Terminal Breakpoints Research Articles

Assessment of quadrivalent characteristics influencing chromosome segregation by analyzing human preimplantation embryos from reciprocal translocation carriers.

Patterns of meiotic chromosome segregation were analyzed in cleavage stage and blastocyst stage human embryos from couples with autosomal reciprocal translocations (ART). The influence of quadrivalent asymmetry degree, the presence of terminal breakpoints, and the involvement of acrocentric chromosomes in the rearrangement were analyzed to evaluate their contribution to the formation of non-viable embryos with significant chromosomal imbalance due to pathological segregation patterns and to assess the selection of human embryos by the blastocyst stage. A selection of viable embryos resulting from alternate and adjacent-1 segregation and a significant reduction in the detection frequency of the 3 : 1 segregation pattern were observed in human embryos at the blastocyst stage. The presence of terminal breakpoints increased the frequency of 3 : 1 segregation and was also associated with better survival of human embryos resulting from adjacent-1 mode, reflecting the process of natural selection of viable embryos to the blastocyst stage. The demonstrated patterns of chromosome segregation and inheritance of a balanced karyotype in humans will contribute to optimizing the prediction of the outcomes of in vitro fertilization programs and assessing the risks of the formation of unbalanced embryos for ART carriers.

Evaluating chromosomal segregation in a family where both spouses carry an autosomal translocation

Autosomal reciprocal translocations represent exchanges of chromatin fragments between non-homologous chromosomes. Translocations are facilitated by the creation of quadrivalent structures during the first meiotic division, which are characterized by the length of the translocated and centric segments, asymmetry, and the presence of terminal breakpoints, all of which may impact segregation mode. Here, we report a rare case of multiple reciprocal translocations within a single family. This includes the evaluation of the translocations in each of the spouses and an analysis of their chromosome segregation patterns as determined by the constellation of universal characteristics in each of their quadrivalents. The obtained results will be of interest to fundamental biology, as they will expand the understanding of the factors affecting chromosome segregation during meiosis.

Analysis of meiotic segregation modes in biopsied blastocysts from preimplantation genetic testing cycles of reciprocal translocations

PurposeTo analyse the meiotic segregation modes of chromosomal structural rearrangements (PGT-SR) of reciprocal translocation in biopsied blastocysts from preimplantation genetic testing and to investigate whether any features of reciprocal translocation, such as carrier gender or the presence of acrocentric chromosomes or terminal breakpoints, affect meiotic segregation modes.MethodsComprehensive chromosomal screening was performed by next generation sequencing (NGS) on 378 biopsied blastocysts from 102 PGD cycles of 89 reciprocal translocation carriers. The segregation modes of a quadrivalent in 378 blastocysts were analysed according to the carrier’s gender, chromosome type and the location of chromosome breakpoints.ResultsThe results showed that 122 out of 378 blastocysts (32.3%) were normal or balanced, 209 (55.3%) were translocated chromosomal abnormalities, and 47 (12.4%) were abnormalities of non-translocated chromosomes. The proportion of translocated chromosomal abnormalities in translocations without acrocentric chromosomes was significantly higher than that in blastocysts from carriers with acrocentric chromosomes (14.8% versus 5.9%, P = 0.032). Translocation with acrocentric chromosomes exhibited a significantly higher proportion of 3:1 segregation (24.8% versus 5.1%, P < 0.0001) and a lower rate of 2:2 segregation (70.3% versus 87.0%, P = 0.00028) compared with the proportions in blastocysts from carriers without acrocentric chromosomes. The frequency of adjacent-2 segregation was significantly different in translocations with terminal breakpoints compared to the frequency in blastocysts from carriers without terminal breakpoints (6.7% versus 15.5%, P = 0.013).ConclusionsThis study indicates that the segregation modes in blastocysts were affected by the presence of acrocentric chromosomes and terminal breakpoints, but not by the carrier’s sex. Our data may be useful for predicting the segregation pattern of a reciprocal translocation and could support genetic counselling for balanced translocation carriers for PGT cycles using blastocyst biopsy.

Breath-Hold Diving.

