21Jul2025

Mitochondrial Donation in a Reproductive Care Pathway for mtDNA Disease

Authors: Robert McFarland, Ph.D., Louise A.Hyslop, Ph.D. https://orcid.org/0000-0002-0326-7208, Catherine Feeney, M.Sc., Rekha N. Pillai, Ph.D., Emma L. Blakely, Ph.D., Eilis Moody, M.Sc., Matthew Prior, Ph.D., +5 , and Douglass M. Turnbull, Ph.D.Author Info & Affiliations

Published July 16, 2025

DOI: 10.1056/NEJMoa2503658

Summary

Pathogenic variants in mitochondrial DNA (mtDNA) are a common cause of severe, often fatal, inherited metabolic disease. A reproductive care pathway was implemented to provide women carrying pathogenic mtDNA variants with reproductive options. A total of 22 women with pathogenic mtDNA variants have commenced or completed pronuclear transfer (and thus receipt of a mitochondrial donation), and there have been 8 live births. All 8 children were healthy at birth, with no or low levels of mtDNA heteroplasmy in blood. Hyperlipidemia and cardiac arrhythmia developed in a child whose mother had hyperlipidemia during pregnancy; both of the child’s conditions responded to treatment. Infant myoclonic epilepsy developed in another child, with spontaneous remission. At the time of this report, all the children have made normal developmental progress. (Funded by the U.K. National Health Service and others.)

Pathogenic variants in mitochondrial DNA (mtDNA) are a common cause of severe inherited metabolic disease.^1,2Mitochondrial DNA is present in multiple copies within each cell. For many pathogenic variants, the relative abundance (heteroplasmy) of mutated copies is a key factor in determining the severity of disease, with higher levels of heteroplasmy causing disease in a diverse range of organs.³ However, other pathogenic mtDNA variants are typically homoplasmic (all copies are mutated) and exhibit a characteristic pattern of isolated organ involvement with variable penetrance.^4,5

Despite the prevalence and severity of mtDNA disease, treatment options are limited.⁶ The exclusive maternal transmission of mtDNA means that children of women with homoplasmic pathogenic mtDNA variants are obligate homoplasmic carriers. The outcome for children born to mothers with heteroplasmy is less predictable; during female germ-cell development, a mitochondrial genetic bottleneck leads to a random shift in heteroplasmy, which results in offspring with variable mutant loads.^7,8

For women considering treatment through assisted reproductive technology to lower the risk of serious mtDNA-related disease in their offspring, options include preimplantation genetic testing (PGT).^9,10 However, for women with high levels of mtDNA heteroplasmy or homoplasmic pathogenic mtDNA variants, PGT is not an option and mitochondrial donation has been considered as a possibility.¹¹

After careful preclinical testing of mitochondrial donation with the use of pronuclear transfer¹² and a change in the Human Fertilisation and Embryology Act (United Kingdom), a new comprehensive National Health Service (NHS) care pathway¹³ was established to provide informed reproductive choices for women; one of these choices is to accept mitochondrial donation through pronuclear transfer. Here, we describe this pathway and present some data on the genetic and clinical outcomes of pregnancies and children born after mitochondrial donation. In an accompanying article in this issue of the Journal, Hyslop et al.¹⁴ describe the corresponding embryology data.

Methods

The mitochondrial reproductive care pathway that was implemented in 2017 (the NHS Mitochondrial Reproductive Care Pathway) is available to all women living in the United Kingdom who harbor pathogenic mtDNA variants, with an underlying principle to provide these women with an informed reproductive choice that can be implemented in a regulated environment.^13,15 The pathway comprises the Mitochondrial Reproductive Advice Clinic (MRA-C) and the Mitochondrial Assisted Reproductive Technology Clinic (MART-C) where women and their partners undergo detailed clinical review (see the Methods section in the Supplementary Appendix, available with the full text of this article at NEJM.org).

As part of the licensing conditions, women who proceed with mitochondrial donation are offered close monitoring of the pregnancy and follow-up of the offspring (Fig. S1 in the Supplementary Appendix). Throughout the process, any health concerns are discussed and, when indicated, assessed by the NHS Highly Specialised Mitochondrial Reproductive Care Pathway team, including assessment of the long-term health consequences for all children born.

To obtain funding for mitochondrial donation, we devised and registered a clinical study to assess the children at 18 months of age by means of the Bayley Scales of Infant and Toddler Development, 3rd edition¹⁶ (National Institute of Health and Care Research [NIHR] Central Portfolio Management System identification number, 31755; ClinicalTrials.gov number, NCT04113447). At the time that the manuscript was submitted, only two children had completed the 18-month assessment; therefore, we are not reporting the results of the clinical trial but the efficacy and safety of mitochondrial donation before that point.

