Nondisjunction — A View from Ringside
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《新英格兰医药杂志》
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Chromosome nondisjunction lands a heavy blow on the chin of humanity. The improper segregation of chromosomes during meiosis leads to chromosomally unbalanced eggs or sperm. If these gametes participate in fertilization, the outcome is an aneuploid embryo, with either trisomy (one chromosome too many) or monosomy (one chromosome too few). Since most such embryos are inviable, one might expect that these errors would be extremely rare. This is true for most organisms, but our own species is a notable exception: aneuploidy is identified in at least 5 percent of all clinically recognized pregnancies, making it the leading known cause of fetal loss. Furthermore, even though only a small proportion of aneuploid fetuses survive to term (primarily those with trisomy 13, 18, or 21 and those with various sex-chromosome abnormalities), aneuploidy is still the leading genetic cause of mental impairment and developmental disabilities. This "one–two" punch of pregnancy loss and developmental impairment has placed nondisjunction at the center of an intensive research effort.
Despite the obvious clinical importance of nondisjunction, the predisposing genetic and environmental factors remain a mystery. However, it is clear that almost all cases involve errors in meiosis, the complex process in which one round of DNA replication is followed by two cellular divisions to generate haploid gametes. The first cell division (meiosis I) separates homologous chromosomes; the second (meiosis II) segregates the sister chromatids of each homologue. Nondisjunction can occur at either of these stages and can generally be distinguished with the use of polymorphic genetic markers at or near the centromere of the nondisjoined chromosomes (see Figure 1). If both copies of the nondisjoined chromosomes are heterozygous for alleles at these markers, it is likely that the error arose at meiosis I. In contrast, homozygosity at the centromere suggests an error at meiosis II.
Figure 1. Decoding Meiotic Nondisjunction.
Analysis of inheritance of polymorphic DNA markers can be used to determine the meiotic stage and parent of origin of aneuploidy. In this example, different alleles of a chromosome 21 centromeric locus are visualized as peaks in sequencing electrophoretograms. The trisomic offspring (+21) has inherited one paternal allele and two different maternal alleles; thus, the extra chromosome originated from an error at maternal meiosis I.
During the past decade, more than 1000 trisomic or monosomic conceptions have been studied to determine the parental origin and meiotic stage of the nondisjunction error.1 Since monosomy almost always results in the loss of the embryo at an early stage, most of the available data derive from cases of trisomy; not surprisingly, the largest data set involves trisomy 21, the condition that is responsible for Down's syndrome. Taken together, these studies indicate remarkable chromosome-to-chromosome variation in nondisjunction, but there is also one unifying and inescapable fact: regardless of the chromosome involved, most cases of human trisomy originate from errors in maternal meiosis I. Given the biology of the human egg, this is not entirely unexpected: the first stage of female meiosis is initiated in the fetal ovary and is followed by a long "arrest" phase that lasts until the time of ovulation. Thus, the first meiotic division is amazingly protracted, taking at least 10 to 15 years and as many as 45 to 50 years to complete.
Although the association between maternal age and trisomy has long been recognized, other predisposing factors have been elusive. However, studies conducted during the past few years have identified the first molecular correlate of nondisjunction, aberrant meiotic recombination. Recombination is familiar to most of us as the process that "shuffles" genetic material during the meiotic prophase, but it plays another equally crucial role: recombinational exchanges lock the homologues together in proper register and thereby facilitate proper segregation at meiosis I. In model organisms (e.g., flies and yeast), it has long been recognized that the absence or reduced numbers of exchanges increase the likelihood of nondisjunction.
With the advent of the Human Genome Project, it became possible to "map" recombination sites in normal and abnormal meioses and to ask whether this was the case for human trisomies as well. The results of these mapping studies have been remarkable: for all the trisomies studied to date (trisomies 15, 16, 18, 21, and sex-chromosome trisomies), significant alterations in recombination have been identified. These alterations take on one of two general forms (see Figure 2). First, for some conditions — especially meioses leading to trisomy 21 — exchanges simply fail to occur between the homologues. Thus, the two chromosomes are left to drift along independently, and if they drift to the same spindle pole (as they will half the time), an aneuploid gamete will ensue. Second, exchanges may occur, but at suboptimal locations. For example, trisomies 16 and 21 frequently result from exchanges that are located very distally on the chromosome, and trisomy 21 and sex-chromosome trisomies from exchanges that occur close to the centromere. Thus, it appears that to maximize the chances of successful segregation, exchanges should be medially placed on the chromosome. The reasons for this are not yet clear; possibly, distal crossovers are unable to lock homologues together, so that they move independently, whereas pericentromeric crossovers lock homologues too tightly, so that they are unable to separate from one another.
