The “mosaic” embryo: misconceptions and misinterpretations in preimplantation genetic testing for aneuploidy

Preimplantation genetic testing for aneuploidy (PGT-A) remains one of the most controversial topics in reproductive medicine. With more than 40% vitro fertilization cycles United States reportedly involving PGT, both those favor and opposed to PGT-A have significant interest efficacy PGT-A. Ongoing issues include what patient population, if any, benefits from PGT-A, true frequency chromosomal mosaicism, whether embryonic aneuploidies self-correct, how practitioners manage embryos designated as “mosaic.” This review addresses several misconceptions misinterpretations data surrounding analysis prediction mosaicism preimplantation embryo. DIALOG: You can discuss this article with its authors other readers at https://www.fertstertdialog.com/posts/33151 (PGT) requires application often error-prone DNA amplification methods obtain a result. In addition, process by which cells are obtained embryo quality PGT vary widely. These methodological challenges result difficulty distinguishing signal technical noise. The diagnosis monogenic disorders (PGT-M) serves an example ability develop that improve accuracy overcome these challenges. Several important advances, including parallel mutation-linked markers prevention detection contamination, helped avoid misdiagnoses accuracy. contrast, uninformed introduction “mosaic" diagnoses has led reduction (PGT-A). Clinical manifestation type inaccuracy is exemplified STAR trial (1Munné S. Kaplan B. Frattarelli J.L. Child T. Nakhuda G. Shamma F.N. et al.Preimplantation versus morphology selection criteria single frozen-thawed transfer good-prognosis patients: multicenter randomized clinical trial.Fertil Steril. 2019; 112: 1071-1079.e7Abstract Full Text PDF PubMed Scopus (209) Google Scholar), reported overall 49% (unusually high) rate failed demonstrate utility patients under age 35 years. Laboratories study included rates 33% 72%, whereas laboratories not 0% 43% same group. indicates inclusion mosaic results overestimation presence abnormalities. False positive undoubtedly plays role failure improved outcomes particularly Mosaicism defined mixture different karyotypes. commonly used predicting involve use arbitrary chromosome copy number thresholds (i.e., 20% 80%) (2Cram D.S. Leigh D. Handyside A. Rechitsky L. Xu K. Harton al.PGDIS position statement on 2019.Reprod Biomed Online. 39: e1-e4PubMed (61) Scholar) next-generation sequencing (Fig. 1). If chromosome’s falls outside threshold normal disomic 2 between 1.8 2.2) uniform monosomy or trisomy below 1.2 above 2.8), determined fall “mosaic range” (1.2–1.8 2.2–2.8). fact, studies published after analyzing artificial mixtures cell lines karyotypes, thereby mimicking aneuploid/euploid scenario trophectoderm biopsy (3Goodrich Xing Tao X. Lonczak Zhan Y. Landis J. al.Evaluation comprehensive screening platforms segmental aneuploidy.J Assist Reprod Genet. 2017; 34: 975-981Crossref (27) Scholar, 4Greco E. Minasi M.G. Fiorentino F. Healthy babies intrauterine aneuploid blastocysts.N Engl J Med. 2015; 373: 2089-2090Crossref (312) 5Maxwell S.M. Colls P. Hodes-Wertz McCulloh D.H. McCaffrey C. Wells al.Why do euploid miscarry? A case-control comparing within presumed resulted miscarriage live birth using sequencing.Fertil 2016; 106: 1414-1419.e5Abstract (119) Scholar). Intermediate profiles indeed observed line mixtures, researchers considered “preclinical validation” biopsy. Unfortunately, it inaccurate assume truly according “intermediate number” strategy (6Paulson R.J. Treff N.R. Isn't time stop calling mosaic?.F S Rep. 2020; 1: 164-165Abstract (13) Although be detected thresholds, inadequate representation variation introduced rigorous system investigate blastocyst involves performing multiple biopsies (7Capalbo Ubaldi F.M. Rienzi Scott R. N. Detecting biopsies: current future possibilities.Hum Reprod. 