Optical genome mapping for detection of chromosomal aberrations in prenatal diagnosis

Retrospective study on the utility of optical genome mapping as a follow-up method in genetic diagnostics

BACKGROUND

Current standard-of-care (SOC) methods for genetic testing are capable of resolving deletions and sequence variants, but they mostly fail to provide information on the breakpoints of duplications and balanced structural variants (SV). However, this information may be necessary for their clinical assessment, especially if the carrier's phenotype is difficult to assess and/or carrier analysis of relatives is not viable. A promising approach to solving such challenging cases arises with access to optical genome mapping (OGM) but has not been systematically explored as of yet.

METHODS

In this retrospective study, we evaluated diagnostic cases from a 1-year period (2023) in which an SV discovery by SOC methods (microarray, karyotyping and whole-exome sequencing) was followed up by OGM, with the objective to unlock clinically relevant information about the SV.

RESULTS

Seven cases were shown by SOC methods to bear potential pathogenic SVs and were consequently followed up by OGM. Of these, six were solved by the additional use of OGM alone. One case required sequencing after OGM analysis to further specify the SV's breakpoints. In all seven cases, OGM was crucial for determining the clinical relevance of the detected SV.

CONCLUSION

This study describes the use of OGM as a valuable method for characterising duplications and balanced SVs. Often, this additional information does not add to the quality of a clinical report. However, for a subset of patients, these data are critical, especially in the prenatal setting or when no familial analyses are possible.

Optical genome mapping to decipher the chromosomal aberrations in families seeking for preconception genetic counseling

Optical genome mapping (OGM) offers high consistency in simultaneously detecting structural and copy number variants. This study aimed to retrospectively evaluate the efficacy and potential applications of OGM in preconception genetic counseling. Herein, 74 samples from 37 families were included, and their results of OGM were compared to conventional methods, namely karyotyping (KT) and chromosomal microarray analysis (CMA), which identified 27 variants across 16 positive families. Notably, OGM achieved a concordance rate of 94.7% and 100% with KT and CMA, respectively, presenting an overall concordance of 96.3%, as it missed detecting a centromeric translocation. Additionally, OGM detected two cryptic balanced translocations and a small deletion in three families that were missed by conventional methods, improving the diagnostic rate by 5.4%, along with assisting in the diagnoses of six families (16.2%) by identifying complex rearrangements and confirming cryptic translocations. The combination of KT with OGM yielded the highest diagnostic rate in all families. Overall, the findings of this study present the notable potential of OGM for its application, combined with KT per requirement, in clinical settings to improve the efficiency and accuracy of diagnoses and rapid screening of individuals seeking preconception genetic counseling.

Complex chromosomal rearrangements in female carriers experiencing recurrent pregnancy loss or poor obstetric history and literature review

PURPOSE

Complex chromosomal rearrangements (CCRs) often remain unidentified as they are rarely observed in the general population. Females with CCRs are generally recognized on the identification of an affected child with multiple congenital anomalies (MCA) or having a history of repeated pregnancy loss/bad obstetric history (RPL/BOH). In contrast, males with CCRs are diagnosed primarily due to infertility. This study aimed to carry out a systematic epidemiological analysis of CCRs in one of the largest series from the Indian population. In addition, a review of the literature on female CCR carriers experiencing RPL/BOH has been compiled in an attempt to identify the genomic landscape of breakpoints, commonly involved chromosomes, and the breakpoint regions.

METHODS

A total of 8560 healthy individuals with normal physical and mental well-being and had no history of any obvious genetic disorder at the time of presentation were referred for chromosome analysis in view of RPL/BOH between 1994 and 2024. Of them, 8158 had a normal chromosome complement whereas, 402 (4.7%) showed chromosomal aberrations. CCRs were detected in seven individuals, i.e., one partner in each of seven couples with structural rearrangements, all of whom were females. Comprehensive characterization of CCR was carried out using various molecular cytogenetic techniques.

RESULTS

Seven CCR carriers had a total of 25 pregnancies: 20 leading to miscarriages (80%), one leading to the birth of an abnormal child (4%), two medically terminated pregnancies (8%) due to abnormal antenatal findings, and the remaining two were healthy (8%). A total of 13 different chromosomes with 24 non-recurring breakpoints were identified in these cases. Chromosome (#) 2 showed four breaks (16.7%), followed by #1, #4, #6, and #13 with three breaks each (12.5% each), while one break each (4.2% each) was seen on the remaining eight chromosomes (#3, #5, #8, #11, #14, #15, #17, and #21). Type I and type IV CCRs were observed in five (71.4%) and one case (14.3%), respectively, along with a "not a true" CCR (14.3%) in the present study group. Overall, the prevalence of CCRs in couples with RPL/BOH was 0.16%.

