The impact of paternal age on new mutations and disease in the next generation

Advanced paternal age is associated with an increased risk of fathering children with genetic disorders and other adverse reproductive consequences. However, the mechanisms underlying this phenomenon remain largely unexplored. In this review, we focus on the impact of paternal age on de novo mutations that are an important contributor to genetic disease and can be studied both indirectly through large-scale sequencing studies and directly in the tissue in which they predominantly arise—the aging testis. We discuss the recent data that have helped establish the origins and frequency of de novo mutations, and highlight experimental evidence about the close link between new mutations, parental age, and genetic disease. We then focus on a small group of rare genetic conditions, the so-called “paternal age effect” disorders that show a strong association between paternal age and disease prevalence, and discuss the underlying mechanism (“selfish selection”) and implications of this process in more detail. More broadly, understanding the causes and consequences of paternal age on genetic risk has important implications both for individual couples and for public health advice given that the average age of fatherhood is steadily increasing in many developed nations. Advanced paternal age is associated with an increased risk of fathering children with genetic disorders and other adverse reproductive consequences. However, the mechanisms underlying this phenomenon remain largely unexplored. In this review, we focus on the impact of paternal age on de novo mutations that are an important contributor to genetic disease and can be studied both indirectly through large-scale sequencing studies and directly in the tissue in which they predominantly arise—the aging testis. We discuss the recent data that have helped establish the origins and frequency of de novo mutations, and highlight experimental evidence about the close link between new mutations, parental age, and genetic disease. We then focus on a small group of rare genetic conditions, the so-called “paternal age effect” disorders that show a strong association between paternal age and disease prevalence, and discuss the underlying mechanism (“selfish selection”) and implications of this process in more detail. More broadly, understanding the causes and consequences of paternal age on genetic risk has important implications both for individual couples and for public health advice given that the average age of fatherhood is steadily increasing in many developed nations. DIALOG: You can discuss this article with its authors and other readers at https://www.fertstertdialog.com/posts/35819 It has long been known that older parents have a high risk of having children with genetic disorders. The link between advanced maternal age and congenital abnormalities, particularly those associated with chromosomal aneuploidies, in offspring has received considerable attention, e.g., the strong association between maternal age effect and Down syndrome (trisomy 21) prevalence (1Cuckle H. Morris J. Maternal age in the epidemiology of common autosomal trisomies.Prenat Diagn. 2021; 41: 573-583Crossref PubMed Scopus (5) Google Scholar). However, there is a growing body of evidence indicating that independent of the maternal age, elevated paternal age is associated with difficulties conceiving, complications in pregnancy, an increased susceptibility of offspring to a wide range of conditions including spontaneous dominant disorders, congenital abnormalities, neurodevelopmental conditions, and various malignancies (2Risch N. Reich E.W. Wishnick M.M. McCarthy J.G. Spontaneous mutation and parental age in humans.Am J Hum Genet. 1987; 41: 218-248PubMed Google Scholar, 3Malaspina D. Harlap S. Fennig S. Heiman D. Nahon D. Feldman D. et al.Advancing paternal age and the risk of schizophrenia.Arch Gen Psychiatry. 2001; 58: 361-367Crossref PubMed Google Scholar, 4Murray L. McCarron P. Bailie K. Middleton R. Davey Smith G. Dempsey S. et al.Association of early life factors and acute lymphoblastic leukaemia in childhood: historical cohort study.Br J Cancer. 