The first successful renal transplant was carried out between “identical” twins almost six decades ago and since that landmark case over 200 kidney transplants between identical twins have been recorded in the United States (1, 2). The early cases of renal transplantation between identical twins were carried out without immunosuppression by necessity because at the time safe and effective immunosuppression was not available. In subsequent years, successful transplantation of kidneys between identical twins has continued with the use of minimal or no immunosuppression (3–5). Interestingly, however, our current understanding of the molecular basis of development and the in utero environment suggests that monozygous (MZ) twins may not be as identical as previously believed. The clinical relevance of these differences in organ transplantation is unclear, and there is little study data pertaining to necessity of immunosuppressive therapy in identical twin transplantation (6). We provide a brief review of the current status of MZ twin transplantation and examine the practical and theoretical implications of current genetic, epigenetic, and immunologic research, with a particular focus on renal transplantation.
HOW IDENTICAL ARE IDENTICAL TWINS?
MZ twins arise from a single ovum fertilized by a single sperm. Traditionally they are considered genetically identical but phenotypically MZ twins do show differences. Historically, these phenotypic differences have been attributed to environmental influences, including effects on epigenetic reprogramming (7). The interaction between genes and modulating environmental stimuli begins early in utero and lifestyle differences give rise to genetic variation that increases with age (epigenetic drift). Genome-wide and single locus DNA methylation patterns leads to epigenetic variability in gene promoters that cause differences in transcriptional regulation and complex phenotypes (8). Furthermore, in the last decade, a number of studies have shown that MZ twins can and do differ genetically, with changes affecting single base pairs, and rarely at larger genomic regions and whole chromosomes (9–12). This phenomenon is related to the concept of somatic variation, whereby the genetic and epigenetic contents of cells in a single individual differ. Somatic variation and MZ twin discordance arises through three main mechanisms: somatic mosaicism, chimerism, and epigenetic alterations accumulated throughout life (epigenetic drift) (Fig. 1).
Somatic mosaicism is not a new concept and is vital for the normal functioning of the immune system with the controlled reshuffling of immunoglobulin and T-cell receptor genes; MZ twins are confronted with different foreign antigens and hence their T-cell reservoirs will vary. However, the presence and prevalence of somatic mosaicism in other tissues is a more recent observation. Somatic mosaics originate from a single zygote (the earliest stage of an embryo development) but contain cells with differing genetic content. Somatic mosaics can arise through a number of mechanisms, including mutations and structural changes occurring after the zygote stage, heteroplasmy (unequal division of mitochondrial DNA within the cellular cytoplasm) and uniparental disomy (where both copies of a chromosome or genomic region are inherited from a single parent) (13). In addition, development and maintenance of any single cell within an organism is stochastic; genetic and epigenetic modifications will continue throughout life in response to the environment and ageing. Indeed, studies of MZ twins have shown that both epigenetic and genetic changes occur in response to the in utero environment and increase with age (14–17).
A chimera originates from the mixing of cells from individuals originating from two distinct zygotes. Cells must mix, implant and survive in the new organism. This may occur “naturally” in utero, maternal cells entering the fetal circulation, and conversely during pregnancy, whereby fetal cells enter the maternal circulation (18, 19). It may also take place between embryos in multiple pregnancies (20). The extent of this phenomenon is unknown but has been seen in hematopoietic tissue as well as hepatocytes, myocytes and islet β cells (20–22). Indeed, in one recent review, the author suggests “that a shift is needed from the conventional paradigm of ‘self versus other’ to a view of the normal ‘self’ as constitutively chimeric” (20). Iatrogenic chimerism occurs transiently after blood transfusion or more permanently after tissue and organ transplantation (23, 24).
The full extent of somatic mosaicism or chimerism in the MZ twin population has not been studied: case report data tends to focus on overtly discordant twins. Indeed, it is technically difficult to estimate the prevalence of such variation in the larger non-twin population, and recent studies have only begun to assess the level of genomic and epigenomic diversity within a single individual (17, 25). However, with the construction of large databases containing phenotypic and genetic information from twins, our understanding of normal twin variation is set to expand (15). The twin model is particularly powerful method for assessing somatic variation, and as such, the understanding we gain from studying twin-twin variation may well prove useful in the future to the transplant team (26).
A comprehensive review of twin zygosity testing is beyond the scope of this article, but it remains an important consideration in the context of organ transplantation between twins. In general monochorionicty is a good screen for monozygosity (27). Standard human leukocyte antigen (HLA) typing is not sufficient alone to determine monozygosity because 25% of dizygotic twins will match at all HLA loci (28). More extensive haplotyping, such as that used in bone marrow transplantation, may add strength to choronicity, as would analysis of additional genetic markers, such as short tandem repeats, used in DNA fingerprinting. However, genetic markers, as with any other genomic region, may be affected by somatic variation, and therefore any anomalies detected by genetic testing must be considered with this in mind. The complexity of zygosity testing arises from many of the concepts that have been discussed in the preceding paragraphs; a review by Machin (27) offers a thorough discussion of the subject.
