Balanced Chromosomal Abnormalities Research Articles

P-714 Prevalence of chromosomal abnormalities in oocyte donor candidates: a French survey of over 8200 karyotypes

Abstract Study question Is karyotyping worth considering for fertile and childless oocyte donor candidates? Summary answer Karyotyping reduces the genetic risk of utilizing oocytes with balanced structural chromosomal rearrangements for recipient couples. What is known already While some countries including France consistently advocate for karyotype to be routinely performed, others suggest it as optional, specifically advising it for donors with a medical history indicating chromosomal rearrangements. Ravel et al. have demonstrated a higher incidence of balanced structural chromosomal rearrangements among 1196 french fertile oocyte donor candidates (FODC) compared to unselected newborns, raising the question whether oocyte donor recruitment could be biased towards chromosomally at-risk women. In 2016, France authorized childless oocyte donor candidates (CODC) to enhance recruitment, yet as of now, there is no reported study on their genetic status. Study design, size, duration A retrospective observational multicentric study conducted from January 2005 to October 2021. Participants/materials, setting, methods ODC were recruited within the French CECOS (Centre d’Etude et de Conservation des Œufs et du Sperme humain) network. Both FODC and CODC were compared to a control group composed of female newborns from literature. Quantitative and qualitative data were expressed as the median [25%–75%] or n (‰), respectively. Non-parametric Mann-Whitney tests were employed for quantitative parameters, while Chi-square tests or Fisher’s exact tests were used, as appropriate, to compare frequencies between groups. Main results and the role of chance A total of 8229 ODC from 22 CECOS centers were included in this study. FODC (n = 6339) and CODC (n = 1890) were compared to 8102 control female newborns. In total, 65 chromosomal abnormalities (7.9‰) were identified, leading to the exclusion of the ODC. This outcome was comparable to the 53 abnormalities observed in the control group (6.54‰, p = 0.3059). The occurrence of balanced structural chromosomal rearrangements was increased in ODC population (4.8‰) compared to controls (2.34‰, p = 0.0086). The occurance of reciprocal translocations was increased 5-fold in CODC (3.70‰) compared to the control group (0.74‰, p = 0.013). The number of gonosomal mosaics was notably increased in FOCD, with 17 cases (2.68‰) compared to the control group with six cases (0.74‰, p = 0.0052). Among chromosomal aberration carriers only two childless women (one reciprocal translocation and one gonosomal mosaïc) had fertility issues in their medical history with a diagnosis of premature ovarian failure. Limitations, reasons for caution This retrospective study was designed to examine ODC within the framework of French bioethics laws. ODC should adhered to specific criteria: being volunteers, anonymous, aged 18 to 38, and without any financial compensation. Since 2016, donors were permitted to retain around 50% of their gametes for personal oocyte cryopreservation. Wider implications of the findings Karyotyping is a straightforward and cost-effective approach to identify structurally balanced chromosomal abnormalities. Thus, chromosomal analysis limits genetic risks whithout resorting to the ethically debated expanded genetic screening in the ODC population. Nevertheless, it is crucial to communicate to gamete recipients the inherent genetic risks associated with any conception. Trial registration number not applicable

Chromosomal structural rearrangements implicate long non-coding RNAs in rare germline disorders