Figure 2. Optimal and Suboptimal Homologue-Crossover Configurations.
Crossovers facilitate proper segregation of chromosomes during meiosis. This example shows an optimal crossover configuration for a pair of homologues (A) and three suboptimal configurations: one in which crossovers failed to occur (B), one with a single, distally placed exchange (C), and one with a single, pericentomeric exchange (D).
Whatever the basis for this effect, the mapping studies make it clear that altered recombination is a major contributor to nondisjunction in humans. This finding raises an obvious question: How is altered recombination linked to the only other known risk factor for human aneuploidy, older maternal age? A simple explanation would be that meiotic recombination patterns change as a woman ages. However, this is almost certainly not the case: meiotic exchanges for oocytes are established during the fetal period, and there is little reason to think that a second round of recombination occurs in the adult ovary. Thus, the associations among age, recombination, and nondisjunction must be more complex.
We have suggested that nondisjunction may require two "hits."2 According to this model, the first hit occurs during the fetal development of the oocyte, with the chance occurrence of pericentromeric or very distal crossovers, or the absence of any crossovers, between the homologues. The second hit occurs many years later around the time of ovulation, when the oocyte resumes meiosis. Because of age-related perturbations in the meiotic machinery, homologues with susceptible crossover configurations are less likely to segregate correctly. In other words, the young ovary is able to process most susceptible exchange configurations correctly, but over time, the degradation of meiotic proteins leads to an increasing likelihood of nondisjunction involving homologues that have "at risk" crossover configurations.
This model suggests a number of predictions that are now being tested to determine whether human trisomies are indeed the result of the two well-timed blows or, rather, the consequence of multiple hits accumulated over the years. In addition, several laboratories are now trying to generate appropriate mouse models, either by knocking out recombination-related genes or by experimentally eliminating crossovers or "moving" them to suboptimal locations. Finally, recent advances in immunofluorescence methods now make it possible to directly view meiotic recombinational events as they are happening (see Figure 3), allowing us to characterize normal and abnormal exchange patterns in individual gametes. This combination of approaches may not win the day immediately. However, it should allow us to focus on something other than maternal age alone — arguably the most relentless opponent of successful human reproduction.
Figure 3. Visualizing Crossovers.
Recently developed immunofluorescence methods make it possible to examine sites of recombination directly in spermatocytes and oocytes. This example shows a human spermatocyte (obtained from a testicular biopsy) captured during the prophase of meiosis I. Antibodies against the protein SCP3 highlight the synaptonemal complex (red), the "glue" that tethers homologues during this phase of meiosis; CREST antiserum localizes to centromeric regions of the chromosomes (blue); and antibodies against the mismatch-repair protein MLH1 pinpoint the sites at which crossovers will develop (yellow). Thus, this approach makes it possible to count the number of crossovers on the short and long arms of individual chromosomes and throughout the cell as a whole. One obvious application of this technique is determining whether infertile men have different levels or locations of crossovers than men with normal spermatogenesis; such studies are now being actively pursued by a number of groups.
Source Information
From the Department of Human Genetics, Emory University, Atlanta (N.E.L.); and the Department of Genetics and Center for Human Genetics, Case Western Reserve University and University Hospitals of Cleveland, Cleveland (T.J.H.).
References
Hassold T, Hunt P. To err (meiotically) is human: the genesis of human aneuploidy. Nat Rev Genet 2001;2:280-291.