Results

REPRODUCTIVE CARE PATHWAY

A total of 196 referrals, which broadly reflected the demographic characteristics of the U.K. population (Table S1), were reviewed by the Referral Assessment Multidisciplinary Team. A total of 4 referrals were not considered further owing to insufficient referral information or an incorrect diagnosis (Figure 1). Before the offer of an appointment in the MRA-C, 23 women withdrew from the process, of whom 9 were already pregnant. A total of 6 women have appointments pending.

FIGURE 1

Reproductive Care Pathway for Women with Pathogenic mtDNA Variants.

Of the 163 women evaluated in the MRA-C (not including the 6 women in whom evaluation is pending), 133 have received or are receiving care in the MART-C, 2 of whom are participating in healthy-lifestyle programs to decrease their body-mass index (Figure 1). A total of 30 women were unable or opted not to proceed further; most of these women viewed the process as “fact finding” to inform their future reproductive decision making, and 3 had conceived naturally before their planned consultation in the MART-C.

After MART-C assessment of their reproductive options, 51 women did not proceed in the program. Reasons included limitations to undergoing in vitro fertilization, such as a very low ovarian reserve (6 women) and age (1 woman), or risks involved in pregnancy owing to poor maternal health. Many women declined at this stage because of personal priorities and timing, often with a desire to proceed in the future. A total of 5 women were offered a reproductive intervention but declined without providing a reason.

Of the 70 women who proceeded with a reproductive option, 36 (in whom 10 different pathogenic mtDNA variants were represented) were offered PGT. The use of PGT resulted in the birth of 13 children (Figure 1) since the start of the care pathway.

MITOCHONDRIAL DONATION

Applications for Mitochondrial Donation

All 32 applications that were submitted to the Human Fertilisation and Embryology Authority for mitochondrial donation with the use of pronuclear transfer were approved; 4 applications are pending (Figure 1). Ten pathogenic mtDNA variants (all in Cambridge reference sequence number NC_012920.1) were represented across the 32 women: m.11778G→A (in 9 women), m.3243A→G (in 7), m.3460G→A (in 4), m.8344A→G (in 4), m.4300A→G (in 3), m.3250T→C (in 1), m.3260A→G (in 1), m.3635G→A (in 1), m.7510T→C (in 1), and m.14484T→C (in 1). Of these 32 women, 3 had previously had embryos genetically tested (1 of them, before the introduction of the care pathway) but did not have any transferred owing to high levels of heteroplasmy in each tested embryo.

Pregnancies after Mitochondrial Donation

Ultrasonography at 7 weeks confirmed eight pregnancies, from which there have been eight live births (four female and four male), including a set of monozygotic twins, and one ongoing pregnancy (Table 1). There have been no miscarriages after ultrasonographic confirmation of pregnancy at 7 weeks’ gestation. Six pregnancies were without complication. In one woman who was homoplasmic for the m.4300A→G variant and had a preexisting hypertrophic cardiomyopathy, paroxysmal atrial fibrillation developed during preparation for embryo transfer, which led to treatment with a beta-blocker and thromboprophylaxis. This patient also had a clinically significant complication of severe hypertriglyceridemia during pregnancy (serum triglyceride level, 77.17 mmol per liter [6835 mg per deciliter; normal range, 0.55 to 1.69 mmol per liter {49 to 150 mg per deciliter}]; serum total cholesterol level, 23.2 mmol per liter [897 mg per deciliter; normal value, <5.2 mmol per liter {200 mg per deciliter}]), which improved with restriction of dietary fat to less than 20 g per day and resolved within a week post partum.

TABLE 1

Characteristics of Women with Pregnancies after Pronuclear Transfer.

FOLLOW-UP OF CHILDREN

Assessment of mtDNA Variant Levels

All the babies were born between 36 weeks 1 day and 42 weeks 2 days by normal vaginal delivery or elective cesarean section for potential maternal-health concerns or, in one case, placenta previa. Newborn and placental weights and Apgar scores were within normal ranges (Table 2). The level of heteroplasmy, available in each of the eight children, was below the threshold for clinical disease. Five children had undetectable levels at birth. A child whose mother was homoplasmic for the m.4300A→G variant had levels of 5% and 9% in blood and urine, respectively, at birth and undetectable levels in blood at 18 months. A child (born to a mother with the m.3260A→G variant) had 16% and 20% heteroplasmy in blood and urine, respectively, at birth, and another child (born to a mother with the m.11778G→A variant) had 12% and 13% heteroplasmy in blood and urine, respectively, at birth.

