When a person has two genetically different populations of red blood cells

This is the story of a 10-year-old Chinese boy hospitalized for acute appendicitis. Before undergoing surgery, doctors determined his blood type. It was then that they discovered they were facing a blood typing problem. This child permanently and naturally possesses two genetically different populations of red blood cells, each of the Rhesus type.

This young patient has what is known as blood group mosaicism. His case was reported in March 2025 in the journal Transfusion and Apheresis Science by doctors and biologists from the Hangzhou Institute of Transfusion Medicine (Zhejiang Province, East China).

Mosaicism is defined as the coexistence of at least two genetically different cell populations within the same individual. However, this child has never received a blood transfusion, nor undergone an organ transplant or hematopoietic stem cell transplant to treat a malignant hematological disease (blood cancer). He has no twins. Finally, his parents did not use in vitro fertilization to conceive him. These are all circumstances in which several genetically different cell populations can coexist in the same individual.

Mosaic refers to an individual or organism in which two or more cell populations coexist, each with different genomes, but originating from the same fertilized egg (zygote). Mosaicism should not be confused with chimerism , which refers to the coexistence within the same individual of two or more genetically distinct cell populations originating from at least two zygotes. These individuals are called chimeras , in reference to the creature in Greek mythology whose body was the result of a mixture of a lion, a goat, and a snake.

Mosaicism occurs through DNA mutations that inevitably occur during cell division. Therefore, everyone can be expected to have some form of mosaicism because DNA alterations are abundant in most cells of the body and accumulate throughout life. These DNA mutations occur after conception in somatic cells (non-germline cells, i.e., cells that will not produce gametes).

Mosaic cell populations correspond to the clonal expansion of mutated cells, produced continuously throughout life. Depending on when these cells appear and their ability to survive, their proportion can fluctuate: some may spread throughout the body, while others remain confined to certain tissues or organs.

In mosaicism, the mutations involved are not limited to the substitution of a single nucleotide (replacing one letter with another in the DNA), but also include small insertions and deletions. An insertion is when one or more letters are mistakenly added to the DNA, and a deletion is when one or more letters are removed. It can also involve variations in the number of copies of a region, small or large, of DNA. A particular type of mutation is loss of heterozygosity, which corresponds to the disappearance of one of the two copies of a gene that a cell normally possesses (one copy from the father, the other from the mother), the remaining copy possibly being altered.

Mosaicism and chimerism are known causes of blood typing difficulties due to the existence of a double population of red blood cells. Many cases go undetected when the minority population is 5% or less.

It is important to know that the Rh system is a blood group system carried only by red blood cells (also called erythrocytes). The Rh system includes five antigens, namely D, C ("large C"), E ("large E"), c ("small c") and e ("small e"). These antigens are highly immunogenic, meaning they are capable of inducing the production of antibodies against them if they are recognized as foreign by a person who does not possess them.

A person is said to be Rh positive (Rh+) when the D antigen is present on the surface of their red blood cells, which is the case for approximately 85% of the European population. In this case, the anti-D antibody binds to the D antigen on the red blood cells, causing them to clump together, a phenomenon that manifests itself by the formation of visible clumps. This same clumping mechanism is observed for other Rh antigens (C, c, E, e) when anti-C, anti-c, anti-E and anti-e antibodies are used for Rh grouping.

But let's return to the case of this Chinese child who was to be operated on for appendicitis and whose blood type it was important to know before surgery. When determining the Rh grouping, a surprise awaited the Chinese biologists: some red blood cells were agglutinated by the anti-C antibody, while others were not. An identical phenomenon occurred when the young boy's red blood cells were exposed to an anti-e antibody. Subsequently, it was shown that the non-agglutinated cells possessed the c ("small c") and E ("large E") antigens on their surface.

Flow cytometry, a technique used to characterize and count different cell types, showed that 47.26% of the boy's red blood cells carried the C antigen and 52.74% did not. Serological methods therefore showed that there were two different populations of red blood cells in this child's blood.

The researchers studied DNA from blood samples, oral mucosa, and hair shafts of the young patient, as well as DNA from the parents' blood. Analysis of about twenty short DNA sequences present at specific locations in the genome detected genetic profiles consistent with the existence of mosaicism. Furthermore, since the child's 46, XY chromosomal formula (or karyotype) showed no abnormalities, the biologists deduced that the cell lines were likely derived from a single zygote rather than the fusion of different zygotes. This is therefore mosaicism, not chimerism.

If a blood group mosaic is discovered, the transfusion strategy is based on the most dominant blood group in order to minimize the transfusion risk. That said, when possible, an autologous transfusion should be performed, which consists of taking blood from the patient before or during surgery, to reinject it, if necessary, during or immediately after the operation.

Blood group mosaicism does not only affect the Rhesus (Rh) system; it can also affect the ABO system. This situation is most common in cases of blood grouping difficulties due to the existence of a double population of red blood cells. It is rare to detect ABO group mosaicism in healthy individuals. Located on the long arm of chromosome 9, the ABO gene determines blood groups based on the presence or absence of two antigens, A and B, on the surface of red blood cells. Depending on whether they have the A antigen, the B antigen, both, or neither, individuals are classified as having blood group A, B, AB, or O.