In recent years, there has been increased focus on exploring the role the non-protein-coding genome plays in Mendelian disorders. One class of particular interest is long non-coding RNAs (lncRNAs), which has recently been implicated in the regulation of diverse molecular processes. However, because lncRNAs do not encode protein, there is uncertainty regarding what constitutes a pathogenic lncRNA variant, and thus annotating such elements is challenging. The Developmental Genome Anatomy Project (DGAP) and similar projects recruit individuals with apparently balanced chromosomal abnormalities (BCAs) that disrupt or dysregulate genes in order to annotate the human genome. We hypothesized that rearrangements disrupting lncRNAs could be the underlying genetic etiology for the phenotypes of a subset of these individuals. Thus, we assessed 279 cases with BCAs and selected 191 cases with simple BCAs (breakpoints at only two genomic locations) for further analysis of lncRNA disruptions. From these, we identified 66 cases in which the chromosomal rearrangements directly disrupt lncRNAs. In 30 cases, no genes of any other class aside from lncRNAs are directly disrupted, consistent with the hypothesis that lncRNA disruptions could underly the phenotypes of these individuals. Strikingly, the lncRNAs MEF2C-AS1 and ENSG00000257522 are each disrupted in two unrelated cases. Furthermore, we experimentally tested the lncRNAs TBX2-AS1 and MEF2C-AS1 and found that knockdown of these lncRNAs resulted in decreased expression of the neighboring transcription factors TBX2 and MEF2C, respectively. To showcase the power of this genomic approach for annotating lncRNAs, here we focus on clinical reports and genetic analysis of seven individuals with likely developmental etiologies due to lncRNA disruptions.

Rare germline disorders implicate long non-coding RNAs disrupted by chromosomal structural rearrangements.

In recent years, there has been increased focus on exploring the role the non-protein-coding genome plays in Mendelian disorders. One class of particular interest is long non-coding RNAs (lncRNAs), which has recently been implicated in the regulation of diverse molecular processes. However, because lncRNAs do not encode protein, there is uncertainty regarding what constitutes a pathogenic lncRNA variant, and thus annotating such elements is challenging. The Developmental Genome Anatomy Project (DGAP) and similar projects recruit individuals with apparently balanced chromosomal abnormalities (BCAs) that disrupt or dysregulate genes in order to annotate the human genome. We hypothesized that rearrangements disrupting lncRNAs could be the underlying genetic etiology for the phenotypes of a subset of these individuals. Thus, we assessed 279 cases with BCAs and selected 191 cases with simple BCAs (breakpoints at only two genomic locations) for further analysis of lncRNA disruptions. From these, we identified 66 cases in which the chromosomal rearrangements directly disrupt lncRNAs. Strikingly, the lncRNAs MEF2C-AS1 and ENSG00000257522 are each disrupted in two unrelated cases. Furthermore, in 30 cases, no genes of any other class aside from lncRNAs are directly disrupted, consistent with the hypothesis that lncRNA disruptions could underly the phenotypes of these individuals. To showcase the power of this genomic approach for annotating lncRNAs, here we focus on clinical reports and genetic analysis of two individuals with BCAs and additionally highlight six individuals with likely developmental etiologies due to lncRNA disruptions.

Abstract 1751: Deep multi-omics analysis of uterine leiomyosarcoma reveals defective HRD signature, a novel gene fusion and the amplification of the poor-prognosis CTHRC1 gene