2002; 86: 356-361Crossref PubMed Scopus (0) Google Scholar, 5Choi J.Y. Lee K.M. Park S.K. Noh D.Y. Ahn S.H. Yoo K.Y. et al.Association of paternal age at birth and the risk of breast cancer in offspring: a case control study.BMC Cancer. 2005; 5: 143Crossref PubMed Scopus (72) Google Scholar, 6Yip B.H. Pawitan Y. Czene K. Parental age and risk of childhood cancers: a population-based cohort study from Sweden.Int J Epidemiol. 2006; 35: 1495-1503Crossref PubMed Scopus (127) Google Scholar, 7Green R.F. Devine O. Crider K.S. Olney R.S. Archer N. Olshan A.F. et al.Association of paternal age and risk for major congenital anomalies from the National Birth Defects Prevention Study, 1997 to 2004.Ann Epidemiol. 2010; 20: 241-249Crossref PubMed Scopus (94) Google Scholar, 8Lan K.C. Chiang H.J. Huang T.L. Chiou Y.-J. Hsu T.Y. Ou Y.C. et al.Association between paternal age and risk of schizophrenia: a nationwide population-based study.J Assist Reprod Genet. 2021; 38: 85-93Crossref PubMed Scopus (5) Google Scholar, 9Bray I. Gunnell D. Davey Smith G. Advanced paternal age: how old is too old?.J Epidemiol Community Health. 2006; 60: 851-853Crossref PubMed Scopus (164) Google Scholar, 10Toriello H.V. Meck J.M. Professional Practice and Guidelines CommitteeStatement on guidance for genetic counseling in advanced paternal age.Genet Med. 2008; 10: 457-460Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, 11Kovac J.R. Addai J. Smith R.P. Coward R.M. Lamb D.J. Lipshultz L.I. The effects of advanced paternal age on fertility.Asian J Androl. 2013; 15: 723-728Crossref PubMed Scopus (71) Google Scholar, 12Janecka M. Mill J. Basson M.A. Goriely A. Spiers H. Reichenberg A. et al.Advanced paternal age effects in neurodevelopmental disorders—review of potential underlying mechanisms.Transl Psychiatry. 2017; 7e1019Crossref Scopus (71) Google Scholar). The American Society for Reproductive Medicine, the British Andrology Society, and the Canadian Fertility and Andrology Society have advised that the upper limit for sperm donors for assisted conception should be 40 years old as a precautionary measure “so that the potential hazards related to aging are diminished” on the basis of increased risk of genetic abnormalities in children (13Guidelines for sperm donation.Fertil Steril. 2004; 82: 9-12Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar, 14British Andrology SocietyBritish Andrology Society guidelines for the screening of semen donors for donor insemination (1999).Hum Reprod. 1999; 14: 1823-1826Crossref PubMed Scopus (0) Google Scholar, 15Havelock J. Liu K. Levitan S. Petropanagos A. Khan L. Guidelines for the third-party reproduction.in: Canadian Fertility & Andrology Society Clinical Practice Guidelines. 2016https://cfas.ca/_Library/clinical_practice_guidelines/Third-Party-Procreation-AMENDED-.pdfGoogle Scholar). In many developed countries, the average age of fatherhood has been steadily increasing, despite considerable demographic variations. In the United Kingdom, e.g., the standardized mean age of fathers in 2020 was 33.7 years, the highest since data collection began and an increase from 29.7 years in 1970, whereas over the same period mean maternal age rose from 26.7 to 30.7 years (16Office for National StatisticsBirth characteristics in England and Wales [Internet].https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/livebirths/bulletins/birthcharacteristicsinenglandandwales/2020Date accessed: April 12, 2022Google Scholar). A similar picture is apparent in the United States, with one study indicating that mean paternal age has risen from 27.4 years in 1972 to 30.9 years in 2015 (with variation attributed to ethnicity or race, geographic location, and education level), with 8.9% and 0.9% of fathers over the age of 40 and 50, respectively (17Khandwala Y.S. Zhang C.A. Lu Y. Eisenberg M.L. The age of fathers in the USA is rising: an analysis of 168 867 480 births from 1972 to 2015.Hum Reprod. 2017; 32: 2110-2116Crossref PubMed Scopus (123) Google Scholar). Given this trend prevalent across the developed world, there is an ever-increasing need to evaluate the impact, and understand the causes and consequences, of advanced paternal age on genetic risk for both individual couples and public health advice. Moreover, this information is also crucial given the popularity of assisted reproductive technologies that offer couples the option to reproduce later in life, to provide accurate risks regarding delayed parenthood (18Chan P.T.K. Robaire B. Advanced paternal age and future generations.Front Endocrinol. 