Methods and limitations
Data collection
Individual embryo heteroplasmy values were extracted from published studies. The current dataset contains 187 values for 7 mtDNA variants; 40 blastocysts with measurements from several samples (TE biopsy, ICM, remaining TE or combined ICM+TE); and prenatal or postnatal measurements from 16 foetuses and children with a matched heteroplasmy result from the transferred embryo. Primary publications were used where available. Values obtained from reviews because the primary source was unavailable are explicitly identified.
Studies do not always report every embryo in a cycle, so the dataset does not represent all embryos from the included cycles. Pooling published cases does not provide a clinical success rate for PGT-mt.
32 records, including 30 matched embryo–tissue pairs from 16 foetuses and children. 22 publications, of which 12 report embryo values.
Dataset size
| Measure | Count |
|---|---|
| embryo heteroplasmy values | 187 |
| mtDNA variants in the heteroplasmy landscape | 7 |
| blastocysts with measurements from several samples | 40 |
| blastocyst sample records | 91 |
| cleavage-stage embryos (two blastomeres) | 22 |
| embryo → offspring records | 32 |
| foetuses and children in all embryo–offspring records (including excluded records) | 17 |
| foetuses and children with matched embryo–tissue pairs in the plotted dataset | 16 |
| publications | 22 |
How values are represented
- Where “0” represents a result reported as not detected, it does not establish complete molecular absence of the variant. The limit of detection is shown only when reported by the source.
- If only an interval was published, its bounds are stored; no midpoint is imputed.
- Each value is accompanied by information on how it was obtained. A derivation label is assigned only when the corresponding rule is documented in the supplementary workbook:
- Derivation not recorded — 137 Values in the “Between different embryos” section
- Mean of blastomere and TE measurements — 1 Values in the “Between different embryos” section
- Mean of two blastomeres — 19 Values in the “Between different embryos” section
- Mean of two embryo halves — 9 Values in the “Between different embryos” section
- Mean of several biopsy measurements — 11 Values in the “Between different embryos” section
- Value digitised from a published figure — 9 Values in the “Between different embryos” section
- Value obtained from reviews (primary source: abstract) — 1 Values in the “Between different embryos” section
Every point leads back to its row in the supplementary workbook and to the publication (DOI).
Reference lines on the charts
- 18% · risk estimate from a specific study — Calculated estimate from a systematic review: a muscle mutation level of ≤18% is associated with a ≥95% chance of being unaffected (159 mtDNA point variants; familial data from 137 pedigrees; a priori probability of being affected 0.477). For m.3243A>G the chance of being unaffected is lower. The estimate was not derived from embryos and is not a threshold for an embryo biopsy. (Hellebrekers et al., 2012 doi:10.1093/humupd/dms008)
Hum Reprod Update 2012;18(4):341–349, abstract (PubMed 22456975), Results. Checked: 2026-10-11, checked against the PubMed abstract. The full text is unavailable in the repository and in PMC; the sample composition and exclusions were checked only to the extent described in the abstract. - 15–30% · range cited in the literature — Range of the symptomatic threshold for m.3243A>G that Ji et al. cite as generally accepted from the literature; the authors could not determine a threshold for m.3243A>G from their own data. (Ji et al., 2023 doi:10.1007/s10815-023-02880-2)
J Assist Reprod Genet 2023;40:2185–2196, p. 2190 (Results) and p. 2193 (Discussion); reference [23] in the article. Checked: 2026-10-11, checked against the PDF (references/Ji et al 2023 PGTmt risk and chance.pdf) - 30% · working criterion of a clinical programme — Working criterion of the Newcastle programme: embryos with less than 30% heteroplasmy, measured in a single day-3 blastomere, were preferentially used for transfer or vitrification. It is the criterion of one clinical programme, not a universal threshold. (Hyslop et al., 2025 doi:10.1056/NEJMoa2415539)
N Engl J Med 2025, Methods (PDF p. 3: “single blastomere was removed on day 3”; “less than 30% heteroplasmy”) and PDF p. 5. Checked: 2026-10-11, checked against the PDF (references/Hyslop et al 2025 MRT PGTmt.pdf)
None of these values defines a safe threshold for an individual embryo. The three published reference values (18, 15–30 and 30%) address different questions and derive from different tissues, methods and outcomes.