Abstract Uterine leiomyosarcoma (uLMS) is a rare and aggressive cancer representing approximately 2-5% of all uterine malignancies. The molecular heterogeneity and pathogenesis of uLMS are notwell understood. To unravel the intricate molecular landscape of uLMS, we conducted a multi-omics workflow consisting of 4 high-throughput sequencing technologies; short- and long-readwhole genome sequencing (WGS), RNA-Seq, and Tandem mass tag (TMT) labeling enablingmassively parallel proteome proteomic analyses. Our study cohort consisted of 97 fresh frozentissue samples collected from 68 female patients, with 19 uLMS, 37 leiomyoma (LM), and 41normal myometrium (MM). A deep analysis of somatic variants with clinical impact revealed 46variants present in 90% of samples and two actionable therapeutic targets: IDH1_p.Arg132Cysand KRAS_p.Gly12Val. Also, 80% of the samples presented a chromothripsis signature and 60%presented the SBS3 COSMIC signature related to homologous recombination deficiency (HRD).Analysis of the proliferation score in RNA-Seq data revealed a notable difference among thetumor groups, signifying a higher proliferation index in the uLMS group, suggesting a moreaggressive growth pattern with implications for clinical behavior and disease progression. Afterapplying a customized workflow to identify balanced chromosomal abnormalities of genefusions with clinical impact, the proto-oncogene eukaryotic elongation factor 1 alpha (EEF1A1)emerged as a prominent fusion partner. Eight fusions in 7 samples were identified withtranscriptional overexpression in positive cases. At the proteomic level the enrichment analysisunveiled activated pathways associated with the innate immune system, while suppressedpathways pertained to extracellular matrix (ECM) organization and smooth muscle contraction.With an integrated bioinformatics pipeline, we identified copy number amplification of theCTHRC1 gene in 80% of the samples, accompanied by increased gene expression and proteinlevels. This gene is related to collagen remodeling and degradation and has been demonstratedto be upregulated in some solid tumors and is closely associated with malignancy. Survivalcurve analysis highlighted a significantly poorer prognosis in uLMS exhibiting CTHRC1 geneamplification with potential clinical relevance of CTHRC1 amplification as a prognostic indicatorin uLMS. Taken together these results show the overall suppression of ECM and collagenremodeling in uLMS associated with poor prognosis. Additionally, these data provide the firstevidence that the EEF1A1 gene has a strong selective pressure in uLMS underscoring theindispensable role of precision genomics in advancing precision medicine. Citation Format: Raul Maia Falcao, Jorge Estefano de Souza, William Mathieson, Joseph Woodward Carlson, Tirzah Braz Petta. Deep multi-omics analysis of uterine leiomyosarcoma reveals defective HRD signature, a novel gene fusion and the amplification of the poor-prognosis CTHRC1 gene [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1751.

The Main Causes of Recurrent Spontaneous Miscarriages

Summary: Repeated spontaneous miscarriages are defined as at least three consecutive episodes of spontaneous miscarriage, less than 14 weeks of amenorrhea, with the same partner, in a patient under 40 years of age. 80% of these patients have an identifiable cause justifying an etiological approach. A guided history and clinical examination are necessary. An etiological assessment is to be carried out: a pelvic ultrasound and a hormonal assessment in the woman, as well as a karyotype of the two members of the couple. The other examinations are requested as a second intention. The main causes are balanced chromosomal abnormalities of the parents, sperm DNA fragmentation, congenital malformations and lesions of the uterine cavity, anti-phospholipid syndrome, unbalanced diabetes, dysthyroidism and luteal insufficiency. In the event of an identified cause, certain therapeutic or preventive measures must be discussed in order to limit the risk of recurrence.

Semen parameters in male carriers of Robertsonian translocations (13;14)

Background.Robertsonian translocations (RTs) are the most common type of balanced chromosomal abnormalities. RT male carriers have an increased risk of reproductive disorders, but the causes of this still are not clear enough.Aim.To evaluate semen parameters in RT(13;14) male carriers.Materials and methods.24 men with 45,XY,der(13;14)(q10;q10) karyotype and infertility (n = 23) or recurrent pregnancy loss in spouse (n = 1).Results.Oligoasthenoteratozoospermia was detected in 18 (75 %) patients, asthenoteratozoospermia in 3 (13 %) patients, and azoospermia in 2 (8 %) patients. Normozoospermia was detected in 1 (4 %) patient, whose wife had a history of recurrent pregnancy loss.Conclusion.Infertility in the examined RT male carriers is associated with spermatogenesis disorders, in most cases resulted in oligozooasthenoteratozoospermia.

The Precise Breakpoint Mapping in Paracentric Inversion 10q22.2q23.3 by Comprehensive Cytogenomic Analysis, Multicolor Banding, and Single-Copy Chromosome Sequencing.

Detection and precise genomic mapping of balanced chromosomal abnormalities in patients with impaired fertility or a clinical phenotype represent a challenge for current cytogenomics owing to difficulties with precise breakpoint localization in the regions enriched for DNA repeats and high genomic variation in such regions. Here, we present a comprehensive cytogenomic approach to breakpoint mapping in a rare paracentric inversion on 10q (in a patient with oligoasthenoteratozoospermia and necrozoospermia) that does not affect other phenotype traits. Multicolor banding, chromosomal microarray analysis, chromosome microdissection with reverse painting, and single-copy sequencing of the rearranged chromosome were performed to determine the length and position of the inverted region as well as to rule out a genetic imbalance at the breakpoints. As a result, a paracentric 19.251 Mbp inversion at 10q22.2q23.3 was described. The most probable location of the breakpoints was predicted using the hg38 assembly. The problems of genetic counseling associated with enrichment for repeats and high DNA variability of usual breakpoint regions were discussed. Possible approaches for cytogenomic assessment of couples with balanced chromosome rearrangements and problems like reproductive failures were considered and suggested as useful part of effective genetic counseling.