2022; 13897101Crossref Scopus (0) Google Scholar). Although epidemiological studies have shown a convincing correlation between paternal age and disease risk, in many cases, this association is not well defined and (in some cases) not always reproducible (8Lan K.C. Chiang H.J. Huang T.L. Chiou Y.-J. Hsu T.Y. Ou Y.C. et al.Association between paternal age and risk of schizophrenia: a nationwide population-based study.J Assist Reprod Genet. 2021; 38: 85-93Crossref PubMed Scopus (5) Google Scholar, 19Chen X.K. Wen S.W. Krewski D. Fleming N. Yang Q. Walker M.C. Paternal age and adverse birth outcomes: teenager or 40+, who is at risk?.Hum Reprod. 2008; 23: 1290-1296Crossref PubMed Scopus (74) Google Scholar, 20Petersen L. Mortensen P.B. Pedersen C.B. Paternal age at birth of first child and risk of schizophrenia.Am J Psychiatry. 2011; 168: 82-88Crossref PubMed Scopus (103) Google Scholar, 21Thompson J.A. The effects of parent ages on birth defects.Acta Sci Paediatr. 2020; 3: 58-69Crossref PubMed Google Scholar). Often, the exact threshold of what consists of an “advanced” paternal age is also poorly defined and varies from study to study (9Bray I. Gunnell D. Davey Smith G. Advanced paternal age: how old is too old?.J Epidemiol Community Health. 2006; 60: 851-853Crossref PubMed Scopus (164) Google Scholar). Additional factors can further cloud our interpretation of population-based studies, including the fact that maternal and paternal ages are often highly correlated with little variability between the age of the mother and the father, so unpicking the impact of one from the other in terms of disease association can be challenging. Importantly, correlations do not provide direct evidence for causality, and the mechanisms underlying the effect of advanced paternal age on disease remain uncertain and are likely to be moderated by a complex interaction of factors (12Janecka M. Mill J. Basson M.A. Goriely A. Spiers H. Reichenberg A. et al.Advanced paternal age effects in neurodevelopmental disorders—review of potential underlying mechanisms.Transl Psychiatry. 2017; 7e1019Crossref Scopus (71) Google Scholar). Over the last decade, thanks to the advances and falling costs of next generation sequencing technology, it has become possible to interrogate and further dissect the components mediating the effect of paternal age on disease risk. Here, we focus largely on de novo point mutations (DNMs), DNA sequence variations that are “new” in offspring and are not apparently present in either parent. In this review, we examine how, why, where, and how often new mutations are introduced in our genomes, and the link between DNMs, disease, and other negative reproductive outcomes. We then discuss the unusual properties of a small group of specific genetic disorders (“paternal age effect” (PAE) disorders) that have provided a paradigm to study DNMs directly in their tissue of origin and led to the discovery of the process of “selfish selection.” These DNMs are important contributors to human disease, and understanding their origins and the factors that influence their occurrence, such as advanced paternal age, have important implications for public health, assisted reproductive technology treatments, complex disease, and the evolution of our genome. The rate at which new mutations arise is crucial to our understanding of both genetic disease and genome biology. Much insight into the biology of DNMs has been gained from large-scale implementation of whole-genome sequencing (WGS) or whole-exome sequencing (WES) of mother-father-child family trios—sequencing of coding portions (WES) or the whole genomes (WGS) of a child and both biologic parents (Fig. 1A). Such studies have convincingly shown that the number of new point mutations present in a new born is on an average 60 (approximately 30–90, depending on parental age at conception), placing the average human germline mutation rate at approximately 1.2 × 10-8 per nucleotide per generation (22Kong A. Frigge M.L. Masson G. Besenbacher S. Sulem P. Magnusson G. et al.Rate of de novo mutations and the importance of father’s age to disease risk.Nature. 