Phaeochromocytoma and paraganglioma are highly heritable tumours; half of those associated with a germline mutation are caused by mutations in genes for Krebs’s cycle enzymes, including succinate dehydrogenase (SDH). Inheritance of SDH alleles is assumed to be Mendelian (probability of 50% from each parent). The departure from transmission of parental alleles in a ratio of 1:1 is termed transmission ratio distortion (TRD). We sought to assess whether TRD occurs in the transmission of SDHB pathogenic variants (PVs). This study was conducted with 41 families of a discovery cohort from Royal North Shore Hospital, Australia, and 41 families from a validation cohort from St Bartholomew’s Hospital, United Kingdom (UK). Inclusion criteria were a clinically diagnosed SDHB PV and a pedigree available for at least two generations. TRD was assessed in 575 participants with the exact binomial test. The transmission ratio for SDHB PV was 0.59 (p=0.005) in the discovery cohort, 0.67 (p<0.001) in the validation cohort and 0.63 (p<0.001) in the combined cohort. No parent-of-origin effect was observed. TRD remained significant after adjusting for potential confounders: 0.67 (p<0.001) excluding families with incomplete family size data; 0.58 (p<0.001) when probands were excluded. TRD was also evident for SDHD PVs in a cohort of 81 patients from 13 families from the UK. The reason for TRD of SDHB and SDHD PVs is unknown but we hypothesize a survival advantage selected during early embryogenesis. The existence of TRD for SDHB and SDHD has implications for reproductive counselling, and further research into the heterozygote state.
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