Download Tudor – Down`s syndrome

Tudor Oroian
Biology 303 – Dr.Ely
5 November 2010
Correlations in Maternal Telomere Length and the Likelihood of a Down’s Syndrome Birth
Telomeres were discovered in 1978 by Elizabeth Blackburn, and Joseph Gall by studying
extrachromosomal genes coding for ribosomal RNA in the ciliated protozoan Tetrahymena. The terminal
RNA fragment was amplified and found to occur in repeating patterns of C-C-C-C-A-A which were
repeated 20 to 70 times and were heterogeneous in length. The repeating sequence was synthesized
more frequently when RNA was used as the template by Escherichia coli DNA polymerase I. Initiation of
this sequence occurred at specific single-strand discontinuities (Blackburn and Gall, 1978). Subsequently,
the orientation of the strands carrying the repeating hexanucleotide sequence was determined, and a
model for the termini of the DNA based on these findings was created.
Since their discovery in 1978, telomeres have managed to stay in the spotlight of genetics
because of the growing amount of research that is dedicated to them. The reason for this interest lies in
their function. As explained above, telomeres are segments of repeating DNA nucleotides that occur at
the ends of chromosomes. Due to the nature of DNA replication, the ends of DNA molecules shorten
after every division, and having essential genes that are present at the very ends of chromosomes would
mean that they would get slowly eroded over time. Instead, telomeres effectively “cap” chromosomes
in a way that allows for removal of nucleotides at the ends of the chromosome during replication,
without endangering vital coding sequences (Blackburn 2009). Because of this vital role, telomeric
abnormalities are constantly being discovered to be at the root of many cancers and aging-related
disorders.
Unfortunately, the key to a longer life is not just to bypass the erosion of telomeres. Without
telomeric erosion, cell division would go haywire and all cells would be immortal, therefore cancerous.
Essentially, telomeres are a very delicate regulatory mechanism in organisms (Nakagawa et al. 2004).
Since telomeres act as time markers on a cellular lever, they often times offer a more adequate
measurement of age than chronological time does.
Maternal age has for a long time been known to be a risk factor for Down’s syndrome, however,
understanding the genetic basis for this correlation is still limited. The frequency for Down’s syndrome
cases range from 2% for women younger than 25 years of age, to over 35% for women over 40 years of
age. This is over a fifteen-fold increase. Non-disjuncture can occur due to several reasons such as a
faulty spindle apparatus (Hawley et al. 1994) or faulty maintenance of sister chromatid adhesion
(Jeffreys et al. 2003).
In 2003, Jeffreys et al. studied the mechanism behind non-disjunction using the model organism
Drosophila melanogaster. It was previously known that for at least 5% of all documented pregnancies,
meiotic segregation errors give rise to zygotes with the wrong number of chromosomes. Since the
importance of age-dependent nondisjunction in humans was recognized, the underlying mechanisms
remained largely unexplained. In this study, experimenters provided evidence that Drosophila is an
accurate model organism for investigating how oocyte aging contributes to meiotic nondisjunction. In
this experiment it was found that much like human oocytes, nonexchange homologs and bivalents with
a single crossover in Drosophila oocytes are most likely to nondisjoin during meiosis I. The researchers
further showed that in a genetic background in which sister chromatid cohesion is compromised,
(because of things such as degradation due to age), nonrecombinant X chromosomes become
vulnerable to meiotic nondisjunction. The data indicated that the backup pathway that normally ensures
proper segregation of chromosomes deteriorates as Drosophila oocytes age, and this finding provided
the gateway to further studies that focus on age-dependent meiotic nondisjunction in humans.
A previous breakthrough however, provided a crucial key to understanding the multiple parts to
the mechanism that causes non disjunction in sister chromatids. Based on observations in model
organisms such as flies, it was hypothesized that non disjunction of chromosome 21 in human females
results from an age-dependent loss of spindle-forming ability (Hawley et al.1994). This hypothesis did
not mention the involvement of telomeric shortening in the increased rate of non disjunction in later
age; however, it was a keystone study for future studies involving non disjunction.
Non-disjunction can occur either during Meiosis I or Meiosis II. An important theory about
biological age proposes that biological aging would differ among women of the same chronological age
and that the frequency of trisomic conceptions depends on the biological age instead of the
chronological age (Kline et al. 2004). It was discovered that every menstrual cycle in women is
associated with maturation of antral follicles which also decrease in number with increasing age.
