Application of mosaicism ratio to multifetal gestations

Abstract

Mosaicism ratio, or MR, is a laboratory metric that can be calculated using massively parallel sequencing data from cell-free DNA (cfDNA) screening. MR compares the amount of cfDNA present from a particular chromosome or chromosomal region to the overall fetal fraction of the specimen. In singleton gestations, MR may be used to refine the positive predictive value of an abnormal cfDNA screening result by identifying cases that could be impacted by various biological factors, such as placental mosaicism or prior co-twin demise. The current study was designed to examine the behavior of mosaicism ratio (MR) in multifetal gestations. Multifetal cfDNA specimens with positive results for trisomies 21, 18, or 13 and confirmed diagnostic outcomes were compiled to examine MR of the aneuploid chromosome based on the number of affected fetuses/placentas. A second multifetal cohort was assembled to analyze the MR of the Y chromosome in cases with at least one male fetus. For aneuploid cases, the average MR of affected singletons (used as a biological proxy for two affected twins) was significantly higher than the average MR for twins in which one fetus was affected. The average MR of the aneuploid chromosome for one affected twin was 52%, 42%, and 48% of that of singleton gestations for trisomy 21, 18, and 13 cases, respectively. MR cutoffs of 0.7 for trisomy 21, and 0.5 for trisomies 18 and 13 may help predict whether one versus both twins are affected with aneuploidy when clinical concern arises. For male cases, the Y MR of XX/XY gestations was 48% of the Y MR for XY/XY gestations. Using a Y MR cutoff of 0.8 allowed determination of XX/XY versus XY/XY gestations with 92.3–94.9% accuracy. Based on the data presented, MR may have utility in the analysis and interpretation of cfDNA data from multifetal gestations.

Overview of Noninvasive Prenatal Testing (NIPT) for the Detection of Fetal Chromosome Abnormalities; Differences in Laboratory Methods and Scope of Testing

“…More recently, testing has considered detecting and quantifying the level of mosaicism. [76][77][78] These approaches are based on identification on an intermediate amount of DNA expected for the aneuploid chromosome, given an accurate measurement of the fetal fraction. Validation of a quantitative assessment of the level of mosaicism is problematic because there is no alternative reference test.…”

“…Initial designs of cf-DNA tests for CATs did not consider mosaicism; cutoffs were mostly designed with the primary goal of distinguishing between affected and unaffected pregnancies with a single fixed cutoff used. More recently, testing has considered detecting and quantifying the level of mosaicism 76–78 . These approaches are based on identification on an intermediate amount of DNA expected for the aneuploid chromosome, given an accurate measurement of the fetal fraction.…”