Chimerism and mosaicism shape our physical constitution and impact medical conditions

Summary

ABO blood group discrepancies in healthy individuals were caused by body-wide chimerism and mosaicism. They can be evaluated with new diagnostic options for disease-related cell clones that are typically associated with mosaicism. The observations raise the attention for sporadic mixed-field observations of any blood group antigen.

In this issue of the Journal, Dauber et al.¹ presented a case series with ABO discrepancies. They were recognized by mixed-field agglutination patterns in routine ABO blood group typing.² Blood transfusion, hematopoietic progenitor cell transplantation or disease are frequently causing mixed-field agglutination in routine blood typing. The ABO gene is located on the long arm of chromosome 9. No wonder that the – extremely rare – trisomy 9 can cause ABO discrepancies.^3,4 However, healthy individuals are rarely found with mixed-field agglutination.

Chimerism and mosaicism are the known causes for blood group discrepancies in healthy individuals. The mixed-field agglutination patterns induced by either cause may be indistinguishable. Many such cases go unnoticed when the discrepant red cell population is 5% or less.⁵ More sensitive methods are long available for research purposes.⁶ If recognized, few cases are followed up because clinical consequences are supposedly improbable or negligible. Therefore, few case reports among healthy individuals are published and even less researched in depth. The authors conducted substantial serologic and molecular workups and unambiguously distinguished between chimerism and mosaicism while some phenotypes were misleading.

Chimerism and mosaicism are conditions caused by different biologic mechanisms.^7,8 A chimeric human carries 2 or more populations of genetically distinct cells that originate from different zygotes:⁹ Embryos can merge or embryonic cells become exchanged between twins;⁵ during fetal development, hematopoietic progenitor cells can be transferred between twins or the mother and child. Hence, human chimeras are not rare, particularly in multiple pregnancy.¹⁰

Mosaicism develops from a single zygote, when somatic DNA mutations inevitably occur during cell divisions.⁸ Mosaicism can affect any gene and regulatory element. Everyone is expected to present mosaicism in some shape or form, because somatic mutations are abundant in most cells and accumulate during life.¹¹ Based on the chances for the underlying distinct mechanisms to occur, chimerism might be observed less frequently than mosaicism in healthy individuals.

Dauber et al.¹ presented a case series of 9 individuals with ABO discrepancies found in routine blood group typing. Discounting 1 pair of twins,¹ the number of independent cases amounted to 8, mostly healthy blood donors. The documented mechanism in the 2 patients eventually proved that the ABO discrepancy was unrelated to their medical condition prompting the ABO typing. The 8 cases can, hence, be considered independent spontaneous observations in healthy constitution. The survey was unbiased regarding the relative prevalence of chimerism versus mosaicism and their tissue distribution other than blood. The cohort is large for a published case series on the topic. These parameters of the study are important for the interpretation of the data.

Red cell populations that also differed for non-ABO antigens hinted to chimerism as cause in 6 of the 9 ABO discrepancies.¹ A multi-pronged experimental approach resolved the causes and proved a body-wide distribution in all cases.¹ The molecular techniques included genome-wide STR screening, chromosome 9-restricted STR markers, ABO genotyping of individual BFU-E colonies, and genome-wide SNV array analysis. In the end, 7 ABO discrepancies were traced to chimerism; only 2 were caused by mosaicism that, however, was found throughout all tissues.¹ The breakpoints regions¹² of the deletions seemed to differ between the 2 cases with mosaicism.¹

Body-wide distribution of genetically distinct cells in healthy individuals may be expected in chimerism as a consequence of the underlying mechanism. Cells originating from different zygotes merge early in pregnancy to form an embryo and contribute to a wide range of cells and tissues. The earlier the merging the wider the distribution will be. When the transfer of cells occurs late in embryonic or during fetal development between twins or between mother and unborn child, the affected tissues become more and more restricted. Eventually, the chimerism will be reduced to hematopoiesis because the exchanged cells become limited to hematopoietic progenitor cells, as those cells may be preferentially shared by means of joined blood streams. These are, of course, the cell lineages that are typically detected by routine blood typing and can be recognized as ABO discrepancies.

The observed body-wide distribution,¹ indicating events in early embryogenic development, was not necessarily expected in the 2 cases with mosaicism. Both individuals were in their 30ies and had more time after birth than before to accrue somatic mutations leading to mosaicism. Somatic non-functional mutations¹¹ or long-range deletions^{1 13} can occur at any time during pregnancy and throughout life, but mosaicism ensuing after birth did not cause any ABO discrepancy in 8 independent observations.¹ Judged by this result, the ABO gene locus could be considered rather stable. The loss of heterozygosity affecting an entire arm of chromosome 9, found in both cases with ABO mosaicism,¹ may be an ABO gene-specific mechanism after all. However, structural genetic variants⁷ with the greatest impact – when they are large and occur early in life – are more likely to be detected in routine typing. Further studies may address if other blood group systems, such as Rh,¹⁴ are also associated with naturally occurring body-wide mosaicism.

A previous study by the same group¹³ found loss of heterozygosity of parts of a chromosomal arm encompassing the RH gene locus. The ensuing somatic mosaicism could be traced back to 2 distinct stages of cell development during hematopoiesis.¹³ Both cases in that study were recognized by mixed-field agglutination patterns in routine Rh blood group typing.¹³ Increasing attention could be focused to evaluate mechanisms in healthy individuals with blood group discrepancies, not limited to ABO.^15,16

Identical twins cannot remain genetically identical for long when their somatic cell lineages inevitably accumulate distinct mutations with tissue distributions that vary between the twins.¹¹ More subtle differences between twins in health or some diseases, hitherto attributed to the environment or nurture, may eventually find a genetic explanation. New diagnostic tools^17,18 may help identify disease-related cell clones in cases of mosaicism.⁷ If no pathology develops, single-cell genomics may still contribute to the understanding of the physiology.¹⁹

In chimerism as well as mosaicism the physiologic, rather than carcinogenic, growth advantage will determine the tissue distribution of cell lineages among the ‘competing’ genetically distinct cells. Their relative distribution can vary during life.⁵ Drastically smaller stretches of loss of heterozygosity than reported^{1 13} can affect gene regulation, and long-read haplotypes of blood group and other genes will contribute to delineate regulatory elements. Many common benign diseases, but also the initiation of malignant growth, develop concomitant with increasing mosaicism during aging, and the temporal association may also be causal in instances.

The distinguishing genetic characteristics among genetically distinct cells cannot be ascertained by whole genome sequences from germ line cells or from peripheral blood. The true genetic polymorphism within an individual includes the genetic diversity among his or her cells and tissues. The term ‘whole’ genome of an individual encompasses much more than the nucleotide sequences of the diploid set of chromosomes in the embryo. The complete genomic constitution is magnitudes larger than the “whole genome sequence” of an individual.

Details of the body cells’ genetic diversity contribute to the physical constitution and diseases and have only begun to be fathomed with current technology. Until more advanced technology become available, innocuous observations^{1 2 3 4 13 14 15 16} and experimental analysis^{1 13 14 15} of blood group discrepancies in routine blood group typing provided a glimpse into the vast genomic diversity among cells and tissues of an individual.

The substantial experimental workup in the study by Dauber et al.¹ uncovered body-wide chimerism and also body-wide mosaicism as main causes of blood group discrepancies in healthy individuals. Both processes must have occurred during the 9 individuals’ early embryonic development. Improved understanding may raise the attention for sporadic mixed-field observations of any blood group antigen. They can be evaluated with new diagnostic options for disease-related cell clones that are typically associated with mosaicism.¹⁴