First come, first served: Mammalian recombination is timed to replication.

Meiotic recombination drives the formation of new chromosomes in germ cells and is essential for fertility in mammals. In this issue of Cell, Pratto et al. have developed a method to map replication origins directly in mammalian tissue for the first time, revealing evolutionary conservation between replication timing and meiotic recombination in males.

Cas9 in Human Embryos: On Target but No Repair.

In this issue of Cell, Zuccaro and colleagues show that on-target Cas9-mediated double-strand breaks cause chromosome loss or mis-repair of the disease allele in > 90% of human embryos. End joining repair pathways dominate, causing small insertions or deletions, which raises serious questions about using double-strand breaks for "gene surgery".

Errors of the Egg: The Establishment and Progression of Human Aneuploidy Research in the Maternal Germline.

Meiosis, a key process in the creation of haploid gametes, is a complex cellular division incorporating unique timing and intricate chromosome dynamics. Abnormalities in this elaborate dance can lead to the production of aneuploid gametes, i.e., eggs containing an incorrect number of chromosomes, many of which cannot generate a viable pregnancy. For many decades, research has been attempting to address why this process is notoriously error prone in humans compared to many other organisms. Rapidly developing technologies, access to new clinical material, and a mounting public infertility crisis have kept the field both active and quickly evolving. In this review, we discuss the history of aneuploidy in humans with a focus on its origins in maternal meiosis. We also gather current working mechanistic hypotheses, as well as up-and-coming areas of interest that point to future scientific avenues and their potential clinical applications.

Regulation of the MLH1-MLH3 endonuclease in meiosis.

During prophase of the first meiotic division, cells deliberately break their DNA1. These DNA breaks are repaired by homologous recombination, which facilitates proper chromosome segregation and enables the reciprocal exchange of DNA segments between homologous chromosomes2. A pathway that depends on the MLH1-MLH3 (MutLγ) nuclease has been implicated in the biased processing of meiotic recombination intermediates into crossovers by an unknown mechanism3-7. Here we have biochemically reconstituted key elements of this pro-crossover pathway. We show that human MSH4-MSH5 (MutSγ), which supports crossing over8, binds branched recombination intermediates and associates with MutLγ, stabilizing the ensemble at joint molecule structures and adjacent double-stranded DNA. MutSγ directly stimulates DNA cleavage by the MutLγ endonuclease. MutLγ activity is further stimulated by EXO1, but only when MutSγ is present. Replication factor C (RFC) and the proliferating cell nuclear antigen (PCNA) are additional components of the nuclease ensemble, thereby triggering crossing-over. Saccharomyces cerevisiae strains in which MutLγ cannot interact with PCNA present defects in forming crossovers. Finally, the MutLγ-MutSγ-EXO1-RFC-PCNA nuclease ensemble preferentially cleaves DNA with Holliday junctions, but shows no canonical resolvase activity. Instead, it probably processes meiotic recombination intermediates by nicking double-stranded DNA adjacent to the junction points9. As DNA nicking by MutLγ depends on its co-factors, the asymmetric distribution of MutSγ and RFC-PCNA on meiotic recombination intermediates may drive biased DNA cleavage. This mode of MutLγ nuclease activation might explain crossover-specific processing of Holliday junctions or their precursors in meiotic chromosomes4.

CHK1-CDC25A-CDK1 regulate cell cycle progression and protect genome integrity in early mouse embryos.

After fertilization, remodeling of the oocyte and sperm genomes is essential to convert these highly differentiated and transcriptionally quiescent cells into early cleavage-stage blastomeres that are transcriptionally active and totipotent. This developmental transition is accompanied by cell cycle adaptation, such as lengthening or shortening of the gap phases G1 and G2. However, regulation of these cell cycle changes is poorly understood, especially in mammals. Checkpoint kinase 1 (CHK1) is a protein kinase that regulates cell cycle progression in somatic cells. Here, we show that CHK1 regulates cell cycle progression in early mouse embryos by restraining CDK1 kinase activity due to CDC25A phosphatase degradation. CHK1 kinase also ensures the long G2 phase needed for genome activation and reprogramming gene expression in two-cell stage mouse embryos. Finally, Chk1 depletion leads to DNA damage and chromosome segregation errors that result in aneuploidy and infertility.