2012; 488: 471-475Crossref PubMed Scopus (1320) Google Scholar, 23Jónsson H. Sulem P. Kehr B. Kristmundsdottir S. Zink F. Hjartarson E. et al.Parental influence on human germline de novo mutations in 1,548 trios from Iceland.Nature. 2017; 549: 519-522Crossref PubMed Scopus (209) Google Scholar, 24Rahbari R. Wuster A. Lindsay S.J. Hardwick R.J. Alexandrov L.B. Al Turki S. et al.Timing, rates and spectra of human germline mutation.Nat Genet. 2016; 48: 126-133Crossref PubMed Scopus (318) Google Scholar, 25Goldmann J.M. Wong W.S.W. Pinelli M. Farrah T. Bodian D. Stittrich A.B. et al.Parent-of-origin-specific signatures of de novo mutations.Nat Genet. 2016; 48: 935-939Crossref PubMed Scopus (160) Google Scholar, 26Sasani T.A. Pedersen B.S. Gao Z. Baird L. Przeworski M. Jorde L.B. et al.Large, three-generation human families reveal post-zygotic mosaicism and variability in germline mutation accumulation.eLife. 2019; 8e46922Crossref PubMed Scopus (47) Google Scholar, 27Kaplanis J. Ide B. Sanghvi R. Neville M. Danecek P. Coorens T. et al.Genetic and chemotherapeutic influences on germline hypermutation.Nature. 2022; 605: 503-508Crossref PubMed Scopus (2) Google Scholar). Overall, the number of DNMs increases steadily and relatively monotonically with parental age. It is also possible to determine the parental origin of DNMs using a haplotype phasing strategy. This can be performed directly using the WGS data from the family trio when an informative heterozygous single nucleotide polymorphism (SNP) is present in the vicinity of the DNM that allows the maternally and paternally-derived alleles in the child to be distinguished (Fig. 1B) (28Goldmann J.M. Veltman J.A. Gilissen C. De Novo mutations reflect development and aging of the human germline.Trends Genet. 2019; 35: 828-839Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 29Bernkopf M, Abdullah UB, Bush SJ, Wood K, Ghaffari S, Giannoulatou E, et al. The PREGCARE study: personalized recurrence risk assessment following the birth of a child with a pathogenic de novo mutation [pre-print] 2022. Available at: https://www.biorxiv.org/content/10.1101/2022.07.26.501520v1.Google Scholar). Such phasing methods have shown that approximately 80% of all DNMs are present on the paternally-derived allele, and the number of DNMs in a child is predominantly influenced by the age of the father at conception (Fig. 1C) (23Jónsson H. Sulem P. Kehr B. Kristmundsdottir S. Zink F. Hjartarson E. et al.Parental influence on human germline de novo mutations in 1,548 trios from Iceland.Nature. 2017; 549: 519-522Crossref PubMed Scopus (209) Google Scholar, 24Rahbari R. Wuster A. Lindsay S.J. Hardwick R.J. Alexandrov L.B. Al Turki S. et al.Timing, rates and spectra of human germline mutation.Nat Genet. 2016; 48: 126-133Crossref PubMed Scopus (318) Google Scholar, 25Goldmann J.M. Wong W.S.W. Pinelli M. Farrah T. Bodian D. Stittrich A.B. et al.Parent-of-origin-specific signatures of de novo mutations.Nat Genet. 2016; 48: 935-939Crossref PubMed Scopus (160) Google Scholar, 28Goldmann J.M. Veltman J.A. Gilissen C. De Novo mutations reflect development and aging of the human germline.Trends Genet. 2019; 35: 828-839Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). On average, approximately 1–2 additional DNMs arise in the genome of a child per additional year in the age of the father (22Kong A. Frigge M.L. Masson G. Besenbacher S. Sulem P. Magnusson G. et al.Rate of de novo mutations and the importance of father’s age to disease risk.Nature. 2012; 488: 471-475Crossref PubMed Scopus (1320) Google Scholar, 23Jónsson H. Sulem P. Kehr B. Kristmundsdottir S. Zink F. Hjartarson E. et al.Parental influence on human germline de novo mutations in 1,548 trios from Iceland.Nature. 2017; 549: 519-522Crossref PubMed Scopus (209) Google Scholar, 24Rahbari R. Wuster A. Lindsay S.J. Hardwick R.J. Alexandrov L.B. Al Turki S. et al.Timing, rates and spectra of human germline mutation.Nat Genet. 