Lamb NE, Feingold E, Savage A, et al. Characterization of susceptible chiasma configurations that increase the risk for maternal nondisjunction of chromosome 21. Hum Mol Genet 1997;6:1391-1399.(Neil E. Lamb, Ph.D., and )
Chromosome nondisjunction lands a heavy blow on the chin of humanity. The improper segregation of chromosomes during meiosis leads to chromosomally unbalanced eggs or sperm. If these gametes participate in fertilization, the outcome is an aneuploid embryo, with either trisomy (one chromosome too many) or monosomy (one chromosome too few). Since most such embryos are inviable, one might expect that these errors would be extremely rare. This is true for most organisms, but our own species is a notable exception: aneuploidy is identified in at least 5 percent of all clinically recognized pregnancies, making it the leading known cause of fetal loss. Furthermore, even though only a small proportion of aneuploid fetuses survive to term (primarily those with trisomy 13, 18, or 21 and those with various sex-chromosome abnormalities), aneuploidy is still the leading genetic cause of mental impairment and developmental disabilities. This "one–two" punch of pregnancy loss and developmental impairment has placed nondisjunction at the center of an intensive research effort.
Despite the obvious clinical importance of nondisjunction, the predisposing genetic and environmental factors remain a mystery. However, it is clear that almost all cases involve errors in meiosis, the complex process in which one round of DNA replication is followed by two cellular divisions to generate haploid gametes. The first cell division (meiosis I) separates homologous chromosomes; the second (meiosis II) segregates the sister chromatids of each homologue. Nondisjunction can occur at either of these stages and can generally be distinguished with the use of polymorphic genetic markers at or near the centromere of the nondisjoined chromosomes (see Figure 1). If both copies of the nondisjoined chromosomes are heterozygous for alleles at these markers, it is likely that the error arose at meiosis I. In contrast, homozygosity at the centromere suggests an error at meiosis II.
Figure 1. Decoding Meiotic Nondisjunction.
Analysis of inheritance of polymorphic DNA markers can be used to determine the meiotic stage and parent of origin of aneuploidy. In this example, different alleles of a chromosome 21 centromeric locus are visualized as peaks in sequencing electrophoretograms. The trisomic offspring (+21) has inherited one paternal allele and two different maternal alleles; thus, the extra chromosome originated from an error at maternal meiosis I.
During the past decade, more than 1000 trisomic or monosomic conceptions have been studied to determine the parental origin and meiotic stage of the nondisjunction error.1 Since monosomy almost always results in the loss of the embryo at an early stage, most of the available data derive from cases of trisomy; not surprisingly, the largest data set involves trisomy 21, the condition that is responsible for Down's syndrome. Taken together, these studies indicate remarkable chromosome-to-chromosome variation in nondisjunction, but there is also one unifying and inescapable fact: regardless of the chromosome involved, most cases of human trisomy originate from errors in maternal meiosis I. Given the biology of the human egg, this is not entirely unexpected: the first stage of female meiosis is initiated in the fetal ovary and is followed by a long "arrest" phase that lasts until the time of ovulation. Thus, the first meiotic division is amazingly protracted, taking at least 10 to 15 years and as many as 45 to 50 years to complete.
Although the association between maternal age and trisomy has long been recognized, other predisposing factors have been elusive. However, studies conducted during the past few years have identified the first molecular correlate of nondisjunction, aberrant meiotic recombination. Recombination is familiar to most of us as the process that "shuffles" genetic material during the meiotic prophase, but it plays another equally crucial role: recombinational exchanges lock the homologues together in proper register and thereby facilitate proper segregation at meiosis I. In model organisms (e.g., flies and yeast), it has long been recognized that the absence or reduced numbers of exchanges increase the likelihood of nondisjunction.
With the advent of the Human Genome Project, it became possible to "map" recombination sites in normal and abnormal meioses and to ask whether this was the case for human trisomies as well. The results of these mapping studies have been remarkable: for all the trisomies studied to date (trisomies 15, 16, 18, 21, and sex-chromosome trisomies), significant alterations in recombination have been identified. These alterations take on one of two general forms (see Figure 2). First, for some conditions — especially meioses leading to trisomy 21 — exchanges simply fail to occur between the homologues. Thus, the two chromosomes are left to drift along independently, and if they drift to the same spindle pole (as they will half the time), an aneuploid gamete will ensue. Second, exchanges may occur, but at suboptimal locations. For example, trisomies 16 and 21 frequently result from exchanges that are located very distally on the chromosome, and trisomy 21 and sex-chromosome trisomies from exchanges that occur close to the centromere. Thus, it appears that to maximize the chances of successful segregation, exchanges should be medially placed on the chromosome. The reasons for this are not yet clear; possibly, distal crossovers are unable to lock homologues together, so that they move independently, whereas pericentromeric crossovers lock homologues too tightly, so that they are unable to separate from one another.