Genetic predisposition to mosaic Y chromosome loss in blood.

Mosaic loss of chromosome Y (LOY) in circulating white blood cells is the most common form of clonal mosaicism1-5, yet our knowledge of the causes and consequences of this is limited. Here, using a computational approach, we estimate that 20% of the male population represented in the UK Biobank study (n = 205,011) has detectable LOY. We identify 156 autosomal genetic determinants of LOY, which we replicate in 757,114 men of European and Japanese ancestry. These loci highlight genes that are involved in cell-cycle regulation and cancer susceptibility, as well as somatic drivers of tumour growth and targets of cancer therapy. We demonstrate that genetic susceptibility to LOY is associated with non-haematological effects on health in both men and women, which supports the hypothesis that clonal haematopoiesis is a biomarker of genomic instability in other tissues. Single-cell RNA sequencing identifies dysregulated expression of autosomal genes in leukocytes with LOY and provides insights into why clonal expansion of these cells may occur. Collectively, these data highlight the value of studying clonal mosaicism to uncover fundamental mechanisms that underlie cancer and other ageing-related diseases.

Failure to recombine is a common feature of human oogenesis.

Failure of homologous chromosomes to recombine is arguably the most important cause of human meiotic nondisjunction, having been linked to numerous autosomal and sex chromosome trisomies of maternal origin. However, almost all information on these "exchangeless" homologs has come from genetic mapping studies of trisomic conceptuses, so the incidence of this defect and its impact on gametogenesis are not clear. If oocytes containing exchangeless homologs are selected against during meiosis, the incidence may be much higher in developing germ cells than in zygotes. To address this, we initiated studies of exchangeless chromosomes in fetal ovarian samples from elective terminations of pregnancy. In total, we examined more than 7,000 oocytes from 160 tissue samples, scoring for the number of foci per cell of the crossover-associated protein MLH1. We identified a surprisingly high level of recombination failure, with more than 7% of oocytes containing at least one chromosome pair that lacked an MLH1 focus. Detailed analyses indicate striking chromosome-specific differences, with a preponderance of MLH1-less homologs involving chromosomes 21 or 22. Further, the effect was linked to the overall level of recombination in the cell, with the presence of one or two exchangeless chromosomes in a cell associated with a 10%-20% reduction in the total number of crossovers. This suggests individuals with lower rates of meiotic recombination are at an increased risk of producing aneuploid offspring.

Mosaic human preimplantation embryos and their developmental potential in a prospective, non-selection clinical trial.

Chromosome imbalance (aneuploidy) is the major cause of pregnancy loss and congenital disorders in humans. Analyses of small biopsies from human embryos suggest that aneuploidy commonly originates during early divisions, resulting in mosaicism. However, the developmental potential of mosaic embryos remains unclear. We followed the distribution of aneuploid chromosomes across 73 unselected preimplantation embryos and 365 biopsies, sampled from four multifocal trophectoderm (TE) samples and the inner cell mass (ICM). When mosaicism impacted fewer than 50% of cells in one TE biopsy (low-medium mosaicism), only 1% of aneuploidies affected other portions of the embryo. A double-blinded prospective non-selection trial (NCT03673592) showed equivalent live-birth rates and miscarriage rates across 484 euploid, 282 low-grade mosaic, and 131 medium-grade mosaic embryos. No instances of mosaicism or uniparental disomy were detected in the ensuing pregnancies or newborns, and obstetrical and neonatal outcomes were similar between the study groups. Thus, low-medium mosaicism in the trophectoderm mostly arises after TE and ICM differentiation, and such embryos have equivalent developmental potential as fully euploid ones.

Cell-free fetal DNA for genetic evaluation in Copenhagen Pregnancy Loss Study (COPL): a prospective cohort study.