Breath-hold diving is practiced by recreational divers, seafood divers, military divers, and competitive athletes. It involves highly integrated physiology and extreme responses. This article reviews human breath-hold diving physiology beginning with an historical overview followed by a summary of foundational research and a survey of some contemporary issues. Immersion and cardiovascular adjustments promote a blood shift into the heart and chest vasculature. Autonomic responses include diving bradycardia, peripheral vasoconstriction, and splenic contraction, which help conserve oxygen. Competitive divers use a technique of lung hyperinflation that raises initial volume and airway pressure to facilitate longer apnea times and greater depths. Gas compression at depth leads to sequential alveolar collapse. Airway pressure decreases with depth and becomes negative relative to ambient due to limited chest compliance at low lung volumes, raising the risk of pulmonary injury called "squeeze," characterized by postdive coughing, wheezing, and hemoptysis. Hypoxia and hypercapnia influence the terminal breakpoint beyond which voluntary apnea cannot be sustained. Ascent blackout due to hypoxia is a danger during long breath-holds, and has become common amongst high-level competitors who can suppress their urge to breathe. Decompression sickness due to nitrogen accumulation causing bubble formation can occur after multiple repetitive dives, or after single deep dives during depth record attempts. Humans experience responses similar to those seen in diving mammals, but to a lesser degree. The deepest sled-assisted breath-hold dive was to 214m. Factors that might determine ultimate human depth capabilities are discussed. © 2018 American Physiological Society. Compr Physiol 8:585-630, 2018.

Chromosome Segregation Analysis in Human Embryos Obtained from Couples Involving Male Carriers of Reciprocal or Robertsonian Translocation

The objective of this study was to investigate the frequency and type of chromosome segregation patterns in cleavage stage embryos obtained from male carriers of Robertsonian (ROB) and reciprocal (REC) translocations undergoing preimplantation genetic diagnosis (PGD) at our reproductive center. We used FISH to analyze chromosome segregation in 308 day 3 cleavage stage embryos obtained from 26 patients. The percentage of embryos consistent with normal or balanced segregation (55.1% vs. 27.1%) and clinical pregnancy (62.5% vs. 19.2%) rates were higher in ROB than the REC translocation carriers. Involvement of non-acrocentric chromosome(s) or terminal breakpoint(s) in reciprocal translocations was associated with an increase in the percent of embryos consistent with adjacent 1 but with a decrease in 3∶1 segregation. Similar results were obtained in the analysis of nontransferred embryos donated for research. 3∶1 segregation was the most frequent segregation type in both day 3 (31%) and spare (35%) embryos obtained from carriers of t(11;22)(q23;q11), the only non-random REC with the same breakpoint reported in a large number of unrelated families mainly identified by the birth of a child with derivative chromosome 22. These results suggest that chromosome segregation patterns in day 3 and nontransferred embryos obtained from male translocation carriers vary with the type of translocation and involvement of acrocentric chromosome(s) or terminal breakpoint(s). These results should be helpful in estimating reproductive success in translocation carriers undergoing PGD.

Meiotic segregation analysis of embryos from reciprocal translocation carriers in PGD cycles

