (either by strand-specific exonuclease degradation or by unwinding of the DNA helix), and finally the creation of heteroduplex DNA by the annealing of the single-strand ends. We have analyzed in detail the structure of nonconservative homologous junctions and determined the contribution of each end to the formation of the junction. We have also analyzed multiple descendants from single recombination events. Two types of junctions were found. The majority (90%) of the junctions were characterized by a single crossover site. These crossover sites were distributed randomly throughout the junction. The remaining 10% of the junctions had mosaic patterns of parental markers. Furthermore, in 9 of 10 cases, multiple descendants from a single recombination event were identical. Thus, it appears that in most cases few parental markers were involved in junction formation. This finding suggests that nonconservative homologous junctions are mediated mainly by short heteroduplexes of a few hundred base pairs or less. These results are discussed in terms of the current models of nonconservative homologous recombination.

Alloimmunization to platelet antigen HPA-1a (Zwa): association with HLA-DRw52a is not 100%.

SIR, Neonatal alloimmune thrombocytopenic purpura (NAITP) is a disease of the new-born involving the destruction of the platelets of the fetus by maternal antibodies. In the Caucasian population, the incidence of NAITP is of about one in 2,000 births with an estimated morbidity and mortality of 15-30%. The majority of NAITP cases are associated with the HPA1 alloantigen system (designated previously as Zw or P1*), which consists of two allelic variants HPA-I a and

Direct evidence that transgene integration is random in murine cells, implying that naturally occurring double-strand breaks may be distributed similarly within the genome.

We have examined the distribution of illegitimate integration of a transgene within the genome of cells of a murine fibroblast cell line, LTA, using fluorescence in situ hybridization (FISH) analysis. The transgene vector contained specific sequences for detection via FISH and a hygromycin resistance gene for selection. Cells were transfected via CaPO4, and pools of 250 to 3000 hygromycin-resistant clones were subjected to FISH analysis. The integration of the transgene was scored for chromosome morphology (acrocentric, metacentric or dicentric) and position (relative to centromere or telomere). More than 90% of the hygromycin-resistant clones observed involved integration of the transgene singly or as multiple copies, at a single site within the genome. No bias was observed for integration of the transgene in any particular chromosome morphology or chromosomal position, even in the presence, within the genome, of sequences homologous to the transgene. This study presents direct evidence that illegitimate integration of a transgene occurs randomly in murine fibroblasts. Since it is postulated that initiation of illegitimate recombination involves a double-strand break (DSB), a corollary to the above results would be that naturally occurring DSBs also occur randomly within the murine genome.

Reciprocal homologous junctions generated in mouse cells.

SummaryWe analysed pairs of reciprocal homologous junctions resulting from intermolecular conservative homologous recombination in mouse cells. The assay used did not rely on the reconstitution of a selectable gene. This permitted the introduction of multiple markers in the parental homologous sequences which in turn enabled us to compare the contribution of each parent to the reciprocal products of a given recombination event. In all recombinants analysed we found, when comparing the reciprocal junctions, a middle segment originating from only one parent. This segment of uniparental origin occurred randomly throughout the region of homology and could extend over a thousand base pairs. These results are consistent with a gap repair process like the one proposed for homologous recombination in yeast. However, introducing a double-strand break in the region of homology did not enhance but rather decreased the proportion of recombinants with reciprocal homologous junctions relative to other types of recombinants.