BACKGROUND: One in four pregnancies end in a pregnancy loss. Although the effect on couples is well documented, evidence-based treatments and prediction models are absent. Fetal aneuploidy is associated with a higher chance of a next successful pregnancy compared with euploid pregnancy loss in which underlying maternal conditions might be causal. Ploidy diagnostics are therefore advantageous but challenging as they require collection of the pregnancy tissue. Cell-free fetal DNA (cffDNA) from maternal blood has the potential for evaluation of fetal ploidy status, but no large-scale validation of the method has been done. METHODS: In this prospective cohort study, women with a pregnancy loss were recruited as a part of the Copenhagen Pregnancy Loss (COPL) study from three gynaecological clinics at public hospitals in Denmark. Women were eligible for inclusion if older than 18 years with a pregnancy loss before gestational age 22 weeks (ie, 154 days) and with an intrauterine pregnancy confirmed by ultrasound (including anembryonic sac), and women with pregnancies of unknown location or molar pregnancies were excluded. Maternal blood was collected while pregnancy tissue was still in situ or within 24 h after pregnancy tissue had passed and was analysed by genome-wide sequencing of cffDNA. Direct sequencing of the pregnancy tissue was done as reference. FINDINGS: We included 1000 consecutive women, at the time of a pregnancy loss diagnosis, between Nov 12, 2020, and May 1, 2022. Results from the first 333 women with a pregnancy loss (recruited between Nov 12, 2020, and Aug 14, 2021) were used to evaluate the validity of cffDNA-based testing. Results from the other 667 women were included to evaluate cffDNA performance and result distribution in a larger cohort of 1000 women in total. Gestational age of fetus ranged from 35-149 days (mean of 70·5 days [SD 16·5], or 10 weeks plus 1 day). The cffDNA-based test had a sensitivity for aneuploidy detection of 85% (95% CI 79-90) and a specificity of 93% (95% CI 88-96) compared with direct sequencing of the pregnancy tissue. Among 1000 cffDNA-based test results, 446 (45%) were euploid, 405 (41%) aneuploid, 37 (4%) had multiple aneuploidies, and 112 (11%) were inconclusive. 105 (32%) of 333 women either did not manage to collect the pregnancy tissue or collected a sample classified as unknown tissue giving a high risk of being maternal. INTERPRETATION: This validation of cffDNA-based testing in pregnancy loss shows the potential and feasibility of the method to distinguish euploid and aneuploid pregnancy loss for improved clinical management and benefit of future reproductive medicine and women's health research. FUNDING: Ole Kirks Foundation, BioInnovation Institute Foundation, and the Novo Nordisk Foundation.

Results from the first autologous grafting of adult human testis tissue: a case report.

Fertility restoration using autologous testicular tissue transplantation is relevant for infertile men surviving from childhood cancer and, possibly, in men with absent or incomplete spermatogenesis resulting in the lack of spermatozoa in the ejaculate (non-obstructive azoospermia, NOA). Currently, testicular tissue from pre-pubertal boys extracted before treatment with gonadotoxic cancer therapy can be cryopreserved with good survival of spermatogonial stem cells. However, strategies for fertility restoration, after successful cancer treatment, are still experimental and no clinical methods have yet been developed. Similarly, no clinically available treatments can help men with NOA to become biological fathers after failed attempts of testicular surgical sperm retrieval. We present a case of a 31-year-old man with NOA who had three pieces of testis tissue (each ∼2 × 4 × 2 mm3) extracted and cryopreserved in relation to performing microdissection testicular sperm extraction (mTESE). Approximately 2 years after mTESE, the thawed tissue pieces were engrafted in surgically created pockets bilaterally under the scrotal skin. Follow-up was performed after 2, 4, and 6 months with assessment of reproductive hormones and ultrasound of the scrotum. After 6 months, all engrafted tissue was extracted and microscopically analyzed for the presence of spermatozoa. Furthermore, parts of the extracted tissue were analyzed histologically and by immunohistochemical analysis. Active blood flow in the engrafted tissue was demonstrated by doppler ultrasound after 6 months. No spermatozoa were found in the extracted tissue. Histological and immunohistochemical analysis demonstrated graft survival with intact clear tubules and normal cell organization. Sertoli cells and spermatocytes with normal morphology were located near the basement membrane. MAGE-A and VASA positive spermatogonia/spermatocytes were detected together with SOX9 positive Sertoli cells. Spermatocytes and/or Sertoli cells positive for γH2AX was also detected. In summary, following autologous grafting of frozen-thawed testis tissue under the scrotal skin in a man with NOA, we demonstrated graft survival after 6 months. No mature spermatozoa were detected; however, this is likely due to the pre-existing spermatogenic failure.

