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Principal Investigator: Dr. Paula Cohen
Contact Information: E-mail: pc242@cornell.edu - Phone: 607-253-4301
Sponsor: National Down Syndrome Society
Title: Maternal Mismatch Repair Proteins and Their Role in Etiology of Trisomy 21
Annual Direct Cost: $12,377
Project Period: 04/01/05-07/31/05
A normal cell contains pairs of near-identical chromosomes derived from the maternal and paternal parents. These cells are referred to as being diploid, containing two copies of every chromosome. In order to produce diploid daughter cells, the DNA must replicate to produce twin copies, or sister chromatids, of each chromosome. During cellular division (mitosis) sister chromatids separate and become individual chromosomes in two new diploid daughter cells that are identical to their parental precursor. By contrast, germ cells (oocytes in females, spermatocytes in males) contain half the number of chromosomes, and are therefore haploid. Upon fertilization, the two haploid germ cells fuse to produce embryos that are once again diploid. The formation of haploid germ cells from diploid precursor cells therefore requires a specialized type of cellular division, known as meiosis. Meiosis differs from mitosis in that cells undergo one DNA replication followed by two divisions. In the first, meiosis I, maternal/paternal chromosomes pair and swap genetic information. This recombination process is essential because it tethers the chromosomes together until the first meiotic division. The resulting cells, containing pairs of sister chromatids, but only either the maternal or the paternal copy of each chromosome, undergo the second meiotic division during which one chromatid goes to each of four daughter cells. While this process is quite robust in lower organisms, it is highly variable in humans, particularly women. If recombination/pairing errors occur, in a phenomenon known as non-disjunction, the germ cells will contain too many or too few chromosomes. Such cells are said to be aneuploid and, upon fertilization, will produce embryos that are either not viable (resulting in miscarriage) or that result in babies with genetic disorders. One such example is trisomy 21, or Down syndrome, in which gametes contain an extra copy of chromosome 21.
The high prevalence of Down syndrome in the human population has prompted much research into the etiology of the disorder. The need for such research is underscored by the observation that the majority of Down syndrome cases are a result of errors during maternal meiosis, particularly in meiosis I. However, such studies are limited because of the fact that maternal meiosis I, unlike its male counterpart, occurs during embryonic life. Oocytes enter meiosis almost synchronously in the embryo and progress through much of prophase of meiosis I before arresting at the dictyate stage. Oocytes remain in this state until after puberty when cohorts of oocytes resume meiosis with each ovulation. Thus, in women, the oocytes remain in suspended animation, but must remain viable with recombination events intact up until menopause (average 51 years in women).
Perhaps the most important feature of meiosis that prevents non-disjunction is the correct maintenance of recombination events between maternal and paternal chromosomes during prophase I. Studies in mice have indicated that a major factor in preventing non-disjunction is the correct functioning of the mismatch repair (MMR) family. The MMR proteins bind to pairing chromosomes and assist in selecting and stabilizing sites of recombination. In fact, two proteins in this family, MLH1 and MLH3, are now known to be the molecular markers of the ultimate recombination sites. Without MLH1 or MLH3, recombination fails in mice, and the resulting oocytes become prone to non-disjunction. MLH1 and MLH3 are also essential for maintaining optimal rates of recombination in human oocytes, but our recent observations indicate that human fetal oocytes suffer from highly variable rates of MLH1-MLH3 localization on meiotic chromosome pairs. Thus we hypothesize that this variability of MLH1-MLH3 in human oocytes is a partial explanation for the higher rates of non-disjunction seen in women, and would therefore represent a significant etiological factor for Down syndrome.
Genetic polymorphisms are minute changes in the human genome that occur at variable rates (1:250 to 1:1000) and can be disease-related. For example, women harboring polymorphisms in the folate metabolic pathway have increased likelihood of having a child with Down syndrome. In addition, some infertile couples have been shown to harbor polymorphisms in the genes encoding members of the MMR pathway. Such observations have prompted us to ask whether other genetic alterations in MMR genes might also lead to an increased risk of aneuploid oocytes and trisomic offspring. Therefore, the current studies will examine the relationship between high and low frequency of MLH1-MLH3 accumulation on meiotic chromosomes in human fetal oocytes and the occurrence of known single nucleotide polymorphisms (SNPs) in the MMR genes. Oocytes will be obtained from human fetal ovaries, through the Einstein Human Fetal Tissue Repository from women undergoing elective pregnancy terminations. Oocytes will be stained for MLH1-MLH3 at recombination sites, and samples from individuals exhibiting particularly low or high frequency of these proteins will be subjected to SNP analysis using the sophisticated sequencing equipment present within the Human Genetics core facility at Albert Einstein College. At the same time, the age of the mother, smoking and drug usage will be correlated with any changes in recombination rates in the embryonic ovaries. These studies are highly novel and will provide an exciting new avenue for research into the etiology of trisomic disorders, and, as such, will vastly improve our understanding of the genetic causes of Down syndrome.
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