Meiotic segregation patterns of 278 embryos from 41 preimplantation genetic diagnosis cycles of 34 reciprocal translocation carriers were analysed to investigate whether some characteristics of reciprocal translocation, including terminal breakpoints, acrocentric chromosome or carrier gender, are related to meiotic segregation patterns. The incidence of normal/balanced karyotypes in translocations with terminal breakpoints was significantly lower than those without terminal breakpoints (6.5% versus 14.4%, P = 0.005). The incidences of adjacent-1 (21.0% versus 29.6%), adjacent-2 (16.1% versus 11.1%) and 3:1 (41.9% versus 30.6%) segregation were not statistically significantly different in translocations with terminal breakpoints versus those without. Translocation with acrocentric chromosomes showed a significantly lower rate of 2:2 segregation (39.2% versus 60.2%, P = 0.001) and a higher rate of 3:1 segregation (43.1% versus 27.3%, P = 0.005) than those without acrocentric chromosomes. The incidence of 2:2 segregation was significantly higher in male than in female carriers (58.2% versus 45.0%, P = 0.019). This study suggested that reciprocal translocation involving terminal breakpoints resulted in a lower rate of normal/balanced karyotype in preimplantation embryos. Some characteristics of reciprocal translocation, such as terminal breakpoints, acrocentric chromosome and carrier gender, are related to the segregation patterns. Reciprocal translocations result from exchange of two terminal segments from different chromosomes. During meiosis I of reciprocal translocations, translocated chromosomes and their normal homologues segregate by alternate, adjacent-I, adjacent-II, 3:1 or 4:0 modes. Genetically normal/balanced gametes are only produced by alternate segregation while other segregation modes would lead to the production of chromosomally unbalanced gametes. Preimplantation genetic diagnosis (PGD) has been offered to carriers of reciprocal translocations to reduce the frequency of spontaneous abortions and affected newborns by selecting chromosomally normal/balanced embryos. In the present study, meiotic segregation patterns of 278 embryos from 41 PGD cycles of 34 reciprocal translocation carriers were analysed to investigate whether some characteristics of reciprocal translocation, including terminal breakpoints, acrocentric chromosome or carrier gender are related to meiotic segregation patterns. The incidence of normal/balanced karyotypes in translocations with terminal breakpoints was significantly lower than those without terminal breakpoints. The incidence of adjacent-1 segregation was lower and the incidences of adjacent-2 and 3:1 segregation were higher in translocations with terminal breakpoints. Translocation with acrocentric chromosomes showed a significantly lower rate of 2:2 segregation and a higher rate of 3:1 segregation than those without acrocentric chromosomes. The incidence of 2:2 segregation was significantly higher in male than in female carriers. Our study suggests that reciprocal translocations involving terminal breakpoints resulted in a lower rate of normal/balanced karyotype in preimplantation embryos. Some characteristics of reciprocal translocation, such as terminal breakpoints, acrocentric chromosome and carrier gender, are related to the segregation patterns.

Chromosomal location of genes for resistance to powdery mildew in common wheat (Triticum aestivum L. em Thell.). 9. Gene MlZec1 from the Triticum dicoccoides-derived wheat line Zecoi-1

The Triticum dicoccoides-derived wheat line Zecoi-1 provides effective protection against powdery mildew. F3 segregation analysis of Chinese Spring × Zecoi-1 hybrids showed that resistance in line Zecoi-1 is controlled by a single dominant gene. Amplified fragment length polymorphism (AFLP) analysis of bulked segregants from F3s showing the homozygous resistant and susceptible phenotypes identified eight markers, of which four were associated with the resistance allele in repulsion phase. Following the assignment of these four repulsion phase AFLP markers to wheat chromosome 2B with the aid of Chinese Spring nulli-tetrasomic lines, they were physically mapped in the terminal breakpoint interval 0.89 (2BL-6)–1.00 (telomere) of chromosome 2BL. Genetic and physical mapping of simple sequence repeat markers from the distal half of chromosome 2BL located the wild emmer-derived powdery mildew resistance gene distal of breakpoint 0.89 in deletion line 2BL-6. Based on disease response patterns, genomic origin and chromosomal location the resistance gene in Zecoi-1 is temporarily designated MlZec1.

Telomere dysfunction triggers extensive DNA fragmentation and evolution of complex chromosome abnormalities in human malignant tumors.

Although mechanisms for chromosomal instability in tumors have been described in animal and in vitro models, little is known about these processes in man. To explore cytogenetic evolution in human tumors, chromosomal breakpoint profiles were constructed for 102 pancreatic carcinomas and 140 osteosarcomas, two tumor types characterized by extensive genomic instability. Cases with few chromosomal alterations showed a preferential clustering of breakpoints to the terminal bands, whereas tumors with many changes showed primarily interstitial and centromeric breakpoints. The terminal breakpoint frequency was negatively correlated to telomeric TTAGGG repeat length, and fluorescence in situ hybridization with telomeric TTAGGG probes consistently indicated shortened telomeres and >10% of chromosome ends lacking telomeric signals. Because telomeric dysfunction may lead to formation of unstable ring and dicentric chromosomes, mitotic figures were also evaluated. Anaphase bridges were found in all cases, and fluorescence in situ hybridization demonstrated extensive structural rearrangements of chromosomes, with terminal transferase detection showing fragmented DNA in 5-20% of interphase cells. Less than 2% of cells showed evidence of necrosis or apoptosis, and telomerase was expressed in the majority of cases. Telomeric dysfunction may thus trigger chromosomal fragmentation through persistent bridge-breakage events in pancreatic carcinomas and osteosarcomas, leading to a continuous reorganization of the tumor genome. Telomerase expression is not sufficient for completely stabilizing the chromosome complement but may be crucial for preventing complete genomic deterioration and maintaining cellular survival.