Prenatal diagnosis of 21 fetuses with balanced chromosomal abnormalities (BCAs) using whole-genome sequencing.

Balanced chromosomal abnormalities (BCAs) are the most common chromosomal abnormalities and the frequency of congenital abnormalities is approximately twice as high in newborns with a de novo BCA, but a prenatal diagnosis based on BCAs is subject to evaluation. To detect translocation breakpoints and conduct a prenatal diagnosis, we performed whole-genome sequencing (WGS) in 21 subjects who were found BCAs, 19 balanced chromosome translocations and two inversions, in prenatal screening. In 16 BCAs on non-N-masked regions (non-NMRs), WGS detected 13 (81.2%, 13/16) BCAs, including all the inversions. All the breakpoints of 12 (12/14) cases of sufficient DNA were confirmed by Sanger sequencing. In 13 interrupted genes, CACNA1E (in case 12) and STARD7 (in case 17) are known causative and PDCL was found in subject (case 11) with situs inversus for the first time. Case 12 with abnormal ultrasound reached a definitive genetic diagnosis of CACNA1E-disease, while STARD7 exon deletion has never been found causative in patients. WGS provides the possibility of prenatal diagnosis in fetuses with BCAs, and its clinical significance also lies in providing data for postnatal diagnosis.

Preimplantation Genetic Testing for Couples with Balanced Chromosomal Rearrangements.

Chromosomal rearrangements play an important role in infertility. Carriers of chromosomal rearrangements have a lower chance of producing normal or balanced gametes due to abnormal segregation of chromosomes at meiosis, which leads to recurrent spontaneous abortions and infertility. Preimplantation genetic testing for structural chromosome rearrangements (PGT-SR) is offered to couples who have balanced chromosomal rearrangements in order to select embryos with a balanced karyotype prior to implantation, thereby increasing the chances of pregnancy. The purpose of the current study was to assess the outcomes of PGT-SR in patients carrying various balanced chromosomal rearrangements and to assess their clinical pregnancy outcome after in vitro fertilization (IVF). In this study, infertile couples with balanced chromosomal abnormalities undergoing PGT-SR were retrospectively analyzed at a single fertility center from January 2016 to December 2019. PGT-SR was performed on 87 embryos from 22 couples in whom one partner carried a balanced translocation or an inversion. Fifty-seven (65.5%) of these embryos had unbalanced or sporadic aneuploidies, 30 (34.5%) embryos were normal or chromosomally balanced, which were then transferred in 18 couples. A higher rate of unbalanced translocations in comparison to sporadic aneuploidies was observed in couples with reciprocal translocation. The live birth rate per embryo transfer was found to be 66.6% (12/18). PGT-SR is a useful tool in selecting normal or balanced embryos for transfer in IVF, which could lead to a pregnancy by reducing the chance of miscarriages due to chromosome aneuploidy in couples with balanced chromosomal rearrangements.