Determining the ABO group is essential to ensure compatibility in blood transfusions or certain solid organ transplants. It is based on an agglutination reaction of red blood cells using anti-A and anti-B antibodies.

A Brazilian team reported in 2024, in the journal Hematology, Transfusion and Cell Therapy , the case of mosaicism during the determination of the ABO group in a 24-year-old blood donor. Here again, not all of his red blood cells were found to be agglomerated by the same antibody. This only caused partial agglutination in this individual who had never been transfused or transplanted before.

When determining the ABO group, it turned out that 52% of this person's red blood cells carried the B antigen but did not possess the A antigen, and the remaining 48% had both antigens on their surface. Specifically, this individual had mosaicism with a double population of red blood cells: one type B, the other type A1B. Data from DNA sequencing of blood samples and oral mucosa also supported mosaicism.

In 2024, researchers from the Department of Transfusion Medicine at the Vienna Medical School (Austria) reported in the British Journal of Haematology a series of nine cases of blood grouping difficulties in the ABO system.

Among them, seven cases were chimerism, two were mosaicism. The latter affected not only the ABO blood system but also all tissues of the body. The fact that mosaicism affects the entire organism indicates that the triggering genetic event occurred early in the individual's life and affected all somatic cell lineages. The effects of this phenomenon depend on the number of cells affected and their spread throughout the body.

The two individuals with ABO mosaicism were a 35-year-old woman and a 31-year-old man who were blood donors. Both had a double population of red blood cells. In the woman, 10% of the red blood cells were A1, the rest O. In the man, 25% of the red blood cells were A1 and 75% O. In both cases, genetic analyses showed a loss of heterozygosity affecting the entire long arm of chromosome 9, which carries the ABO gene.

This same mechanism was described in 2017 as possibly being responsible for a double population of red blood cells for the Rh system. The individual is then both Rh positive and Rh negative.

In these cases, loss of heterozygosity affects the short arm of chromosome 1. It can also occur early in embryonic development. This genetic abnormality is the most common cause of mosaicism in the Rh system, whether limited to the blood line or present more widely throughout the body. This phenomenon is thought to affect at least one in 500 people over the age of 60 who are apparently healthy.

In 2007, a team of German and Austrian hematologists reported in the journal Blood the case of nine patients (three of whom suffered from hematological disease) whose routine Rh group determination revealed the presence in their blood of red blood cells carrying the D antigen and others that were devoid of it. It should be remembered that the presence of this antigen corresponds to Rh positive and its absence corresponds to Rh negative. These patients had not recently been transfused and had not received a hematopoietic stem cell transplant. Genetic analyses had excluded any chimerism.

Depending on the patient, red blood cells that did not carry the D antigen were also either negative for the C antigen or negative for the E antigen. In these individuals, the cause of Rh group mosaicism was shown to be a loss of heterozygosity of several regions of chromosome 1, including the RHD gene encoding the D antigen and the RHCE gene encoding the C and E antigens.

A double population of red blood cells for the Rh system, or even a progressive loss of Rh antigens, can sometimes be observed in patients suffering from a hematological disease, such as acute myeloid leukemia (AML) or chronic myeloid leukemia (CML), a myeloproliferative disease or a myelodysplastic syndrome (conditions characterized by the production of abnormal stem cells by the bone marrow).

Blood types can hold other surprises for transfusion physicians and hematologists, as evidenced by observations published in the medical literature.

In 1998, a Franco-Danish team reported in the British Journal of Haematology the case of a woman (group O) who had a change in her Rh group. She went from Rh positive to Rh negative over the course of three years, between 1991 and 1994. Chronic myeloid leukemia was diagnosed in 1994. This patient had never received a transfusion. Consistently identified as Rh-positive between 1965 and 1991, she was systematically found to be Rh-negative after 1994. This change in Rh group was most likely due to the occurrence of a somatic mutation (deletion of a single letter) that the researchers discovered in the gene encoding the D antigen of the Rh system.

More recently, in 2019, in the journal Haematologica , a German-Austrian team described the case of two individuals whose routine Rh group determination showed that their red blood cells were discordant for the c antigen. Approximately half of their red blood cells were c-positive, while the other half were negative for this antigen. These two women, in apparent good health, had no history of blood transfusion or transplant. Again, any chimerism was excluded by the genetic data.

In these two patients, the existence of a double population of red blood cells for the Rh group was due to a complex genetic anomaly involving a loss of genetic material (deletion) on the short arm of chromosome 1 on which the RHD gene encoding the D antigen is located. To determine whether mosaicism existed, the researchers used genetic markers specific to chromosome 1. In one case, the mosaicism was confined to myeloid stem cells, while in the other case this phenomenon affected not only the myeloid lineage but also lymphocytes (white blood cells) and hair root cells, thus indicating that the responsible genetic anomaly, associated with a loss of heterozygosity, had occurred at an earlier embryonic stage.

As we can see, determining blood groups may, in rare cases, not be as simple as it seems. Between mosaicism in the ABO group, interpretation difficulties in the Rh system, and enigmatic disappearances of Rh antigens in certain blood diseases, hematologists can sometimes find themselves faced with extremely surprising results, which require a real investigation to understand the cause.

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