Because of this decrease in antral follicles and a general decrease in the number of eggs, change occurs
in the hormonal environment in the ovary, which makes gametes more prone to errors during meiosis.
One critical ovarian hormone is follicle stimulating hormone. Therefore, van Montfrans et al.
(2002) used the knowledge from two previous studies (Nasseri et al., 1999; van Montfrans et al., 1999)
which reported elevated basal FSH concentrations in women with a history of aneuploid pregnancies to
determine if there was any statistical evidence supporting the impact of hormonal environments on the
likelihood of a DS birth. The experiment was performed by measuring inhibin B and estradiol
concentrations which were able to used as indicators for depletion of the follicle pool in women with a
history of a Down's syndrome pregnancy. It was found that in the women with a history of a Down's
syndrome pregnancy, there was a significant inverse correlation between basal FSH and inhibin B
concentrations. In the control group, this correlation was not conclusive. These data indicated that the
higher basal FSH concentrations observed in women with a history of a Down's syndrome pregnancy are
likely to reflect early an early decrease of the follicle pool. In addition to this, the study discussed two
possible mechanisms for this observed phenomenon. Both of these two mechanisms have at their
foundation granulose cells, which act as hormonal feedback monitors, and which are found in
significantly fewer number among women with a history of Down’s Syndrome births. This experiment
was a cornerstone study in the Ghosh et al. experiment (2010) since it proved that age was a significant
factor in Down’s syndrome births. This experiment mainly had to do with the chemical and hormonal
environment in oocytes, but given previous knowledge of telomeres, environmental factors could affect
telomeric depletion.
In the 2010 paper by Ghosh et. al., the telomere length of Down’s syndrome bearing mothers was
considered over a range or reproductive ages, and compared to age-matched controls using a crosssectional design. The Down’s syndrome mother population was also stratified in accordance to meiotic
outcome groups (meiosis I or meiosis II) to determine if telomere length is associated with types of NonDisjunctuThe subjects for this study were 3 case women and 3 control women for each year of age from
18 to 42 years of age. The total number of subjects in the study was therefore 75 individuals in the
control group and 75 with non disjunction. Blood samples were taken from the mother to determine the
telomere length. Cytogenic analysis was also performed in order to exclude individuals with hidden
mosaicism. After this, genotyping was performed in non disjunction cases in order to isolate the origin of
the trisomy (meiosis I or meiosis II). Ten polymorphic STR markers were used to determine the
contribution of the maternal alleles to the Down’s syndrome child. The differentiation between meiosis I
and meiosis II was determined by two sets of guidelines. First, if parental heterozygosity of the markers
was retained in the trisomic child, it was inferred that the error occurred during Meiosis I. For
determining if the non disjunction occurred during Meiosis II, the parental heterozygosity had to be
reduced in the trisomic child.
After these steps, the most crucial part of the experiment was performed: the determination of
the telomere length. A special kit (TeloTAGGG) was used to estimate telomere length in each of the DNA
samples. Statistical analyses were performed on the resulting data. Separate models were produced for
predictions of telomere length based on age, as well as a separation between Meiosis I and Meiosis II
non disjunction. The rates of telomeric erosion were measured in kilo-base-pairs/year, and statistical
significance was tested using standard regression t-statistics. Furthermore, women were stratified into
three age groups: young (18-25), middle (26-34) and old (35-42), to test if telomere length was different
among the groups in each category.
The results obtained were consistent with the hypotheses that maternal biological age has an
impact on the likelihood of a Down’s syndrome age. The first graph, (below) shows telomere length (in
kilo base pairs) in relation to maternal age. As expected, the older the mother, the shorter the telomere.
The second graph exhibits telomere length among Meiosis I and Meiosis II (combined) in relation to
maternal age. The third and fourth graphs exhibit the differences in telomeric erosion when Meiosis I
and Meiosis II non disjunction are separated. It is to be noted that in the case of meiosis II the rate of
erosion is much faster than in Meiosis I or control.