Limitations
- The dataset is descriptive and does not support a meta-analysis: studies differ in biopsy stage, analytical methods and limits of detection.
- Successful cycles are published more often than unsuccessful ones; there are no data to correct for this bias.
- Embryos are not independent observations. Many come from the same patient and cycle, and variants are unevenly represented. A pooled proportion does not estimate individual risk.
- In embryo–child comparisons several measurements belong to the same child (different tissues, ages, methods, laboratories).
- The PGT-mt × PGT-A model calculates scenarios rather than individual predictions. Dependence between embryos is an assumed scenario parameter, not a correlation estimated from family data.
Variant classification
Variant classifications are taken from the supplementary workbook. The Atlas does not independently classify variants, including on the basis of heteroplasmy level. Each classification is accompanied by its supporting statement in the primary publication and the date checked. This review did not cross-check the variants against MITOMAP or ClinVar.
| Variant | Class | Basis |
|---|---|---|
| HV2 polymorphism | Neutral polymorphism | as in the supplementary workbook; no explicit statement found in the primary publication (checked 2026-10-11) |
| m.10158T>C | Pathogenic variant | basis: Hyslop et al., 2025, Table 1, Maternal Pathogenic Variant Heteroplasmy in Infants; PGT; checked 2026-10-11 |
| m.10191T>C | Pathogenic variant | as in the supplementary workbook; no explicit statement found in the primary publication (checked 2026-10-11) |
| m.10197G>A | Pathogenic variant | basis: Mizuguchi et al., 2025, PDF p. 1: “The m.10197G>A mutation… is a rare pathogenic mutation of Leigh encephalopathy”; checked 2026-10-11 |
| m.13094T>C | Pathogenic variant | basis: Hyslop et al., 2025, Table 1, Maternal Pathogenic Variant Heteroplasmy in Infants; PGT; checked 2026-10-11 |
| m.14487T>C | Pathogenic variant | basis: Tofilo et al., 2026, Tofilo et al., 2026, main text, Introduction — pathogenic mtDNA variant m.14487T>C; doi:10.1007/s10815-026-04012-y, PMID 42649365.; checked 2026-10-11 |
| m.3243A>G | Pathogenic variant | basis: Spath et al., 2021, PDF p. 8: “Unlike many other pathogenic mtDNA mutations, m.3243A>G heteroplasmy levels are poorly predictive…”; checked 2026-10-11 |
| m.3250T>C | Variant of uncertain significance | basis: Hu et al., 2026, PDF p. 3: “the variant is annotated as a variant of uncertain significance (VUS) due to incomplete penetrance and likely benign in silico predictions” (the authors cite MITOMAP and Wong et al.); checked 2026-10-11 |
| m.3697G>A | Pathogenic variant | inferred from Ji et al., 2021, PDF p. 7: “the inheritance of different pathogenic mtDNA mutations… previous analysis has not included m.3697G>A”; checked 2026-10-11 |
| m.8344A>G | Pathogenic variant | as in the supplementary workbook; no explicit statement found in the primary publication (checked 2026-10-11) |
| m.8993T>C | Pathogenic variant | basis: Hyslop et al., 2025, Table 1, Maternal Pathogenic Variant Heteroplasmy in Infants; PGT; checked 2026-10-11 |
| m.8993T>G | Pathogenic variant | inferred from Spath et al., 2021, PDF p. 9: “clear differences in transmission patterns between pathogenic mutations… m.8993T>G…”; checked 2026-10-11 |
What to report about a PGT-mt cycle
- Patient and cycle: age; family history; heteroplasmy levels in the carrier’s tissues; oocyte count; fertilisation and embryo development.
- Embryo: biopsy day; morphology; PGT-A result; heteroplasmy in every sample — for every embryo, including those not transferred.
- Analytical method: whole-genome amplification; target region; linearity; limit of detection; controls.
- Outcome: prenatal diagnosis and postnatal follow-up with tissue, age and method.