Genome diversity and instability in human germ cells and preimplantation embryos.

Genome diversity is essential for evolution and is of fundamental importance to human health. Generating genome diversity requires phases of DNA damage and repair that can cause genome instability. Humans have a high incidence of de novo congenital disorders compared to other organisms. Recent access to eggs, sperm and preimplantation embryos is revealing unprecedented rates of genome instability that may result in infertility and de novo mutations that cause genomic imbalance in at least 70% of conceptions. The error type and incidence of de novo mutations differ during developmental stages and are influenced by differences in male and female meiosis. In females, DNA repair is a critical factor that determines fertility and reproductive lifespan. In males, aberrant meiotic recombination causes infertility, embryonic failure and pregnancy loss. Evidence suggest germ cells are remarkably diverse in the type of genome instability that they display and the DNA damage responses they deploy. Additionally, the initial embryonic cell cycles are characterized by a high degree of genome instability that cause congenital disorders and may limit the use of CRISPR-Cas9 for heritable genome editing.

Incidence, Origin, and Predictive Model for the Detection and Clinical Management of Segmental Aneuploidies in Human Embryos.

Despite next-generation sequencing, which now allows for the accurate detection of segmental aneuploidies from in vitro fertilization embryo biopsies, the origin and characteristics of these aneuploidies are still relatively unknown. Using a multifocal biopsy approach (four trophectoderms [TEs] and one inner cell mass [ICM] analyzed per blastocyst; n = 390), we determine the origin of the aneuploidy and the diagnostic predictive value of segmental aneuploidy detection in TE biopsies toward the ICM's chromosomal constitution. Contrary to the prevalent meiotic origin of whole-chromosome aneuploidies, we show that sub-chromosomal abnormalities in human blastocysts arise from mitotic errors in around 70% of cases. As a consequence, the positive-predictive value toward ICM configuration was significantly lower for segmental as compared to whole-chromosome aneuploidies (70.8% versus 97.18%, respectively). In order to enhance the clinical utility of reporting segmental findings in clinical TE biopsies, we have developed and clinically verified a risk stratification model based on a second TE biopsy confirmation and segmental length; this model can significantly improve the prediction of aneuploidy risk in the ICM in over 86% of clinical cases enrolled. In conclusion, we provide evidence of the predominant mitotic origin of segmental aneuploidies in preimplantation embryos and develop a risk stratification model that can help post-test genetic counseling and that facilitates the decision-making process on clinical utilization of these embryos.

Childhood BMI after ART with frozen embryo transfer.