TABLE 2

Details of Pregnancy and Births of Infants after Pronuclear Transfer.

Assessment of Children

All eight neonates were healthy at birth and appear to be making normal developmental progress (see the Results section in the Supplementary Appendix). At the time of this report, three children are between 0 and 5 months of age, two are between 6 and 11 months of age, one is between 12 and 17 months of age, one is between 18 and 23 months of age, and one is at least 24 months of age. To date, five children have had no reported medical problems. An infant born to a mother with the m.3460G→A variant, who had no detectable maternal mtDNA at birth, had brief startles (head and neck flexion with eye blink) from 7 months of age. Neurologic, developmental, and general examinations were normal, as were routine blood tests and an interictal electroencephalogram. The child received a diagnosis of myoclonic epilepsy of infancy, a rare, typically self-limiting condition that occurs in otherwise healthy and developmentally normal children.¹⁷ After 3 months, myoclonic jerks ceased without treatment, and developmental progress remains normal in all domains. Another child had a urinary tract infection that responded promptly to antibiotic treatment.

One breast-fed infant, born to the woman with the m.4300A→G variant who had gestational severe hypertriglyceridemia, had prolonged jaundice with mildly elevated levels of liver enzymes (elevated alanine aminotransferase and γ-glutamyltransferase levels with conjugated hyperbilirubinemia), an enlarged hyperechogenic liver (indicative of hepatic steatosis) on ultrasonography, and hyperlipidemia (serum triglyceride level, 7.5 mmol per liter [664 mg per deciliter; normal value, <1.15 mmol per liter {102 mg per deciliter}]). Dietary restriction of fat intake led to resolution of the hyperlipidemia and fatty liver over the course of 3 months. An echocardiogram, obtained at approximately 4 months of age, revealed a dilated left ventricle. This finding prompted a detailed pediatric cardiology assessment that showed ventricular preexcitation (Wolff–Parkinson–White pattern) and atrial tachycardia with secondary left ventricular dilatation, although the patient was asymptomatic. Extensive genetic investigations, including genome sequencing, did not identify a hereditary cause for this cardiac phenotype. Two independent pediatric cardiologists concluded that the atrial tachyarrhythmia was responsible for the enlarged left ventricle with reduced contractility. Treatment with antiarrhythmic medications has resulted in the left ventricle returning to normal morphologic features and function. This child was assessed at an adjusted age of 18 months 12 days, with the use of the Bayley Scales of Infant and Toddler Development, 3rd edition¹⁶: the scores were within the normal range. Both hearing and vision were assessed as normal by bedside testing. At the time of this report, the child remains well and continues to develop normally. Antiarrhythmic treatment is being slowly weaned with a view to possible ablative therapy in the future. One other child has had an 18-month neurodevelopmental assessment that was normal in all the domains. Planned cardiology assessments have been normal.

Discussion

Women at risk for transmitting severe mtDNA disease to their children should have the opportunity to make informed choices about their reproductive options: prenatal testing, PGT, mitochondrial donation, egg donation, adoption, and deciding not to have children. Multidisciplinary advice regarding the optimal choice for an individual woman is tailored to the specific mtDNA variant and the woman’s own views on risk reduction. In this study, many of the women who were eligible for mitochondrial donation or PGT decided, after counseling, not to proceed in the pathway at this time. Because mitochondrial donation and PGT involve in vitro fertilization, the success of which is inversely correlated with age,^18,19 we would encourage this type of early fact-finding discussion. Although this study involved women from the United Kingdom, we believe that the results are generalizable to other countries.

There is a theoretical risk of deterioration in health during pregnancy among women with pathogenic mtDNA variants, with an increased risk of pregnancy complications and early delivery.²⁰ In seven pregnancies, including one that is ongoing, no complications have been reported, although one patient with hypertrophic cardiomyopathy had intermittent atrial fibrillation. This patient also had a clinically significant complication of severe hyperlipidemia during pregnancy.²¹ Despite extensive metabolic and genetic investigations (see the Results section in the Supplementary Appendix), the cause of this hyperlipidemia has not been identified. We are not aware of reports of extreme hyperlipidemia associated with this or any other mtDNA variant during pregnancy.

