By Leslie Feldman
The average age of fatherhood is increasing in the US and in Western Europe. Some research shows that offspring of older fathers are at increased risk for diseases and conditions (Bray et al., 2006). Some experts predict an upswing in cases of schizophrenia will accompany the increasing average paternal age. “The actual percentage of cases with paternal germ line-derived schizophrenia in a given population will depend on the demographics of paternal childbearing age, among other factors. With an upswing in paternal age, these cases would be expected to become more prevalent” (Malaspina et al., 2006). Approximately 25-33% of all cases of schizophrenia may be due to the father’s age at conception, according to Malaspina (2006). Malaspina sees a connection between advancing paternal age and neural functioning difficulties in people with autism and with schizophrenia. According to Tarin et al. (1998), there are well over 30 known conditions that the offspring of older fathers are more at risk for (see chart on paternal aging in the linked article).
The diagnosis of autism is increasing in the US and elsewhere (Centers for Disease Control, 2006). In a population study of 1990 through 1999, a total of 669,995 children, Atladóttir and colleagues (2007) reported increased diagnosese of autism, Torrette Syndrome, and hyperkinetic disorder. Is there a connection between increased cases of disorders such as autism and increased average paternal age? Psychiatrist Michael Craig Miller (2006), editor of the Harvard Mental Health Letter is convinced there is. Although a connection between the two would be corelational (not causal), the relationship encourages examination of the possibility that something related to paternal age (e.g. mutations in gametes) may contribute to the occurrence of autism. If there is a potential causal relationship, the new study by the Centers for Autism and Developmental Disabilities Research and Epidemiology (CADDRE) Network would provide a valuable opportunity to test the hypothesis.
Observations of a connection between advanced paternal age and difficulties for offspring go way back. Earlier research looking for a link between maternal age and autism also found the average paternal age (34) was much higher than the average age in the general population (Gillberg, 1980). Geneticist James F. Crow (1997) cites Wilhelm Weinberg (1862-1937) as noticing, during his 42 years of medical practice and helping 3,500 births, that the mutation rate might be a function of paternal age. Crow said, the evidence suggested that the greatest mutational health hazard in the population is fertile old men.
A study by Reichenberg et al. (2006) found a strong connection between cases of autism and advancing paternal age. Reichenberg and colleagues, who found more autism as paternal age increased, also found that the ratio of girls to boys in this cohort was 1:1, suggesting that this was a special subset of autism, maybe de novo rather than familial autism.
What might be the mechanism that produces higher rates of disorders among children of older fathers? The DNA in a 20 year-old male has been copied approximately100 times but in a 50 year-old father it has been copied over 800 times. Singh and colleagues (2003) studied differences in the sperm of older and younger men. Men over age 35 have sperm with lower motility and more highly damaged DNA in the form of double-strand breaks. The older group also had fewer apoptotic cells, an important discovery. (Apoptosis is form of cell death that protects the parent organism from problems or that permits differentiation, as in resorption of a tadpole’s tail.) A really key factor that differentiates sperm from other cells in the body is that they do not repair their DNA damage, as most other cells do. As a result, the only way to avoid passing DNA damage to a child is for the damaged cells to undergo apoptosis, a process that the study indicates declines with age. Singh is quoted in Science Blog (Sullivan, 2002) as explaining that, “In older men, the sperm are accumulating more damage, and those severely damaged sperm are not being eliminated.”