P-549 Paraclinical and spermogram evaluation of infertile males with balanced chromosomal rearrangements referred to Royan institute from 2009-2021

Abstract Study question Whether balanced chromosomal rearrangements are associated with altered semen parameters/increased DNA Fragmentation Index (DFI), are the common breakpoints related to male infertility in patients ? Summary answer Balanced chromosomal rearrangements is associated with abnormal semen parameters and increased DFI in carriers. Chromosomes 9,1,3 showed the highest incidence of chromosomal rearrangement What is known already Among many factors, balanced chromosomal rearrangements are thought to be one of the major genetic factors influencing male infertility which can cause disorders related to spermogram, DNA fragmentation, increased risk of miscarriage and ART failure. Finding common chromosomal breakpoints in infertile men carrying balanced chromosomal translocations is significant since these patients usually possess normal clinical features. Numerous studies have been performed on the various chromosome translocations. However, there has not been any report on the common breakpoints in balanced chromosomal abnormalities related to male infertility in Iran. Study design, size, duration This cross-sectional study performed on 25000 patients’ files admitted to the Royan institute during the period 2009- 2021.Infertile men carrying balanced chromosomal rearrangements were included as case group. Sperm parameters, DFI and common chromosomal break points were evaluated Participants/materials, setting, methods 25000 files of patients were reviewed. 252 infertile men carrying balanced chromosomal rearrangements and 252 men with normal karyotype were included (case and control group, respectively). Sperm concentration, motility (CASA), sperm morphology (Papanicolaou Staining),DFI (SCSA) and cytogenetic analysis of peripheral blood lymphocytes (G-banding) were evaluated. Numerical chromosomal abnormalities, deletions, marker chromosomes, alcohol or drug addiction, smoking, chemotherapy and radiation, varicocele and exposure to environmental pollution were excluded. The common chromosomal breakpoints in translocations were investigated. Main results and the role of chance Sperm concentration, total motility and normal morphology were significantly lower in balanced chromosomal rearrangements carriers compared to control group(p < 0.05). DFI in patient was significantly higher in comparison to control group(p < 0.05). Among different kind of chromosomal rearrangement carriers’, sperm concentration and total motility were lower in Robertsonian translocation carriers comparing to the Reciprocal translocation and inversion ones. The reciprocal translocation and inversion carriers did not differ regarding to sperm concentration and total motility. Meanwhile, normal morphology was lower in the translocation compared to inversion carriers. No difference was observed in DFI among carriers of balanced chromosomal rearrangements. Among case group, inversion accounted for the largest share, followed by Reciprocal, Robertsonian translocation and insertion. Group C had the highest frequency in reciprocal translocation, inversion and insertion among the chromosomal groups. The highest balanced translocation prevalence was related to three chromosomes 9,3, and1. 9q12,1p36,3p11 were the most frequent breakpoints among them. Chromosome 9 had the largest contribution to inversions,22 to reciprocal translocations and14 to Robertsonian translocations. In the Robertsonian translocation, group D showed the highest frequency. Interestingly, patients carrying normal variations such as pericentric inversion of chromosome 9showed a significant alteration in sperm parameters, therefore normal variations showed a possible relationship to infertility. Limitations, reasons for caution In order to achieve stronger statistical results on DFI, a larger sample size would be more helpful. Wider implications of the findings The results may be useful to identify the most common breakpoints and consequently genes related to male infertility and can suggest appropriate molecular mechanism for their infertility. Trial registration number not applicable

Evaluating the Clinical Utility of Genome Sequencing for Cytogenetically Balanced Chromosomal Abnormalities in Prenatal Diagnosis.

Balanced chromosomal abnormalities (BCAs) are changes in the localization or orientation of a chromosomal segment without visible gain or loss of genetic material. BCAs occur at a frequency of 1 in 500 newborns and are associated with an increased risk of multiple congenital anomalies and/or neurodevelopmental disorders, especially if it is a de novo mutation. In this pilot project, we used short read genome sequencing (GS) to retrospectively re-sequence ten prenatal subjects with de novo BCAs and compared the performance of GS with the original karyotyping. GS characterized all BCAs found by conventional karyotyping with the added benefit of precise sub-band delineation. By identifying BCA breakpoints at the nucleotide level using GS, we found disruption of OMIM genes in three cases and identified cryptic gain/loss at the breakpoints in two cases. Of these five cases, four cases reached a definitive genetic diagnosis while the other one case had a BCA interpreted as unknown clinical significance. The additional information gained from GS can change the interpretation of the BCAs and has the potential to improve the genetic counseling and perinatal management by providing a more specific genetic diagnosis. This demonstrates the added clinical utility of using GS for the diagnosis of BCAs.