2016; 48: 126-133Crossref PubMed Scopus (318) Google Scholar, 30Acuna-Hidalgo R. Veltman J.A. Hoischen A. New insights into the generation and role of de novo mutations in health and disease.Genome Biol. 2016; 17: 241Crossref PubMed Scopus (222) Google Scholar). Juxtaposed to this, a smaller (but significant) maternal age effect has been reported (23Jónsson H. Sulem P. Kehr B. Kristmundsdottir S. Zink F. Hjartarson E. et al.Parental influence on human germline de novo mutations in 1,548 trios from Iceland.Nature. 2017; 549: 519-522Crossref PubMed Scopus (209) Google Scholar, 25Goldmann J.M. Wong W.S.W. Pinelli M. Farrah T. Bodian D. Stittrich A.B. et al.Parent-of-origin-specific signatures of de novo mutations.Nat Genet. 2016; 48: 935-939Crossref PubMed Scopus (160) Google Scholar, 26Sasani T.A. Pedersen B.S. Gao Z. Baird L. Przeworski M. Jorde L.B. et al.Large, three-generation human families reveal post-zygotic mosaicism and variability in germline mutation accumulation.eLife. 2019; 8e46922Crossref PubMed Scopus (47) Google Scholar, 31Wong W.S.W. Solomon B.D. Bodian D.L. Kothiyal P. Eley G. Huddleston K.C. et al.New observations on maternal age effect on germline de novo mutations.Nat Commun. 2016; 710486Crossref Scopus (103) Google Scholar) (Table 1). Aside from DNM datasets derived from family trios, WGS studies of individual multi-sibling families with large age differences between the first and last child have again shown that the predominant factor determining the number of DNMs in a child is the paternal age, and overall only a modest variability in familial mutation rates has been reported (24Rahbari R. Wuster A. Lindsay S.J. Hardwick R.J. Alexandrov L.B. Al Turki S. et al.Timing, rates and spectra of human germline mutation.Nat Genet. 2016; 48: 126-133Crossref PubMed Scopus (318) Google Scholar, 26Sasani T.A. Pedersen B.S. Gao Z. Baird L. Przeworski M. Jorde L.B. et al.Large, three-generation human families reveal post-zygotic mosaicism and variability in germline mutation accumulation.eLife. 2019; 8e46922Crossref PubMed Scopus (47) Google Scholar, 32Jónsson H. Sulem P. Arnadottir G.A. Pálsson G. Eggertsson H.P. Kristmundsdottir S. et al.Multiple transmissions of de novo mutations in families.Nat Genet. 2018; 50: 1674-1680Crossref PubMed Scopus (55) Google Scholar, 33Goldmann J.M. Hampstead J.E. Wong W.S.W. Wilfert A.B. Turner T.N. Jonker M.A. et al.Differences in the number of de novo mutations between individuals are due to small family-specific effects and stochasticity.Genome Res. 2021; 31: 1513-1518Crossref PubMed Scopus (0) Google Scholar).Table 1Characteristics and effects of parental age on prevalence of different classes of DNMs.Origin of mutational eventCharacteristicsRate of increase with parent’s ageNo. of DNMs in child (with 20–25-year-old parent)No. of DNMs in child (with 40–45-year-old parent)DNM occurring in the paternal germlineMost of the DNMs (∼80%) across the genome are on the paternally-derived allele.Most paternal DNMs have a specific mutational pattern or signature, characteristic of processes involving stem cell cycling, and induction of copy errors (23Jónsson H. Sulem P. Kehr B. Kristmundsdottir S. Zink F. Hjartarson E. et al.Parental influence on human germline de novo mutations in 1,548 trios from Iceland.Nature. 2017; 549: 519-522Crossref PubMed Scopus (209) Google Scholar, 24Rahbari R. Wuster A. Lindsay S.J. Hardwick R.J. Alexandrov L.B. Al Turki S. et al.Timing, rates and spectra of human germline mutation.Nat Genet. 2016; 48: 126-133Crossref PubMed Scopus (318) Google Scholar, 25Goldmann J.M. Wong W.S.W. Pinelli M. Farrah T. Bodian D. Stittrich A.B. et al.Parent-of-origin-specific signatures of de novo mutations.Nat Genet. 2016; 48: 935-939Crossref PubMed Scopus (160) Google Scholar, 26Sasani T.A. Pedersen B.S. Gao Z. Baird L. Przeworski M. Jorde L.B. et al.Large, three-generation human families reveal post-zygotic mosaicism and variability in germline mutation accumulation.eLife. 2019; 8e46922Crossref PubMed Scopus (47) Google Scholar, 27Kaplanis J. Ide B. Sanghvi R. Neville M. Danecek P. Coorens T. et al.Genetic and chemotherapeutic influences on germline hypermutation.Nature. 