Graph 1: Telomere Length in Relation to Maternal Age
Graph 2: All mothers
Graph 3: Meiosis I mothers
Graph 4: Meiosis II mothers
Based on graphical and empirical data, the estimated rate of telomere shortening was 61
bp/year in the euploid mother group. For MII mothers, the estimated TL shortening rate was 111
bp/year and in MII mothers it was 230 bp/year. The model with an interaction term showed statistically
significant differences between these loss rates
Several conclusions may be drawn from these results. First, based on previous studies, it is clear
that telomeric length has a strong correlation with ovarian aging, and therefore maternal reproductive
age, which in turn is found to have an effect on the likeliness of a child to have Down’s syndrome
(Jeffreys et al. 2003). Considering these findings, it is a natural hypothesis that women who had a
trisomy 21 child at a young age, suffer from early and accelerated genetic aging in comparison to the
mothers of euploid children. The results demonstrated that among mothers with Down’s syndrome
children, telomere lengths were much shorter in older mothers than in younger mothers. The rate of
decrease in relation to age was greater in mothers with meiosis II abnormalities than in mothers with
meiosis I abnormalities; however, both showed considerable differences in the rate of decrease
compared to the control mothers.
It is important to realize that the data does not indicate that women experienced rapid genetic
aging over time, since the change in telomere length was not measured over a period of time in a single
mother. However, what was done was a comparison between women of different ages. This study
cannot make predictions about what telomere length could be expected in older women who had a
Down’s syndrome child when they were younger.
Perhaps the most interesting result was the difference in telomere length between the three
groups, which became apparent only in older women. In contrast to the researchers’ expectations, the
young meiosis I and meiosis II cases and controls were not different in genetic age. This result does not
support the theory that young mothers who have Down’s syndrome children have accelerated cellular
aging. One explanation for the increased difference observed in older women is that the control women
who had normal pregnancies at a later age were themselves abnormal for having a euploid pregnancy at
such a late age and essentially had decelerated aging. A second and more likely explanation would be
that the decline in telomere length is correlated with an altered pattern of recombination (Liu et al.
2004). This phenomenon is thought to be strongly associated with non disjunction of human
chromosome 21. It is likely that environmental aneugens accumulate gradually in the genome and
manifest their effects only after women reach their late reproductive years when other natural cellular
and genetic degradations have already started and cellular control mechanisms are less thorough
because of differing hormonal climates.
In conclusion, this study covered a broad range of topics; however, the apparent focus on
trisomy 21 was merely a tool to help explore the differences between chronological age and biological
age. This study opens the door to many other avenues of research in telomeric erosion and the key that
it plays in personal and reproductive health.
Bibliography:
1. Jeffreys CA, Burrage PS, Bickel SE (2003)
A model system for increased meiotic nondisjunction in older oocyte. Curr Biol 13:498–503
2. Hawley RS, Frazier JA, Rasooly R (1994)
Separation anxiety: the etiologyof nondisjunction in flies and people. Hum Mol Genet 3:1521–
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3. Kline J, Kinney A, Reuss ML, Kelly A, Levin B, Ferin M, WarburtonD (2004)
Trisomic pregnancy and the oocyte pool. Hum Reprod 19:1633–1643
4. van Montfrans JM, van HooV MH, Martens F, Lambalk CB (2002)
Basal FSH, estradiol and inhibin B concentrations in women with a previous Down’s syndrome
affected pregnancy. Hum Reprod 17:44–47
5. Nakagawa S, Gemmell NJ, Burke T (September 2004).
Measuring vertebrate telomeres: applications and limitations". Mol. Ecol. 13 (9): 2523–33.
6. Blackburn E. Telomeres and Telomerase: The Means to the End. (2009) Nobel Lecture by Elizabeth
Blackburn http://nobelprize.org/mediaplayer/index.php?id=1214
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Irregular telomeres impair meiotic synapsis and recombination in mice. Proc Natl Acad Sci USA
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8. Wikipedia: http://en.wikipedia.org/wiki/Telomeres
9. Wikipeda: http://en.wikipedia.org/wiki/Down%27s_syndrome
10. Blackburn E., Gall J (1978)
A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in
Tetrahymena . Pub. Elsevier Ltd.
Primary Article:
11. Sujoy Ghosh, Eleanor Feingold, Sumita Chakraborty, Subrata Kumar Dey (2010)
Telomere length is associated with types of chromosome 21 nondisjunction: a new insight into
the maternal age effect on Down syndrome birth. Hum Genet (2010) 127:403–409