The eight infants were healthy at birth and are developing normally. Our data support a marked reduction in the transmission of pathogenic mtDNA variants after mitochondrial donation, with heteroplasmy either undetectable or well below a threshold likely to cause disease. We were able to sample only easily accessible tissues and do not know whether these tissues are entirely representative.

One child born to a mother with m.4300A→G variant–related disease had hyperlipidemia (also noted in her mother during pregnancy) and cardiac arrhythmia; both conditions have been successfully treated. Primary cardiac arrhythmia is not a known feature of the m.4300A→G variant (rather, patients with this variant are at risk for progressive hypertrophic cardiomyopathy).⁵ None of the other children had cardiac problems. We have instigated a pediatric cardiology review at 3 to 6 months in all the children born after mitochondrial donation. Another child had myoclonic epilepsy of infancy, a self-limiting condition that has resolved.

Although concerns exist about the safety of mitochondrial donation, including mismatch between nuclear DNA and mtDNA and reversion to the original maternal mitochondrial genotype,^22,23 it is challenging to establish cause and effect of adverse health outcomes in babies born. First, the procedures of conventional assisted reproductive technology are associated with an increased incidence of congenital anomalies, most notably of the cardiovascular system.²⁴ Second, some pathogenic mtDNA variants are associated with an increased incidence of pregnancy complications that can cause adverse health outcomes in children.²⁰Third, the risk of cardiovascular defects is increased by exposure during early pregnancy to maternal metabolic conditions such as diabetes²⁵ and hyperlipidemia.²⁶ It is of interest that one child born after PGT, performed for m.3243A→G (a pathogenic mtDNA variant strongly associated with the development of diabetes mellitus) had a congenital cardiac defect; the maternal glycated hemoglobin level was normal before conception.

We found that pronuclear transfer, a form of mitochondrial donation, was effective in reducing the level of pathogenic mtDNA variant to substantially below the threshold for clinical disease in the offspring of women with homoplasmic (or high heteroplasmic) levels. We are assessing, over the long term, the health and extent of heteroplasmy (if detectable) of the offspring. Indeed, the role of mitochondrial donation as a choice for women with a heritable pathogenic mtDNA variant will only be established with the availability of additional data.

NOTES

This article was published on July 16, 2025, at NEJM.org.

The Highly Specialised Mitochondrial Reproductive Care Pathway is supported by the NHS at Newcastle upon Tyne Hospitals NHS Foundation Trust, Newcastle upon Tyne, United Kingdom. Support was provided by Wellcome (203105/Z/16/Z and 206001/Z/16/Z). Infrastructural support was provided by Newcastle University; a National Institute for Health and Care Research Biomedical Research Centre award to the Newcastle upon Tyne Hospitals NHS Foundation Trust, Newcastle upon Tyne, United Kingdom; and the NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children.

Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.

We thank Alex Bright, Carol Shaw, Jan Dutton, Corina Moldova, Neil Seller, Satish Adwani, Ed Blair, Rachel Oliver, Bernadette Brent, Ruth Curry, and Fredrik Karpe for their help in managing patients; Lyndsey Butterworth for public engagement in developing the reproductive pathway; and the clinical and laboratory staff of the NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children and the Newcastle Fertility Centre at the International Centre for Life.

SUPPLEMENTARY MATERIAL

Supplementary Appendix(nejmoa2503658_appendix.pdf)

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Disclosure Forms(nejmoa2503658_disclosures.pdf)

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REFERENCES

Gorman GS, Schaefer AM, Ng Y, et al. Prevalence of nuclear and mitochondrial DNA mutations related to adult mitochondrial disease. Ann Neurol2015;77:753-759.

Go to Citation

PubMed

Google Scholar

Mancuso M. Complex neurological and multisystem presentations in mitochondrial disease. Handb Clin Neurol2023;194:117-124.

Go to Citation

PubMed

Google Scholar

Stewart JB, Chinnery PF. Extreme heterogeneity of human mitochondrial DNA from organelles to populations. Nat Rev Genet 2021;22:106-118.

Go to Citation

PubMed

Google Scholar

Carelli V, La Morgia C, Yu-Wai-Man P. Mitochondrial optic neuropathies. Handb Clin Neurol 2023;194:23-42.

Go to Citation

PubMed

Google Scholar

Taylor RW, Giordano C, Davidson MM, et al. A homoplasmic mitochondrial transfer ribonucleic acid mutation as a cause of maternally inherited hypertrophic cardiomyopathy. J Am Coll Cardiol2003;41:1786-1796.

Crossref

PubMed

Google Scholar