Time to reduce the rate of idiopathic recurrent pregnancy losses

Recurrent pregnancy loss (RPL) is a polyetiological pathology, with the majority of causes and risk factors still not fully understood. The paper provides an overview of the current clinical guidelines on RPL, which shows the contradictions of recommendations for certain positions of examination and treatment. Taking into account the differences in the recommendations for genetic testing a detailed review of primary sources on the contribution of chromosomal pathology to RPL was done that confirms the value of cytogenetic testing of the conception product and need for attention to study of other than mother’s age factors that increase the risk of recurrent quantitative chromosomal abnormalities (aneuploidies, polyploidies). Balanced structural chromosomal abnormalities are the cause 5% of RPL. Carriers of balanced structural abnormalities do not phenotypically differ from people with a normal karyotype, but have a high risk of infertility, recurrent miscarriage, stillbirth, and birth of a child with chromosomal abnormalities. Examination of spouses with RPL for balanced structural chromosome abnormalities is the first and mandatory stage of examination, especially if cytogenetic examination of the conception products was not performed or was not informative.This article also includes a review of studies in 2019–2020 years on improving diagnostic algorithms for the RPL causes to reduce the idiopathic cases. Scientific researches prove that a complete examination to identify all possible causes of RPL regardless of the result of the conception product karyotype determining can reduce the frequency of idiopathic RPL to 10–15%.Thus, the exhaustive examination of all couples with RPL (diagnosis of genetic, anatomical, autoimmune, hormonal and microbiological causes, as well as a thorough assessment of risk factors) can significantly reduce the proportion of idiopathic forms of RPL. This reduces the stress of uncertainty and unreasonable empirical treatment in patients and provides a possibility to develop an individual plan for reproduction, using assisted reproductive technologies if necessary.

Clinical Utility of SNP Array Analysis in Prenatal Diagnosis: A Cohort Study of 5000 Pregnancies.

BackgroundSingle nucleotide polymorphism array (SNP-array) has been introduced for prenatal diagnosis. We aimed to evaluate the clinical value of SNP-array in the diagnosis of fetal chromosomal anomalies.MethodsA retrospective study was conducted on 5000 cases tested by SNP-array, and the results of 4022 cases analyzed by both karyotyping and SNP-array were compared.ResultsSNP-array analysis of 5000 samples revealed that the overall abnormality detection rate by SNP-array was 12.3%, and the overall detection rate of clinically significant copy number variations (CNVs) by SNP-array was 2.6%. SNP-array identified clinically significant submicroscopic CNVs in 4.5% fetuses with anomaly on ultrasonography, in 1.6% of fetuses with advanced maternal age (AMA), in 2.5% of fetuses with abnormal result on maternal serum screening, in 2.9% of fetuses with abnormal non-invasive prenatal testing (NIPT) results and in 3.0% of fetuses with other indications. Of the 4022 samples analyzed by both karyotyping and SNP-array, SNP-array could identify all the aneuploidy and triploidy detected by karyotyping but did not identify balanced structural chromosomal abnormalities and low-level mosaicism detected by karyotyping.ConclusionSNP-array could additionally identify clinically significant submicroscopic CNVs, and we recommend the combination of SNP-array analysis and karyotyping in prenatal diagnosis.

Comprehensive clinically oriented workflow for nucleotide level resolution and interpretation in prenatal diagnosis of de novo apparently balanced chromosomal translocations in their genomic landscape.