STUDY QUESTION: Does BMI at 7-10 years of age differ in children conceived after frozen embryo transfer (FET) compared to children conceived after fresh embryo transfer (fresh-ET) or natural conception (NC)? SUMMARY ANSWER: BMI in childhood does not differ between children conceived after FET compared to children conceived after fresh-ET or NC. WHAT IS KNOWN ALREADY: High childhood BMI is strongly associated with obesity and cardiometabolic disease and mortality in adulthood. Children conceived after FET have a higher risk of being born large for gestational age (LGA) than children conceived after NC. It is well-documented that being born LGA is associated with an increased risk of obesity in childhood, and it has been hypothesized that ART induces epigenetic variations around fertilization, implantation, and early embryonic stages, which influence fetal size at birth as well as BMI and health later in life. STUDY DESIGN, SIZE, DURATION: The study 'Health in Childhood following Assisted Reproductive Technology' (HiCART) is a large retrospective cohort study with 606 singletons aged 7-10 years divided into three groups according to mode of conception: FET (n = 200), fresh-ET (n = 203), and NC (n = 203). All children were born in Eastern Denmark from 2009 to 2013 and the study was conducted from January 2019 to September 2021. PARTICIPANTS/MATERIALS, SETTING, METHODS: We anticipated that the participation rate would differ between the three study groups owing to variation in the motivation to engage. To reach the goal of 200 children in each group, we invited 478 in the FET-group, 661 in the fresh-ET-group, and 1175 in the NC-group. The children underwent clinical examinations including anthropometric measurements, whole-body dual-energy x-ray absorptiometry-scan, and pubertal staging. Standard deviation scores (SDS) were calculated for all anthropometric measurements using Danish reference values. Parents completed a questionnaire regarding the pregnancy and the current health of the child and themselves. Maternal, obstetric, and neonatal data were obtained from the Danish IVF Registry and Danish Medical Birth Registry. MAIN RESULTS AND THE ROLE OF CHANCE: As expected, children conceived after FET had a significantly higher birthweight (SDS) compared to both children born after fresh-ET (mean difference 0.42, 95% CI (0.21; 0.62)) and NC (mean difference 0.35, 95% CI (0.14; 0.57)). At follow-up (7-10 years), no differences were found in BMI (SDS) comparing FET to fresh-ET, FET to NC, and fresh-ET to NC. Similar results were also found regarding the secondary outcomes weight (SDS), height (SDS), sitting height, waist circumference, hip circumference, fat, and fat percentage. In the multivariate linear regression analyses, the effect of mode of conception remained non-significant after adjusting for multiple confounders. When stratified on sex, weight (SDS), and height (SDS) were significantly higher for girls born after FET compared to girls born after NC. Further, FET-girls also had significantly higher waist, hip, and fat measurements compared to girls born after fresh-ET. However, for the boys the differences remained insignificant after confounder adjustment. LIMITATIONS, REASONS FOR CAUTION: The sample size was decided in order to detect a difference of 0.3 SDS in childhood BMI (which corresponds to an adult cardiovascular mortality hazard ratio of 1.034). Thus, smaller differences in BMI SDS may be overlooked. As the overall participation rate was 26% (FET: 41%, fresh-ET: 31%, NC: 18%), selection bias cannot be excluded. Regarding the three study groups, many possible confounders have been included but there might be a small risk of selection bias as information regarding cause of infertility is not available in this study. WIDER IMPLICATIONS OF THE FINDINGS: The increased birthweight in children conceived after FET did not translate into differences in BMI, however, for the girls born after FET, we observed increased height (SDS) and weight (SDS) compared to the girls born after NC, while for the boys the results remained insignificant after confounder adjustment. Since body composition in childhood is a strong biomarker of cardiometabolic disease later in life, longitudinal studies of girls and boys born after FET are needed. STUDY FUNDING/COMPETING INTEREST(S): The study was funded by the Novo Nordisk Foundation (grant number: NNF18OC0034092, NFF19OC0054340) and Rigshospitalets Research Foundation. There were no competing interests. TRIAL REGISTRATION NUMBER: ClinicalTrials.gov identifier: NCT03719703.

Maternal exome analysis for the diagnosis of oocyte maturation defects and early embryonic developmental arrest.