Preimplantation genetic diagnosis of structural abnormalities

Preimplantation genetic diagnosis (PGD) of translocations can be achieved through a variety of methods. For female carriers, a possibility is by polar body biopsy and analysis of its metaphase chromosomes using painting probes. For male carriers or female carriers with terminal breakpoints, metaphase chromosomes can also be studied by fusing blastomeres to enucleated oocytes. Otherwise, interphase analysis of the translocation can be performed using distal, subtelomeric or breakpoint spanning probes. The results obtained after PGD of translocations indicate a significant decrease in spontaneous abortions after the procedure, a good selection against unbalanced oocytes and embryos, and pregnancy rates that depend on the type of translocation involved. Balanced translocations occur in 0.2% of the neonatal population, but at a higher rate among infertile couples and patients with recurrent abortions. In a recent report, balanced translocations were found in 0.6% of infertile couples, 3.2% of couples that failed over ten IVF cycles, and 9.2% among fertile couples experiencing three or more consecutive first-trimester abortions (Hum. Reprod. 14 (1999) 2097). They were also found in 2–3.2% of males requiring ICSI (Hum. Reprod. 11 (1996) 2609; Hum. Reprod. 13 (1998) 576). PGD can be offered to carriers of balanced translocations as an alternative to prenatal diagnosis and pregnancy termination of unbalanced fetuses. In recent years, PGD for structural chromosome abnormalities has been attempted by a variety of approaches. The aims of PGD for translocations are to reduce the rate of spontaneous abortions that this population suffers and to minimize the risk of conceiving an unbalanced baby.

Telomeres and their possible role in chromosome stabilization

The evidence to date generally supports the hypothesis that telomere capping makes chromosome fragments refractory to subsequent rejoining events, but this control may be somewhat relaxed after chromosome breakage. Cell survival requires that the fragments rejoin before metaphase. Unprotected ends such as those produced by DNA damage are subject to degradation, presumably by endogenous cellular exo- and endonucleases. Telomere repeat sequences may be added to broken chromosome ends to protect the ends from further degradation. That telomeric DNA does not always prevent rejoining raises interesting questions as to what constitutes capping, and how rapidly it occurs after DNA damage in relation to chromosome break rejoining. The prevention of degradation and control of rejoining may be mediated by telomere-specific binding proteins, especially the telomere terminal binding protein [Gualberto et al., 1992; Longtine et al., 1989; Price, 1990; Price and Cech, 1989]. Some of these proteins may be involved in scavenging telomeric DNA when the cell senses that chromosomal breaks have occurred. This mechanism is consistent with the observations of Murnane and Yu [1993], who found that a plasmid with telomere sequences was stably integrated in vivo into a chromosome terminal breakpoint lacking telomere repeats. It is also consistent with the high frequency of interstitial telomere sequences observed in normal cells; a history of DNA damage and repair may be recorded by these sequences (Ijdo et al., 1991]. Although chromosome break rejoining is an efficient process in eukaryotic cells, some breaks are never rejoined and can result in terminal deletions and chromatid and isochromatid deletions at metaphase. It is unclear why these breaks are not rejoined, but it may be due to one or more of the following: 1) chance: broken chromosomes are separated, do not approach sufficiently close to one another, and are consequently physically unable to rejoin; 2) a large number of added telomere repeat sequences indicating to the cell that the chromosome has an authentic telomere; 3) some other DNA modification event that protects DNA ends from degradation, e.g., folding back of DNA ends to form a hairpin, as has been implicated in VDJ recombination [Lieber, 1993].