CONCLUSIONS

To the best of our knowledge, this is the first study on the epidemiology of CCRs in couples with RPL/BOH of Indian origin. The incidence of CCRs in couples experiencing RPL/BOH in the present cohort was found to be 0.16% with type I CCR being the most predominant of all types, which is congruent with observations from non-Hispanic white and South East Asian populations. The uniqueness and rarity of each CCR pose a challenge in genetic and reproductive counseling.

Tissue-specific TCF4 triplet repeat instability revealed by optical genome mapping

BACKGROUND

Fuchs endothelial corneal dystrophy (FECD) is the most common repeat-mediated disease in humans. It exclusively affects corneal endothelial cells (CECs), with ≤81% of cases associated with an intronic TCF4 triplet repeat (CTG18.1). Here, we utilise optical genome mapping (OGM) to investigate CTG18.1 tissue-specific instability to gain mechanistic insights.

METHODS

We applied OGM to a diverse range of genomic DNAs (gDNAs) from patients with FECD and controls (n = 43); CECs, leukocytes and fibroblasts. A bioinformatics pipeline was developed to robustly interrogate CTG18.1-spanning DNA molecules. All results were compared with conventional polymerase chain reaction-based fragment analysis.

FINDINGS

Analysis of bio-samples revealed that expanded CTG18.1 alleles behave dynamically, regardless of cell-type origin. However, clusters of CTG18.1 molecules, encompassing ∼1800-11,900 repeats, were exclusively detected in diseased CECs from expansion-positive cases. Additionally, both progenitor allele size and age were found to influence the level of leukocyte-specific CTG18.1 instability.

INTERPRETATION

OGM is a powerful tool for analysing somatic instability of repeat loci and reveals here the extreme levels of CTG18.1 instability occurring within diseased CECs underpinning FECD pathophysiology, opening up new therapeutic avenues for FECD. Furthermore, these findings highlight the broader translational utility of FECD as a model for developing therapeutic strategies for rarer diseases similarly attributed to somatically unstable repeats.

FUNDING

UK Research and Innovation, Moorfields Eye Charity, Fight for Sight, Medical Research Council, NIHR BRC at Moorfields Eye Hospital and UCL Institute of Ophthalmology, Grantová Agentura České Republiky, Univerzita Karlova v Praze, the National Brain Appeal's Innovation Fund and Rosetrees Trust.

Complex chromosomal 6q rearrangements revealed by combined long-molecule genomics technologies

Technologies for detecting structural variation (SV) have advanced with the advent of long-read sequencing, which enables the validation of SV at a nucleotide level. Optical genome mapping (OGM), a technology based on physical mapping, can also provide comprehensive SVs analysis. We applied long-read whole genome sequencing (LRWGS) to accurately reconstruct breakpoint (BP) segments in a patient with complex chromosome 6q rearrangements that remained elusive by conventional karyotyping. Although all BPs were precisely identified by LRWGS, there were two possible ways to construct the BP segments in terms of their orders and orientations. Thus, we also used OGM analysis. Notably, OGM recognized entire inversions exceeding 500 kb in size, which LRWGS could not characterize. Consequently, here we successfully unveil the full genomic structure of this complex chromosomal 6q rearrangement and cryptic SVs through combined long-molecule genomic analyses, showcasing how LRWGS and OGM can complement each other in SV analysis.

Optical Genome Mapping for Chromosomal Aberrations Detection-False-Negative Results and Contributing Factors

Optical genome mapping (OGM) has been known as an all-in-one technology for chromosomal aberration detection. However, there are also aberrations beyond the detection range of OGM. This study aimed to report the aberrations missed by OGM and analyze the contributing factors. OGM was performed by taking both GRCh37 and GRCh38 as reference genomes. The OGM results were analyzed in blinded fashion and compared to standard assays. Quality control (QC) metrics, sample types, reference genome, effective coverage and classes and locations of aberrations were then analyzed. In total, 154 clinically reported variations from 123 samples were investigated. OGM failed to detect 10 (6.5%, 10/154) aberrations with GRCh37 assembly, including five copy number variations (CNVs), two submicroscopic balanced translocations, two pericentric inversion and one isochromosome (mosaicism). All the samples passed pre-analytical and analytical QC. With GRCh38 assembly, the false-negative rate of OGM fell to 4.5% (7/154). The breakpoints of the CNVs, balanced translocations and inversions undetected by OGM were located in segmental duplication (SD) regions or regions with no DLE-1 label. In conclusion, besides variations with centromeric breakpoints, structural variations (SVs) with breakpoints located in large repetitive sequences may also be missed by OGM. GRCh38 is recommended as the reference genome when OGM is performed. Our results highlight the necessity of fully understanding the detection range and limitation of OGM in clinical practice.