RESEARCH QUESTION: Can a methodology be developed for case selection and whole-exome sequencing (WES) analysis of women who are infertile owing to recurrent oocyte maturation defects (OOMD) and/or preimplantation embryo lethality (PREMBL)? DESIGN: Data were collected from IVF patients attending the Istanbul Memorial Hospital (2015-2021). A statistical methodology to identify infertile endophenotypes (recurrent low oocyte maturation rate, low fertilization rate and preimplantation developmental arrest) was developed using a large IVF dataset (11,221 couples). Twenty-eight infertile women with OOMD/PREMBL were subsequently enrolled for WES on their genomic DNA. Pathogenic variants were prioritized using a custom-made bioinformatic pipeline set to minimize false-positive discoveries through resampling in control cohorts (the Human Genome Diversity Project and 1343 whole-exome sequences from oocyte donors). Individual single-cell RNA sequencing data from 18 human metaphase II (MII) oocytes and antral granulosa cells was used for genome-wide validation. WES and bioinformatics were performed at Igenomix and the National Research Council, Italy. RESULTS: Variant prioritization analysis identified 265 unique variants in 248 genes (average 22.4 per sample). Of the genes harbouring high-impact variants 78% were expressed by MII oocytes and/or antral granulosa cells, significantly higher than for random sample of controls (odds ratio = 5, Fisher's exact P = 0.0004). Seven of the 28 women (25%) were homozygous carriers of missense pathogenic variants in known candidate genes for OOMD/PREMBL, including PATL2, NLRP5 (n = 2),TLE6, PADI6, TUBB8 and TRIP13. Furthermore, novel gene-disease associations were identified. In fact, one woman with a low oocyte maturation rate was a homozygous carrier of high-impact variants in ENSA, an essential gene for prophase I meiotic transition in mice. CONCLUSIONS: This analytical framework could reveal known and new genes associated with isolated recurrent OOMD/PREMBL, providing essential indications for scaling this strategy to larger studies.

Impact of implementation of the Maternal-Fetal Triage Index on patients presenting with severe hypertension.

BACKGROUND: Severe hypertension remains one of the leading preventable causes of maternal mortality in the United States. Timeliness to response to severe hypertension in pregnancy is a crucial quality indicator tracked by state and national organizations. We hypothesized that the implementation of the Maternal-Fetal Triage Index, a validated acuity tool, would improve care performance in women with severe hypertension in an urban, inner-city hospital setting. OBJECTIVE: This study aimed to assess the impact of the Maternal-Fetal Triage Index on the management of women presenting with severe preeclampsia diagnosed by severe hypertension as measured by time to provider assessment, administration of magnesium sulfate, and immediate administration of acute antihypertensives. STUDY DESIGN: This was a prospective, observational study of pregnant women presenting to the labor and delivery triage unit with severe preeclampsia diagnosed by severe hypertension giving birth at a large urban inner-city academic facility before (epoch 1: January 1, 2019, to December 31, 2019) and after (epoch 2: March 1, 2021, to September 31, 2021) the implementation of the Maternal-Fetal Triage Index. Baseline outcomes of time to assessment, time to magnesium sulfate prophylaxis, and time to antihypertensive medication administration before the implementation of the Maternal-Fetal Triage Index were assessed. The Maternal-Fetal Triage Index tool was implemented on March 1, 2021, following standardized education in 2020 for all triage nurses, unit technicians, healthcare unit coordinators, and healthcare providers. Time to assessment, administration of magnesium sulfate prophylaxis, and time to antihypertensive administration after the implementation of the Maternal-Fetal Triage Index were compared with measures before the implementation of the Maternal-Fetal Triage Index. Statistical analysis included Wilcoxon rank-sum test with P<.05 considered significant when comparing epoch 1 with epoch 2. RESULTS: A total of 370 patients were admitted with severe hypertension in 2019 before the use of the Maternal-Fetal Triage Index, and 254 patients were admitted with severe hypertension in 2021 after the implementation of the Maternal-Fetal Triage Index. There was no difference between epochs across baseline characteristics, including age, race and ethnicity, parity, and body mass index. After the Maternal-Fetal Triage Index was implemented, the time to provider assessment was significantly improved, from a median time of 44 minutes (interquartile range, 0-65) in epoch 1 to 17 minutes (interquartile range, 0-39) in epoch 2 (P<.001). Furthermore, the time from arrival to magnesium sulfate prophylaxis was significantly faster with a median time of 161 minutes (interquartile range, 109-256) in epoch 1 vs 127 minutes (interquartile range, 85-258) in epoch 2 (P=.001). Moreover, there was a decrease in the time from arrival to antihypertensive medication administration for severe blood pressures after the implementation of the Maternal-Fetal Triage Index (101 minutes [interquartile range, 61-177] vs 66 minutes [interquartile range, 35-203]; P<.001). CONCLUSION: The implementation of the Maternal-Fetal Triage Index at a large urban inner-city hospital was associated with improved timeliness of assessment and treatment of women with severe hypertension. The Maternal-Fetal Triage Index is a viable tool to improve the efficiency in triage units, specifically in the management of severe hypertension.

