Mosaic Trisomy 9 in an infant with mixed‐field ABO blood grouping

Abstract

A 44-year-old group A D+ G3P0 woman at 32 weeks' gestation underwent emergent cesarean section delivery of a 1.57-kg singleton male infant due to chorioamnionitis and breech presentation. ABO typing of the infant revealed mixed-field agglutination, with an apparent 50:50 mixture of A+ and O+ cells. There was no evidence of twin gestation, history of intrauterine transfusion, or maternal-fetal hemorrhage. Dextrose column separation of the infant's blood after agglutination with excess anti-A reagent 1 (see figure, left) allowed isolation of the nonreacting cell population. Testing for weak A phenotypes and presence of H antigen confirmed the partial group O phenotype. Serologic testing for D, E, c, e, Fy a , Fy(b), N, S, Jk a , and Jk b antigens disclosed identical expression patterns between the infant's two cell populations. Single-nucleotide polymorphism chromosomal microarray showed a mosaic single-copy genomic gain (trisomy) of Chromosome 9 in approximately 30% of cells. Fluorescence in situ hybridization analysis confirmed the mosaic presence of an additional Chromosome 9 homolog, with a normal (disomic) interphase nucleus (see figure, right, A) adjacent to a metaphase cell with Trisomy 9 (see figure, right, B). ABO mixed-field agglutination is most commonly seen after out-of-group RBC transfusion or hematopoietic stem cell transplantation. Rarely, chimerism resulting from either twin chimerism or tetragametic (dispermic) chimerism may result in dual RBC populations. In this patient, Mosaic Trisomy 9 introduced an extra copy of the ABO gene, located on Chromosome 9q34.2. The observed mixed-field agglutination likely reflects two phenotypically distinct cell lines: a more prevalent disomic line (genotype OO) and a less prevalent trisomic line (genotype AOO). Mosaic Trisomy 9 is a rare finding in liveborn infants resulting in variable outcomes, including cardiac defects. After identification of mosaicism in this infant an echocardiogram was performed, revealing a previously unsuspected atrial septal defect with left-to-right shunt.