In The Case of Neurexin 1b, These Represent Point Mutations (Very Small DNA Rearrangements).

James F. Crow


Dolores Malaspina, M.D.
New Point Mutations in Humans Are Introduced Through The Male Line" This Has Been Known Since the 1950s", "What is Intriguing is why society chooses
to ignore this"

But now it is becoming increasingly clear that the biological clock ticks
for men as well as women, as researchers turn up evidence that as would-be
fathers get older, they have an increased chance of passing on genetic
defects to their children.

"New point mutations in humans are introduced through the male line," says
Dolores Malaspina, MD, professor of clinical psychiatry at Columbia
University and the New York State Psychiatric Institute. Furthermore, she
adds, the number of mutations in sperm increases as men age.

"This has been known since the 50s," said Malaspina. "What is intriguing is
why society chooses to ignore this." Society is starting to pay attention. With many couples now deferring
childbearing until they are older, the issue of paternal age and increased
risk for birth defects is gaining a higher profile. It is also possible, say
some experts, that if current trends of older fatherhood continue, it could
someday become a public health problem as well as a personal one.


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Paternal aging is associated with premeiotic damage to spermatogonia, a mechanism by which new point mutations are introduced into the gene pool.

1: Epidemiology. 2002 Nov;13(6):660-7. Links
Paternal age and preeclampsia.Harlap S, Paltiel O, Deutsch L, Knaanie A, Masalha S, Tiram E, Caplan LS, Malaspina D, Friedlander Y.
Department of Obstetrics and Gynecology and Kaplan Cancer Center, New York University School of Medicine, New York, NY 10016, USA. harlas01@popmail.med.nyu.edu

BACKGROUND: Paternal aging is associated with premeiotic damage to spermatogonia, a mechanism by which new point mutations are introduced into the gene pool. We hypothesized that paternal age might contribute to preeclampsia. METHODS: We studied the incidence of preeclampsia in 81,213 deliveries surveyed in 1964-1976 in the Jerusalem Perinatal Study. We controlled for maternal age, parity and other risk factors using logistic regression. RESULTS: Preeclampsia was reported in 1303 deliveries (1.6%). Compared with fathers age 25-34 years, the odds ratios (ORs) for preeclampsia were 1.24 (95% confidence interval = 1.05-1.46) for age 35-44 and 1.80 (1.40-2.31) for age 45+. For fathers age <25, the OR was 1.25 (1.04-1.51). Although weaker than maternal age effects, paternal effects were consistent within subgroups of other variables. CONCLUSIONS: These findings support the hypothesis that a modest proportion of preeclampsia might be explained by new mutations acquired from fathers and add to a growing body of evidence for paternal age effects in birth defects, neuropsychiatric disease and neoplasia.








Acta Genet Stat Med. 1953;4(2-3):133-9. Related Articles, Links


Schizophrenia as a gene mutation.

BOOK JA.





1: Am J Hum Genet. 1998 Sep;63(3):711-6. Related Articles, Links


Mutations in fibroblast growth-factor receptor 3 in sporadic cases of achondroplasia occur exclusively on the paternally derived chromosome.


"For example, the presence of large, extensive germline mosaicism probably implies that a germ-line mutation event had occurred early (perhaps when the father was in the embryonic stage" CJ
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Nat Rev Genet. 2000 Oct;1(1):40-7. Links
The origins, patterns and implications of human spontaneous mutation.Crow JF.
Genetics Department, University of Wisconsin, Madison, Wisconsin 53706, USA. jfcrow@facstaff.wisc.eduThgermline mutation rate in human males, especially older males, is generally much higher than in females, mainly because in males there are many more germ-cell divisions. However, there are some exceptions and many variations. Base substitutions, insertion-deletions, repeat expansions and chromosomal changes each follow different rules. Evidence from evolutionary sequence data indicates that the overall rate of deleterious mutation may be high enough to have a large effect on human well-being. But there are ways in which the impact of deleterious mutations can be mitigated.


James F. Crow


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Genetic mechanism
Several genetic mechanisms might explain the relationship between paternal age and the risk for schizophrenia (see Malaspina, 2001). It could be due to de novo point mutations arising in one or several schizophrenia susceptibility loci. Paternal age is known to be the principal source of new mutations in mammals, likely explained by the constant cell replication cycles that occur in spermatogenesis (James Crow, 2000). Following puberty, spermatogonia undergo some 23 divisions per year. At ages 20 and 40, a man's germ cell precursors will have undergone about 200 and 660 such divisions, respectively. During a man's life, the spermatogonia are vulnerable to DNA damage, and mutations may accumulate in clones of spermatogonia as men age. In contrast, the numbers of such divisions in female germ cells is usually 24, all but the last occurring during fetal life.
Trinucleotide repeat expansions could also underlie the paternal age effect. Repeat expansions have been demonstrated in several neuropsychiatric disorders, including myotonic dystrophy, fragile X syndrome, spinocerebellar ataxias, and Huntington disease. The sex of the transmitting parent is frequently a major factor influencing anticipation, with many disorders showing greater trinucleotide repeat expansion with paternal inheritance (Lindblad and Schalling, 1999; Schols et al., 2004; Duyao et al., 1993). Larger numbers of repeat expansions could be related to chance molecular events during the many cell divisions that occur during spermatogenesis.

Later paternal age might confer a risk for schizophrenia if it was associated with errors in the "imprinting" patterns of paternally inherited alleles. Imprinting is a form of gene regulation in which gene expression in the offspring depends on whether the allele was inherited from the male or female parent. Imprinted genes that are only expressed if paternally inherited alleles are reciprocally silenced at the maternal allele, and vice versa. Imprinting occurs during gametogenesis after the methylation patterns from the previous generation are "erased" and new parent of origin specific methylation patterns are established. Errors in erasure or reestablishment of these imprint patterns may lead to defective gene expression profiles in the offspring. The enzymes responsible for methylating DNA are the DNA methyltransferases, or DNMTs. These enzymes methylate cytosine residues in CpG dinucleotides, usually in the promoter region of genes, typically to reduce the expression of the mRNA. The methylation may become inefficient for a variety of reasons; one possibility is reduced DNA methylation activity in spermatogenesis, since DNMT levels diminish as paternal age increases (Benoit and Trasler, 1994; La Salle et al., 2004). Another possible mechanism is that this declining DNMT activity could be epigenetically transmitted to the offspring of older fathers. There are a number of different DNMTs that differ in whether they initiate or sustain methylation, and which are active at different ages and in different tissues.

Human imprinted genes have a critical role in the growth of the placenta, fetus, and central nervous system, in behavioral development, and in adult body size. It is an appealing hypothesis that loss of normal imprinting of genes critical to neurodevelopment may play a role in schizophrenia. Indeed, one of the most consistently identified molecular abnormalities in schizophrenia has been theorized to result from abnormal epigenetic mechanisms (Veldic et al., 2004), that is, the reduced GABA and reelin expression in prefrontal GABAergic interneurons. An overexpression of DNMT in these GABAergic interneurons, hypermethylating the reelin and GAD67 promoter regions, might be responsible for reducing their mRNA transcripts and expression levels. These decrements could functionally impair the role of GABAergic interneurons in regulating the activity and firing of pyramidal neurons, thereby causing cognitive dysfunction. Later paternal age could be related to the abnormal regulation or expression of DNMT activity in specific cells.