We present a comprehensive clinically oriented workflow for large-insert genome sequencing (liGS)-based nucleotide level resolution and interpretation of de novo (dn) apparently balanced chromosomal abnormalities (BCA) in prenatal diagnosis (PND). Retrospective or concomitant with conventional PND and liGS, molecular and newly developed clinically inspired bioinformatic tools (TAD-GConTool and CNV-ConTool) are applied to analyze and assess the functional and phenotypic outcome of dn structural variants (dnSVs). Retrospective analysis of four phenotype-associated dnSVs identified during conventional PND precisely reveal the genomic elements disrupted by the translocation breakpoints. Identification of autosomal dominant disease due to the disruption of ANKS1B and WDR26 by t(12;17)(q23.1;q21.33)dn and t(1;3)(q24.11;p25.3)dn breakpoints, respectively, substantiated the proposed workflow. We then applied this workflow to two ongoing prenatal cases with apparently balanced dnBCAs: 46,XX,t(16;17)(q24;q21.3)dn referred for increased risk on combined first trimester screening and 46,XY,t(2;19)(p13;q13.1)dn referred due to a previous trisomy 21 pregnancy. Translocation breakpoints in the t(16;17) involve ANKRD11 and WNT3 and disruption of ANKRD11 resulted in KBG syndrome confirmed in postnatal follow-up. Breakpoints in the t(2;19) are within ATP6V1B1 and the 3' UTR of CEP89, and are not interpreted to cause disease. Genotype-phenotype correlation confirms the causative role of WDR26 in the Skraban-Deardorff and 1q41q42 microdeletion phenocopy syndromes, and that disruption of ANKS1B causes ANKS1B haploinsufficiency syndrome. In sum, we show that an liGS-based approach can be realized in PND care providing additional information concerning clinical outcomes of dnBCAs in patients with such rearrangements.

Subtelomeric Rearrangements: Presentation of 21 Probands with Emphasis on Familial Cases

Intellectual disability affects 2% - 3% of the general population, with a chromosomal abnormality being found in 4% - 28% of these patients and a cryptic subtelomeric abnormality in 3% - 16%. In most cases, these subtelomeric rearrangements are submicroscopic, requiring techniques other than conventional karyotype for detection. They may be de novo or inherited from an affected parent or from a healthy carrier of a balanced chromosomal abnormality. The aim of this study was to characterize patients from our medical genetics center, in whom both a deletion and duplication in subtelomeric regions were found. Clinical and cytogenetic characterization of 21 probands followed at our center, from 1998 until 2017, with subtelomeric rearrangements. There were 21 probands from 19 families presenting with intellectual disability and facial dysmorphisms. Seven had behavior changes, five had epilepsy and 14 presented with some other sign or symptom. Four had chromosomal abnormalities detected by conventional karyotype and four were diagnosed by array-comparative genomic hybridization. In four cases, parental studies were not possible. The online mendelian inheritance in man classification was provided whenever any of the phenotypes (deletion or duplication syndrome) was dominant. Patients and relevant family members were clinically and cytogenetically characterized. Although rare, subtelomeric changes are a substantial cause of syndromic intellectual disability with important familial repercussions. It is essential to remember that a normal array-comparative genomic hybridization result does not exclude a balanced rearrangement in the parents. Parental genetic studies are essential not only for a complete characterization of the rearrangement, but also for accurate genetic counselling and screening of family members at risk for recurrence.

Computational Prediction of Position Effects of Human Chromosome Rearrangements.

Balanced and apparently balanced chromosome abnormalities (BCAs) have long been known to generate disease through position effects, either by altering local networks of gene regulation or positioning genes in architecturally different chromosome domains. Despite these observations, identification of distally affected genes by BCAs is oftentimes neglected, especially when predicted gene disruptions are found elsewhere in the genome. In this unit, we provide detailed instructions on how to run a computational pipeline that identifies relevant candidates of non-coding BCA position effects. This methodology facilitates quick identification of genes potentially involved in disease by non-coding BCAs and other types of rearrangements, and expands on the importance of considering the long-range consequences of genomic lesions.

Balanced Chromosomal Rearrangement Detection by Low-Pass Whole-Genome Sequencing.