32: 492-498PubMed method obviously incompatible diagnosing transferable embryo, but provides superior evidence bona fide mosaicism. systematic performed rebiopsies deemed basis intermediate numbers revealed (57%) were unlikely been (8Marin aneuploidy: reanalysis concordance data.Prenat Diagn. 2021; 41: 545-553Crossref (10) many range" found meiotic origin consistent aneuploidy, further confirming (9Handyside A.H. McCollin Summers M.C. Ottolini C.S. Copy postzygotic mitotic biopsied stage arrested embryos.Prenat 525-535Crossref (4) When originates error, all resulting will possess (uniform). recent demonstrated “low-level mosaic” equivalent potential (10Capalbo Poli M. Girardi Cimadomo Benini al.A prospective double-blinded non-selection normalcy newborns derived putative low/moderate-degree IVF embryos.medRxiv. 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Among transfers, only 1 (0.04%) confirmed pregnancy. reported, investigators indicated 44 before observing (personal communication). child born healthy, Overall, demonstrates remarkable lack confirmation predictions “mosaicism” reflects predict delivery. publications provided guide management 35Chen Darcy Boyd counseling considerations.Curr Opin 33: 100-105Crossref 36Practice Committee Genetic Counseling Professional Group (GCPG) American Society Reproductive Medicine. Electronic address: [email protected]Clinical committee opinion.Fertil 114: 246-254Abstract (36) 37Zwingerman Langlois Opinion No. 406: Prenatal Gynaecol Can. 42: 1437-1443.e1Abstract 38Levy Hoffmann E.R. McCoy R.C. Grati F.R. Chromosomal origins implications diagnosis.Prenat 631-641Crossref 39Sachdev N.M. Maxwell Diagnosis mosaicism.Fertil 107: 6-11Abstract (53) described should emphasized. might argue expected setting low begin with. However, prevalence natural conception estimated 1% 2% (38Levy 40Huang Adusumalli Patel Liem Williams III, Pisarska M.D. Prevalence couples infertility.Fertil 2009; 91: 2355-2360Abstract (39) It transferring 2,500 would 1,000 times lower conception, fact 100% Despite increasing predictors concept fully capable providing definitive continues inappropriately conveyed. Beginning Greco al.(4Greco reports describing outcomes. For example, “Using use” (31Viotti “The meta-analysis” “Degree potential: corrected embryos” (41Kushnir V.A. Darmon S.K. Barad Gleicher Degree embryos.Reprod 16: 6Crossref (37) virtually every case, differences better explained simply considering never actually false positives). aneuploid. 28% evaluated. karyomapping 64% range (uniform aneuploid) Moreover, nucleotide polymorphism genotyping coupled additionally originate triploid (42Marin Jr., R.T. origin, consequences reliability screening.Curr 29: 168-174Crossref (17) Transfer (false negative aneuploidy) lead reduced success (as some studies), given near (27Tiegs 43Scott Ferry Su Comprehensive highly study.Fertil 2012; 97: 870-875Abstract (240) uniformly fail sustain contradict notion unable represent status remaining Proponents worthwhile, alternative explanation well-supported interpretation rates. From molecular biology perspective, diagnostic error could arise suboptimal analyzed. occurs exponential manner. During linear phase, doubles quantity each cycle polymerase chain reaction. components reaction become (saturation), longer able double cycle. decrease relative difference trisomic chromosomes, leading 2). Similarly, too few cells, reach phase quantitation Conversely, addition completely euploid. 29% rebiopsy. 36% “low-level” shown As Table 1, successful and, coincidently, 38%.Table 1Rate (“mosaic").StudyPGT-A platform“Mosaic” transferredSuccess (%)Confirmed mosaicMosaic (%)Greco Scholar)aCGH183300Maxwell (5Maxwell Scholar)NGS183300Fragouli (14Fragouli Scholar)NGS442700Munne (15Munné cytogenet