Relatives' negotiation power in relation to older people's acute hospital admission: A qualitative interview study.

BACKGROUND: Acutely admitted older people are potentially vulnerable and dependent on relatives to negotiate and navigate on their behalf. AIM: This study aimed to explore relatives' experiences of their interactions with healthcare professionals during acute hospital admission of older people to derive themes of importance for relatives' negotiations with these professionals. METHOD: A qualitative design was applied. Relatives of acutely admitted older people at two emergency departments in Denmark were interviewed (n = 17). The qualitative content analysis was guided by Graneheim and Lundman's concepts. RESULTS: The analysis derived four themes: (a) Mandate, (b) Incentive, (c) Capability and (d) Attitude to taking action. These four sources of relatives' negotiation power can be illustrated in the MICA model. CONCLUSION: Four themes were identified as important sources of relatives' negotiation power. Since the four sources of power potentially change according to the situation, relatives' negotiation power seems to be context dependent.

Strategies for Covalent Labeling of Long RNAs.

The introduction of chemical modifications into long RNA molecules at specific positions for visualization, biophysical investigations, diagnostic and therapeutic applications still remains challenging. In this review, we present recent approaches for covalent internal labeling of long RNAs. Topics included are the assembly of large modified RNAs via enzymatic ligation of short synthetic oligonucleotides and synthetic biology approaches preparing site-specifically modified RNAs via in vitro transcription using an expanded genetic alphabet. Moreover, recent approaches to employ deoxyribozymes (DNAzymes) and ribozymes for RNA labeling and RNA methyltransferase based labeling strategies are presented. We discuss the potentials and limits of the individual methods, their applicability for RNAs with several hundred to thousands of nucleotides in length and indicate future directions in the field.

Chromosomal mosaicism: Origins and clinical implications in preimplantation and prenatal diagnosis.

The diagnosis of chromosomal mosaicism in the preimplantation and prenatal stage is fraught with uncertainty and multiple factors need to be considered in order to gauge the likely impact. The clinical effects of chromosomal mosaicism are directly linked to the type of the imbalance (size, gene content, and copy number), the timing of the initial event leading to mosaicism during embryogenesis/fetal development, the distribution of the abnormal cells throughout the various tissues within the body as well as the ratio of normal/abnormal cells within each of those tissues. Additional factors such as assay noise and culture artifacts also have an impact on the significance and management of mosaic cases. Genetic counseling is an important part of educating patients about the likelihood of having a liveborn with a chromosome abnormality and these risks differ according to the time of ascertainment and the tissue where the mosaic cells were initially discovered. Each situation needs to be assessed on a case-by-case basis and counseled accordingly. This review will discuss the clinical impact of finding mosaicism through: embryo biopsy, chorionic villus sampling, amniocentesis, and noninvasive prenatal testing using cell-free DNA.

Origins and mechanisms leading to aneuploidy in human eggs.

The gain or loss of a chromosome-or aneuploidy-acts as one of the major triggers for infertility and pregnancy loss in humans. These chromosomal abnormalities affect more than 40% of eggs in women at both ends of the age spectrum, that is, young girls as well as women of advancing maternal age. Recent studies in human oocytes and embryos using genomics, cytogenetics, and in silico modeling all provide new insight into the rates and potential genetic and cellular factors associated with aneuploidy at varying stages of development. Here, we review recent studies that are shedding light on potential molecular mechanisms of chromosome missegregation in oocytes and embryos across the entire female reproductive life span.