In the context of organ and tissue transplantation, there is perhaps scope for more “functional” assays, akin to skin grafting seen in the early transplantations (1). Of course, choosing an appropriate substrate for these investigations would require substantial research, and the use of renal tissue would pose a practical challenge.
IMPLICATIONS OF MZ TWIN DISCORDANCE IN TRANSPLANTATION
Clinically, MZ twin discordance has been described for single gene disorders, such as hemophilia B and Duchenne Muscular Dystrophy, as well as in complex conditions, such as multiple sclerosis and congenital heart disease (10, 29, 30). There is also a high prevalence of discordance for autoimmune conditions, such as systemic lupus erythematosus and rheumatoid arthritis: somatic and epigenetic variation between MZ twins identified by next generation (DNA) sequencing technologies may hint at potential immunologic discordance between MZ twins and hence have implications for transplantation (31, 32).
Although the mechanisms through which this discordance arises are becoming apparent, translating this information into clinical decisions is a challenge. In the context of MZ twin transplantation, it is the requirement and degree of immunosuppression necessary that remains an uncertainty.
IMMUNOSUPPRESSION IN MZ TRANSPLANTATION
Immunosuppressive regimens must balance the risks of immunosuppression, namely, infection, malignancy, and other specific side effects of agents, with the benefits of preventing graft rejection and disease recurrence. To ascertain the evidence base for immunosuppression in transplants performed between identical twins, we searched the literature to identify studies and reports pertaining to MZ twin organ transplantation (Appendix SI, SDC,https://links.lww.com/TP/B17). The most comprehensive analysis of MZ twin transplant cases was compiled by Krishnan and colleagues who identified close to 200 MZ twin renal transplants in the United States between 1987 and 2006. They reported that 71% of cases were receiving immunosuppressive therapy at the time of discharge from hospital (61% received steroids and 31% calcineurin inhibitors), falling to 34% 1 year and 33% 3 years after transplantation. At 1 year, 21% of recipients were still on calcineurin inhibitors, and 27% were still on steroids. Interestingly, graft function was superior in recipients who were not maintained on immunosuppression.
There are limited data available pertaining to rejection rates in MZ twin transplantation to provide evidence-based guidance on the use of immunosuppression. Kessaris and colleagues (5), who identified 132 living-donor identical twin transplants that took place between 1988 and 2004 in the United Kingdom and United States, recorded similar observations to those of Krishnan et al: a large proportion of the patients remained on immunosuppression and there was no significant difference in graft survival between those on immunosuppression and those not. The findings from these reports suggest that renal transplantation between MZ twins can be performed safely without long-term immunosuppression. Successful living-donor liver and small bowel transplants have also been carried out without immunosuppression in MZ twins (Table 1) (4, 6, 33–42).
DISCUSSION AND CONCLUSION
It has long been observed that graft rejection is not solely a function of immune homology (43). Trauma, ischemia and reperfusion during organ transplantation stimulates cytokine release, activating immune responses and contributing to graft rejection (44). The presence of autoantibodies to cryptic tissue antigens (e.g., vimentin) associated with kidney and heart allograft rejection indicate that tissue damage may be an important initiator of graft rejection regardless of genetic identity (45). For this reason, perioperative steroids are an important component of immunosuppressive regimens and one that should most likely remain in MZ twin transplantation (2).
Twin and HLA-identical sibling transplantation also carries additional considerations, not related to immunosuppression, such as distinct emotional consequences and disease recurrence: one of the most common reasons for graft loss in HLA-identical sibling transplants (46–48).
When planning renal transplantation between apparent MZ twins, zygosity testing using multilocus probes should be carried out to confirm twins are indeed monozygotic. Advances in developmental biology have clearly shown that MZ twins may not be as identical as once believed, and the mechanisms responsible for such differences are now becoming clearer. To what extent such differences are capable of triggering graft rejection remains unknown, but the risks seem low, and long-term transplant outcome is excellent. Available evidence to guide immunosuppressive is limited but a prudent approach would include the use of steroids together with a calcineurin inhibitor after transplantation. However, once the inflammatory response associated with transplant surgery has resolved, cautious reduction and eventual withdrawal of immunosuppression should be possible, thereby avoiding the long-term side effects of immunosuppression, including the detrimental effects of calcineurin inhibitors on graft function.
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