2022; 605: 503-508Crossref PubMed Scopus (2) Google Scholar)1–2 DNMs per year of paternal ageApproximately 35 DNMsApproximately 70 DNMsDNM occurring in the maternal germlineApproximately 20% of DNMs are found on the maternally derived allele. These often exhibit a distinct mutational signatures and are found as “clustered DNMs” in specific regions of the genome—characteristic of accumulation of double-strand break-induced mutations throughout oocyte aging (23Jónsson H. Sulem P. Kehr B. Kristmundsdottir S. Zink F. Hjartarson E. et al.Parental influence on human germline de novo mutations in 1,548 trios from Iceland.Nature. 2017; 549: 519-522Crossref PubMed Scopus (209) Google Scholar, 25Goldmann J.M. Wong W.S.W. Pinelli M. Farrah T. Bodian D. Stittrich A.B. et al.Parent-of-origin-specific signatures of de novo mutations.Nat Genet. 2016; 48: 935-939Crossref PubMed Scopus (160) Google Scholar, 28Goldmann J.M. Veltman J.A. Gilissen C. De Novo mutations reflect development and aging of the human germline.Trends Genet. 2019; 35: 828-839Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 31Wong W.S.W. Solomon B.D. Bodian D.L. Kothiyal P. Eley G. Huddleston K.C. et al.New observations on maternal age effect on germline de novo mutations.Nat Commun. 2016; 710486Crossref Scopus (103) Google Scholar)Approximately 1 DNM every 4 y of maternal ageApproximately 5 DNMsApproximately 10 DNMsMosaic DNM: Mutational event occurring in the primordial germ cells of one of the parents, or in the offspring during early development (the first few mitotic divisions) of the fertilized embryoDNMs caused by germline mosaicism in either one of the 2 parents are associated with an increased recurrence risk in future pregnancies. (28Goldmann J.M. Veltman J.A. Gilissen C. De Novo mutations reflect development and aging of the human germline.Trends Genet. 2019; 35: 828-839Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 29Bernkopf M, Abdullah UB, Bush SJ, Wood K, Ghaffari S, Giannoulatou E, et al. The PREGCARE study: personalized recurrence risk assessment following the birth of a child with a pathogenic de novo mutation [pre-print] 2022. Available at: https://www.biorxiv.org/content/10.1101/2022.07.26.501520v1.Google Scholar, 30Acuna-Hidalgo R. Veltman J.A. Hoischen A. New insights into the generation and role of de novo mutations in health and disease.Genome Biol. 2016; 17: 241Crossref PubMed Scopus (222) Google Scholar)Mosaic DNMs occurring during offspring’s early development can be difficult to distinguish from constitutive (heterozygous) DNMs (26Sasani T.A. Pedersen B.S. Gao Z. Baird L. Przeworski M. Jorde L.B. et al.Large, three-generation human families reveal post-zygotic mosaicism and variability in germline mutation accumulation.eLife. 2019; 8e46922Crossref PubMed Scopus (47) Google Scholar, 30Acuna-Hidalgo R. Veltman J.A. Hoischen A. New insights into the generation and role of de novo mutations in health and disease.Genome Biol. 2016; 17: 241Crossref PubMed Scopus (222) Google Scholar).No change with parental ageNo bias in parental originApproximately 5–10 DNMsApproximately 5–10 DNMsSelfish DNM: causing paternal age effect disorderSmall subset of recurrent functional/pathogenic DNMs in genes clustering in specific spermatogonial stem cell pathways, which present with a high apparent mutation rate.Selfish DNMs are exclusively found on the paternally-derived allele. Associated with increased paternal age (56Goriely A. Wilkie A.O.M. Paternal age effect mutations and selfish spermatogonial selection: causes and consequences for human disease.Am J Hum Genet. 2012; 90: 175-200Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar).Strong correlation between disease prevalence and paternal age: fathers are significantly older than population average (2Risch N. Reich E.W. Wishnick M.M. McCarthy J.G. Spontaneous mutation and parental age in humans.Am J Hum Genet. 1987; 41: 218-248PubMed Google Scholar, 56Goriely A. Wilkie A.O.M. Paternal age effect mutations and selfish spermatogonial selection: causes and consequences for human disease.Am J Hum Genet. 2012; 90: 175-200Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar).Exponential increase of the mutation levels in sperm with age of the donor (56Goriely A. Wilkie A.O.M. Paternal age effect mutations and selfish spermatogonial selection: causes and consequences for human disease.Am J Hum Genet. 