By Anne W. Higgins, Ph.D. & Patricia M. Miron, Ph.D.

July 17, 2018 Insights

In Greek, a chimera is a monster that breathed fire, had a lion’s head, a goat’s body, and a snake’s tail.  In medicine and biology, a chimera is an organism that has two genetically distinct cell lines.  In humans, an additional cell line can be acquired through an allogenic stem cell transplant or, more rarely, through fusion of two embryos, in which case the chimerism is congenital.

Congenital chimeras are rare and often come to attention due to ambiguous genitalia or a discrepancy in the sex chromosome complement (Malan, 2006).  Chromosome analysis can detect chimerism if the two embryos or cell lines are XX and XY. However, until the use of more recent cytogenomic testing, chimeras with the two cell lines of the same sex could not be detected.  Chromosomal oligo-SNP microarray analysis (CMA), which detects chromosome gains and losses across the entire genome, also detects loss of heterozygosity (LOH).  Heterozygosity reflects differences in the maternally and paternally inherited DNA sequences and is normally present across the genome.   LOH can arise through several mechanisms, one of which being uniparental disomy (UPD), inheritance of both chromosomes (or chromosomal regions) from only one parent.  Here we describe a rare fetal chimera discovered through prenatal testing using chromosome, fluorescence in-situ hybridization (FISH), and chromosomal oligo-SNP microarray analyses.

The patient is a 30 year old female presenting with her first pregnancy at 17.0 weeks gestational age.  The patient underwent non-invasive prenatal screening earlier that was interpreted as negative for aneuploidy.  The patient desired cytogenetic and alpha fetoprotein testing of the amniotic fluid from this pregnancy due to abnormal kidney and abnormal placenta seen on fetal ultrasound examination.  The alpha fetoprotein level was within normal limits for the stated gestational age.  FISH analysis of uncultured amniotic fluid cells showed both XX and XY cells; approximately 10% of cells were XX and 90% were XY.  Chromosome analysis of cultured cells also showed both XX and XY cells, with approximately 45% XX and 55% XY.

The CMA of uncultured cells did not detect any chromosome abnormalities was also consistent with the presence of both XX and XY cells, with approximately 35% XX and 65% XY.  The CMA performed on cultured cells again showed no evidence of copy number gains or losses, but only a very small percentage of XY cells, indicating the XX cell line had outgrown the XY cell line in the culture dish used to obtain material for the CMA.  Surprisingly, the SNP allele pattern obtained from this mostly XX cell population showed genome wide UPD.

Concurrent molecular studies to assess the possibility that the XX cells were maternal in origin, showed no evidence of a Y chromosome and homozygosity (LOH) for every marker tested.  Additionally, a number of markers were not present on either chromosome in the mother. Thus, the XX cell line appears to represent genome wide paternal isoUPD.  In other words, for every chromosome pair, the two chromosomes are exactly the same as each other and represent duplication of a single paternally inherited chromosome.

Genome wide paternal isoUPD has been reported in the literature with the majority of patients demonstrating features consistent with Beckwith-Wiedemann syndrome, which is caused by chromosome 11p15 paternal UPD (Kalish, 2013).  Follow up analyses of the fetus and placenta demonstrated one enlarged kidney, male external genitalia, one teste, and placental mesenchymal dysplasia.  Placental mesenchymal dysplasia is a rare condition marked by placentomegaly and abnormal chorionic villi and is associated with intrauterine growth restriction and fetal death.  Placental mesenchymal dysplasia has also shown an association with Beckwith-Weidemann syndrome (Kaiser-Rogers, 2006, Pham, 2006).  In this case the phenotype of the potentially liveborn child is unclear because it is unknown which cells are populating which tissues/organs; however, the placental phenotype is consistent with previous reports of paternal isoUPD.

By definition, chimeras form from fusion of two genetically distinct zygotes into one embryo.  There are multiple proposed mechanisms, including two oocytes fertilized by two spermatozoa forming two embryos that then fuse (tetragametic chimera), parthenogenesis formation of a diploid oocyte that fuses to another fertilized embryo, fertilization of the second (haploid) polar body with fusion to another fertilized embryo, and fertilization with of an egg followed by endoreplication (chromosomal duplication) of the paternal haploid pronucleus.  The SNP pattern seen in the CMA analysis of the case presented here is consistent with fertilization of an “empty” egg (no nucleus) with an X bearing sperm that underwent endoreplication followed by fusion with a normal XY zygote/embryo.  Chimeras have been thought to be rare events.  However, techniques for analysis such as chromosome and FISH analyses did not allow for their detection.  With the use of the most current technologies, such as SNP microarray analysis, that allow for the detection of chimeras, these events may prove to be more frequent than previously understood.

References

Malan V, Vekemans M, Turleau C. Chimera and other fertilization errors. Clin Genet 2006, 70: 363-373.

Kalish JM, Conlin LK, et al. Clinical Features of Three Girls with Mosaic Genome-Wide Paternal Uniparental Isodisomy. Am J Med Genet Part A 2013, 161A:1929-1939.

Kaiser-Rogers KA, McFadden DE, et al. Androgenetic/biparental mosaicism causes placental mesenchymal dysplasia. J Med Genet 2006, 43:187-192.

Pham T, Steele J, et al. Placental Mesenchymal Dysplasia is Associated with High Rates of Intrauterine Growth Restriction and Fetal Demise. Am J Clin Pathol 2006, 126:67-78.