Balanced chromosomal rearrangements (or balanced chromosome abnormalities, BCAs) are common chromosomal structural variants. Emerging studies have demonstrated the feasibility of using whole-genome sequencing (WGS) for detection of BCA-associated breakpoints, but the requirement for a priori knowledge of the rearranged regions from G-banded chromosome analysis limits its application. The protocols described here are based on low-pass WGS for detecting BCA events independent from chromosome analysis, and has been validated using genomic data from the 1000 Genomes Project. This approach adopts non-size-selected mate-pair library (3∼8kb) with 2∼3μg DNA as input, and requires only 30 million read-pairs (50bp, equivalent to 1-fold base-coverage) for each sample. The complete procedure takes 13 days and the total cost is estimated to be less than $600 (USD) per sample. © 2018 by John Wiley & Sons, Inc.

Identification of balanced chromosomal rearrangements previously unknown among participants in the 1000 Genomes Project: implications for interpretation of structural variation in genomes and the future of clinical cytogenetics.

PurposeRecent studies demonstrate that whole-genome sequencing (WGS) enables detection of cryptic rearrangements in apparently balanced chromosomal rearrangements (also known as balanced chromosomal abnormalities, BCAs) previously identified by conventional cytogenetic methods. We aimed to assess our analytical tool for detecting BCAs in The 1000 Genomes Project without knowing affected bands.MethodsThe 1000 Genomes Project provides an unprecedented integrated map of structural variants in phenotypically normal subjects, but there is no information on potential inclusion of subjects with apparently BCAs akin to those traditionally detected in diagnostic cytogenetics laboratories. We applied our analytical tool to 1,166 genomes from the 1000 Genomes Project with sufficient physical coverage (8.25-fold).ResultsOur approach detected four reciprocal balanced translocations and four inversions ranging in size from 57.9 kb to 13.3 Mb, all of which were confirmed by cytogenetic methods and PCR studies. One of DNAs has a subtle translocation that is not readily identified by chromosome analysis due to similar banding patterns and size of exchanged segments, and another results in disruption of all transcripts of an OMIM gene.ConclusionsOur study demonstrates the extension of utilizing low-coverage WGS for unbiased detection of BCAs including translocations and inversions previously unknown in the 1000 Genomes Project.

Computational Prediction of Position Effects of Apparently Balanced Human Chromosomal Rearrangements

Interpretation of variants of uncertain significance, especially chromosomal rearrangements in non-coding regions of the human genome, remains one of the biggest challenges in modern molecular diagnosis. To improve our understanding and interpretation of such variants, we used high-resolution three-dimensional chromosomal structural data and transcriptional regulatory information to predict position effects and their association with pathogenic phenotypes in 17 subjects with apparently balanced chromosomal abnormalities. We found that the rearrangements predict disruption of long-range chromatin interactions between several enhancers and genes whose annotated clinical features are strongly associated with the subjects' phenotypes. We confirm gene-expression changes for a couple of candidate genes to exemplify the utility of our analysis of position effect. These results highlight the important interplay between chromosomal structure and disease and demonstrate the need to utilize chromatin conformational data for the prediction of position effects in the clinical interpretation of non-coding chromosomal rearrangements.

The genomic landscape of balanced cytogenetic abnormalities associated with human congenital anomalies.

Despite the clinical significance of balanced chromosomal abnormalities (BCAs), their characterization has largely been restricted to cytogenetic resolution. We explored the landscape of BCAs at nucleotide resolution in 273 subjects with a spectrum of congenital anomalies. Whole-genome sequencing revised 93% of karyotypes and demonstrated complexity that was cryptic to karyotyping in 21% of BCAs, highlighting the limitations of conventional cytogenetic approaches. At least 33.9% of BCAs resulted in gene disruption that likely contributed to the developmental phenotype, 5.2% were associated with pathogenic genomic imbalances, and 7.3% disrupted topologically associated domains (TADs) encompassing known syndromic loci. Remarkably, BCA breakpoints in eight subjects altered a single TAD encompassing MEF2C, a known driver of 5q14.3 microdeletion syndrome, resulting in decreased MEF2C expression. We propose that sequence-level resolution dramatically improves prediction of clinical outcomes for balanced rearrangements and provides insight into new pathogenic mechanisms, such as altered regulation due to changes in chromosome topology.