2012; 90: 175-200Abstract Full Text Full Text PDF PubMed Scopus (225) Google Scholar)DNA mutational events are rare, but once they occur, they lead to clonal expansion within the testis over time, because they encode functional mutant proteins with oncogenic-like properties, i.e., the relative increase with age depends on the activity and the selective advantage conferred by the mutant protein.DNA = deoxyribonucleic acid; DNM = de novo mutation; No. = number. Open table in a new tab DNA = deoxyribonucleic acid; DNM = de novo mutation; No. = number. It has long been accepted that differences in the biology of the male and female germlines provide a compelling explanation as to why most (approximately 80%) of the DNMs are paternal in origin, in particular the number of germline cell divisions in the life history of a sperm compared with an egg (Fig. 1C) (Table 1) (34Crow J.F. The origins, patterns and implications of human spontaneous mutation.Nat Rev Genet. 2000; 1: 40-47Crossref PubMed Google Scholar). All cell divisions take place during early embryogenesis that are required to produce an oocyte. By contrast, spermatogonial stem cells (SSCs) within the seminiferous tubules of the testis divide continuously to sustain sperm production throughout a man’s reproductive life, and so the number of genome replications increases with age. It can be estimated that the sperm produced by a 25-year-old man would have undergone approximately 350 SSC replications compared with approximately 750 in a 45-year-old man (Fig. 1C) (34Crow J.F. The origins, patterns and implications of human spontaneous mutation.Nat Rev Genet. 2000; 1: 40-47Crossref PubMed Google Scholar). In addition to the number of mitotic divisions in the male germline resulting in incidental copying errors, other factors have been proposed that contribute to the age effect, including damage-associated mutations (particularly oxidative stress) during environmental exposures, age-related reduction in DNA repair and epigenetic reprogramming of germ cells (27Kaplanis J. Ide B. Sanghvi R. Neville M. Danecek P. Coorens T. et al.Genetic and chemotherapeutic influences on germline hypermutation.Nature. 2022; 605: 503-508Crossref PubMed Scopus (2) Google Scholar, 35Crow J.F. Age and sex effects on human mutation rates: an old problem with new complexities.J Radiat Res. 2006; 47: B75-B82Crossref PubMed Scopus (0) Google Scholar, 36Ségurel L. Wyman M.J. Przeworski M. Determinants of mutation rate variation in the human germline.Annu Rev Genomics Hum Genet. 2014; 15: 47-70Crossref PubMed Scopus (171) Google Scholar, 37Aitken R.J. De Iuliis G.N. Nixon B. The sins of our forefathers: paternal impacts on de novo mutation rate and development.Annu Rev Genet. 2020; 54: 1-24Crossref PubMed Scopus (14) Google Scholar). However, molecular evidence derived from large WGS data is consistent with SSC replications being the predominant factor influencing the parental bias in DNM origin and the PAE of DNMs. For example, large WGS mutation datasets have been used to derive “mutational signatures” (defined as specific DNA substitution patterns typically caused by distinct underlying mutational processes, such as DNA replication errors, DNA damage caused by ultraviolet exposure, or other exogenous/endogenous exposure, defective DNA repair pathways) (38Koh G. Degasperi A. Zou X. Momen S. Nik-Zainal S. Mutational signatures: emerging concepts, caveats and clinical applications.Nat Rev Cancer. 2021; 21: 619-637Crossref PubMed Scopus (39) Google Scholar). This approach shows that the most common signatures observed in DNMs are similar to those associated with spontaneous preneoplastic somatic mutations (i.e., “mutation signatures 1 and 5”) (24Rahbari R. Wuster A. Lindsay S.J. Hardwick R.J. Alexandrov L.B. Al Turki S. et al.Timing, rates and spectra of human germline mutation.Nat Genet. 2016; 48: 126-133Crossref PubMed Scopus (318) Google Scholar, 25Goldmann J.M. Wong W.S.W. Pinelli M. Farrah T. Bodian D. Stittrich A.B. et al.Parent-of-origin-specific signatures of de novo mutations.Nat Genet. 2016; 48: 935-939Crossref PubMed Scopus (160) Google Scholar, 26Sasani T.A. Pedersen B.S. Gao Z. Baird L. Przeworski M. Jorde L.B. et al.Large, three-generation human families reveal post-zygotic mosaicism and variability in germline mutation accumulation.eLife. 2019; 8e46922Crossref PubMed