Introduction

This clinical case highlights the direct identification of an embryo with an inherited genetic mutation associated with cardiomyopathy. The identification of the affected embryo was made possible through Orchid's Whole-Genome Preimplantation Genetic Testing (PGT-WGS), and importantly, this genetic mutation was detected without relying on parental genetic information — neither the in vitro fertilization (IVF) center nor the genetic testing company had prior knowledge of this disease-causing mutation in the biological father before the screening occurred. This case emphasizes the vital role of PGT-WGS in preventing the inheritance of genetic conditions.

Explicit consent was obtained from the patients for this anonymized case report.

Background

Many IVF centers in the United States have a requirement for carrier screening before starting fertility treatment. This screening helps in making recommendations for preimplantation genetic testing for monogenic disorders (PGT-M). Carrier screening aims to detect genetic variants associated with autosomal recessive and X-linked conditions and is not designed to screen for autosomal dominant (AD) disorders, which encompass conditions like hereditary cancer and cardiomyopathies, among others^[1]. If there is an autosomal dominant condition that the parents are unaware of or haven't disclosed, PGT-M may not be suggested. Conventional PGT-M exclusively targets a single, previously identified genetic disorder.

Orchid, on the other hand, conducts thorough preimplantation genetic screening of embryos by examining the entire genome. This process screens 1300 genes for genetic variants linked to disease-causing conditions, encompassing not just X-linked and autosomal recessive disorders but also autosomal dominant pathogenic variants within expert-curated gene panels. This approach allows Orchid to detect a wide range of inherited and non-inherited conditions. As a result, couples receive detailed information about their embryos, empowering them to make well-informed decisions when it comes to selecting embryos for implantation^[2].

Case Presentation

A couple (male, 42; female, 36) opted for an IVF cycle and engaged Orchid for a whole-genome preimplantation genetic screening, with the goal of performing generalized monogenic and predisposition screening. IVF was performed for the purposes of fertility preservation due to advancing maternal and paternal age; because carrier screening did not identify any X-linked variants or shared autosomal recessive variants, no specific genes or variants were requested for testing.

During this cycle, 9 embryos were identified as viable candidates for implantation and underwent preimplantation genetic testing.

Methods

• The couple underwent a typical IVF with embryo screening flow including a ~5-cell biopsy that was sent to Orchid for genetic testing
• Whole-genome amplification and screening was performed on these biopsies following Orchid’s standard operating protocols
• Orchid scientific team including geneticists reviewed whole genome data and completed the analysis

Findings

Nine biopsies were reviewed by Orchid and the findings were as follows:

• Four biopsies were chromsomally abnormal (aneuploid) and one was mosaic
• Four biopsies were chromsomally normal (euploid) and underwent monognic screening for ~1300 genes
• One of the four euploid embryos was identified to have a dominant pathogenic variant in a gene (TTN) on the ACMG Secondary Findings panel covered by Orchid. The pathogenic TTN variant is linked to Dilated Cardiomyopathy^[3]. The remaining embryos screened negative for the TTN variant and pathogenic/likely pathogenic variants in ~1300 genes.

Variants within the TTN gene are associated with Dilated Cardiomyopathy (DCM), a genetic disorder leading to progressive heart muscle dysfunction and enlargement, typically presenting between ages 30 and 50^[4].

The presence of this TTN variant, and absence on other embryos, was confirmed via Sanger sequencing.

Analysis

When a variant presents on only a single embryo during PGT-WGS testing, it is generally unknown whether the variant was inherited or occurred by chance (de-novo).  However, during post-test genetic counseling with the couple, the male partner revealed that he has been seen by cardiology specialists due to dilated cardiomyopathy and has had previous genetic testing which revealed the same pathogenic TTN variant identified in the embryo.

Conclusion

Utilizing whole-genome preimplantation genetic screening, Orchid identified an embryo with a pathogenic TTN variant linked to Dilated Cardiomyopathy, all without any prior information provided to the IVF center or the PGT lab.

Non-genetics professionals are typically unaware that carrier screening covers only a fraction of the potential genetic diseases that may be transmitted to their offspring. Especially omitted are autosomal dominant diseases, where a single variant may induce mild to severe disease in a parent but potentially translate into severe disease in a child. In this context, carrier screening may inadvertently give parents confidence that they have thoroughly pre-empted genetic disease transmission to their future children, when in reality carrier screening was not a comprehensive genetic test. As a result, patients may also de-emphasize their health history when building an IVF plan, and instead assume that carrier screening will surface any risks of inherited disease.

The detection of this pathogenic TTN variant underscores the potential blind spots of carrier screening and substantiates the value of comprehensive whole-genome preimplantation genetic screening in mitigating the transmission of genetic disorders during IVF cycles.

References

[1] A.R. Gregg, M. Aarabi, S. Klugman, N.T. Leach, M.T. Bashford,T. Goldwaser, E. Chen, T.N. Sparks, H.V. Reddi, A. Rajkovic, and J.S.Dungan. Screening for autosomal recessive and x-linked conditions duringpregnancy and preconception: a practice resource of the american college of medical genetics and genomics (acmg). Genet Med, 23(10):1793–1806,2021.

[2] Yuntao Xia, Willy Chertman, Dhruva Chandramohan, Maria Katz, ElanBechor, Ben Podgursky, Michael Hoxie, Qinnan Zhang, Jessica Kang,Edwina Blue, Justin Chen, Justin Schleede, Nathan Slotnick, Xiaoli Du,Jonathan Kort, Robert Boostanfar, Eric Urcia, Barry Behr, and Noor Siddiqui. First clinical validation of whole genome screening on standardtrophectoderm biopsies of preimplantation embryos. bioRxiv, 2022.

[3] Online Mendelian Inheritance in Man, OMIM®. Titin; ttn. https://www.omim.org/entry/188840, No year available. Cytogenetic location:2q31.2. Genomic coordinates (GRCh38): 2:178,525,989-178,807,423.

[4] Maria Franaszczyk, Piotr Chmielewski, Grazyna Truszkowska, et al. Titintruncating variants in dilated cardiomyopathy - prevalence and genotype-phenotype correlations. PLoS One, 12(1):e0169007, 2017.