Figure 2. Optimal and Suboptimal Homologue-Crossover Configurations.
Crossovers facilitate proper segregation of chromosomes during meiosis. This example shows an optimal crossover configuration for a pair of homologues (A) and three suboptimal configurations: one in which crossovers failed to occur (B), one with a single, distally placed exchange (C), and one with a single, pericentomeric exchange (D).
Whatever the basis for this effect, the mapping studies make it clear that altered recombination is a major contributor to nondisjunction in humans. This finding raises an obvious question: How is altered recombination linked to the only other known risk factor for human aneuploidy, older maternal age? A simple explanation would be that meiotic recombination patterns change as a woman ages. However, this is almost certainly not the case: meiotic exchanges for oocytes are established during the fetal period, and there is little reason to think that a second round of recombination occurs in the adult ovary. Thus, the associations among age, recombination, and nondisjunction must be more complex.
We have suggested that nondisjunction may require two "hits."2 According to this model, the first hit occurs during the fetal development of the oocyte, with the chance occurrence of pericentromeric or very distal crossovers, or the absence of any crossovers, between the homologues. The second hit occurs many years later around the time of ovulation, when the oocyte resumes meiosis. Because of age-related perturbations in the meiotic machinery, homologues with susceptible crossover configurations are less likely to segregate correctly. In other words, the young ovary is able to process most susceptible exchange configurations correctly, but over time, the degradation of meiotic proteins leads to an increasing likelihood of nondisjunction involving homologues that have "at risk" crossover configurations.
This model suggests a number of predictions that are now being tested to determine whether human trisomies are indeed the result of the two well-timed blows or, rather, the consequence of multiple hits accumulated over the years. In addition, several laboratories are now trying to generate appropriate mouse models, either by knocking out recombination-related genes or by experimentally eliminating crossovers or "moving" them to suboptimal locations. Finally, recent advances in immunofluorescence methods now make it possible to directly view meiotic recombinational events as they are happening (see Figure 3), allowing us to characterize normal and abnormal exchange patterns in individual gametes. This combination of approaches may not win the day immediately. However, it should allow us to focus on something other than maternal age alone — arguably the most relentless opponent of successful human reproduction.
Figure 3. Visualizing Crossovers.
Recently developed immunofluorescence methods make it possible to examine sites of recombination directly in spermatocytes and oocytes. This example shows a human spermatocyte (obtained from a testicular biopsy) captured during the prophase of meiosis I. Antibodies against the protein SCP3 highlight the synaptonemal complex (red), the "glue" that tethers homologues during this phase of meiosis; CREST antiserum localizes to centromeric regions of the chromosomes (blue); and antibodies against the mismatch-repair protein MLH1 pinpoint the sites at which crossovers will develop (yellow). Thus, this approach makes it possible to count the number of crossovers on the short and long arms of individual chromosomes and throughout the cell as a whole. One obvious application of this technique is determining whether infertile men have different levels or locations of crossovers than men with normal spermatogenesis; such studies are now being actively pursued by a number of groups.
Source Information
From the Department of Human Genetics, Emory University, Atlanta (N.E.L.); and the Department of Genetics and Center for Human Genetics, Case Western Reserve University and University Hospitals of Cleveland, Cleveland (T.J.H.).
References
Hassold T, Hunt P. To err (meiotically) is human: the genesis of human aneuploidy. Nat Rev Genet 2001;2:280-291.
Lamb NE, Feingold E, Savage A, et al. Characterization of susceptible chiasma configurations that increase the risk for maternal nondisjunction of chromosome 21. Hum Mol Genet 1997;6:1391-1399.(Neil E. Lamb, Ph.D., and )