Mitosis and Chromosome Segregation (BMS 532)

Mitosis, Errors in Chromosome Segregation, and Aneuploidies BMS 532 MOLECUL AR BIOLOGY AND GENETICS BLOCK 2 LECTURE 3  Objectives 1. Define the following terms: nondisjunction, aneuploidy, euploidy, trisomy, monosomy, chimerism/chimera, mosaicism/mosaic, uniparental dis...

Mitosis, Errors in Chromosome Segregation, and Aneuploidies BMS 532 MOLECUL AR BIOLOGY AND GENETICS BLOCK 2 LECTURE 3  Objectives 1. Define the following terms: nondisjunction, aneuploidy, euploidy, trisomy, monosomy, chimerism/chimera, mosaicism/mosaic, uniparental disomy, isodisomy, and heterodisomy Nondisjunction 2. Summarize the mechanisms of maternal meiotic nondisjunction and mitotic nondisjunction 3. Compare and contrast consequences of mitotic and meiotic nondisjunction in terms of organismal structures, function, and mechanisms of action 4. Explain the role of mosaicism in perpetuation of chromosomal abnormalities and aneuploidies and compare and contrast the different types of mosaicism (CPM and fetal with and without placental involvement) 5. Explain the difference between chimerism and mosaicism Mitosis and Errors in Chromosome Segregation 6. Explain how chromosome segregation can result in chromosome abnormalities during mitosis Uniparental Disomy 7. Explain how uniparental disomy can lead to phenotypic consequences and disease manifestation with emphasis on differentially methylated regions and compare whole chromosome UPD with segmental UPD in terms of these consequences 8. List the main mechanisms of whole chromosome UPD and explain how UPD can develop Aneuploidy Examples 9. Compare and contrast trisomy 21, 18, 13, 16, Turner and Klinefelter syndromes in terms of causes, phenotypic consequences, and viability 10. Explain why there is variable viability across autosomal and sex chromosome aneuplodies LO1, LO3 A Few Reminders and Background Oogenesis is different than spermatogenesis in terms of both timing and ability to evaluate the 4 gamete products Oogenesis remains limited in terms of ability to study ◦ Much of what we know about the process of human gametogenesis is derived from spermatogenesis or analysis in other organisms Nondisjunction can occur in both meiosis and mitosis but with different consequences for the organism ◦ Mitosis = tissue specificity (can be nearly all tissues if it occurs early enough in development) ◦ Meiosis = full organism/create errors in offspring unless corrections occur during embryogenesis mitoses LO1, LO3 Aneuploidies Introduction Aneuploidy = inappropriate chromosome numbers ◦ An individual/cell with unbalanced chromosome content is collectively referred to as an aneuploid ◦ When the content is abnormal but balanced = euploid/euploidy The severity of a particular autosomal aneuploidy correlates with the gene content of the chromosome ◦ This is not a size correlation but rather a content correlation ◦ The more significant the genetic content, the more severe the phenotype Aneuploidies for chromosomes rich in genes (particularly structural genes) are less likely to survive (embryonic lethal or even prevented from implantation) LO1, LO3 Trisomies and Monosomies Whole chromosome aneuploidies can be classified by the amount of increased or decreased information ◦ Trisomy = 3 copies of the corresponding chromosome ◦ Monosomy = 1 copy of the corresponding chromosome Trisomies for all autosomes have been reported in products of conception from spontaneous pregnancy losses ◦ The observed frequencies of trisomies varies greatly ◦ Trisomy for chromosome 1 has been reported in spontaneous pregnancy losses though no fetal pole had developed in the reported cases ◦ Trisomy 16 ~30% of all spontaneous losses ◦ In livebirths, trisomies other than 13, 18, and 21 are RARE and typically only mosaic Monosomies are extremely rare in both spontaneous losses and live births implying lethality before sufficient tissue is present for analysis ◦ Sole Exception for Adult viability = Turner syndrome; sex chromosome monosomy LO1, LO2, LO3 Mechanisms Inappropriate chromosome numbers are most commonly derived from inappropriate chromosome segregation Studies show that nondisjunction of maternal meiosis I accounts for the majority of cases of chromosomal aneuploidy ◦ EXCEPTION = chromosomes 7, 13, and 18 ◦ 13 shows an equal number of maternal meiosis I and II errors Studies link increases in these events with advanced maternal age: DIRECT Correlation Mechanisms for Meiosis I Nondisjuction (see next slide) ◦ Both homologous chromosomes to one pole ◦ Premature separation of one of the homologous chromosomes LO2 Mechanism of Meiosis I Nondisjunction BOTH TO ONE POLE PREMATURE SEPARATION Meiosis I **Studies show that this method is actually more common Caused by premature breakdown of COHESIN LO1, LO2, LO3 Maternal Age and Nondisjunction Production Line Hypothesis ◦ Maturation of oocytes occurs in the same order as original development in fetal life Limited oocyte pool model ◦ The number of follicles in antral state decreases with increasing age ◦ The fewer number of follicles equals an increase in the probability that a lower quality one will be chosen Take Home Message: Our understanding of the process/causes of nondisjunction is INCOMPLETE  Mechanisms of LO1, LO2, LO3, LO6 Mitotic Nondisjunction In mitosis, sister chromatid split to give each daughter cell as close to the same genetic content as possible and ensuring each cell has a copy of material from each source (paternal and maternal) Mitotic Nondisjunction is about failure to split or failure to capture properly and can have multiple outcomes l na al rn er S Phase l te na at al Pa rn M er te at Pa M l Expected na al rn er Outcomes te at Pa M LO1, LO2, LO3, Mosaicism: Nondisjunction LO4, , LO6 in Mitosis Detection typically requires analysis of more than one tissue/cell type First few mitoses are particularly vulnerable ◦ Given the number of mitoses that take place, it is possible for more than one mitotic error and thus more than one unique aneuploidy cell line to develop (rare) Structural Rearrangement Mosaicism = Rare ◦ Can involve two lines of opposite imbalance Gonadal Mosaicism may be common but is not frequently evaluated thus actual numbers are less well known ◦ Evidence that all women may be gonadal mosaic for chromosome 21 aneuploidies Direct relationship between timing of event and fraction of cells exhibiting aneuploidy ◦ Early events = more tissues and broader impact phenotypically Direct relationship between severity of abnormality and probability that cell line will undergo cell death and be removed  Mitotic Error: LO3, LO6 Timing and Consequence  Timing and Consequences LO3, LO6 Continued LO4, LO6 MOSAICISM There are multiple types of mosaicism Can exhibit segmental mosaicism ◦ CPM: confined placental ◦ Fetal mosaicism (with and without normal placenta) NOTE: mosaicism does NOT have to be orderly or even organized LO5 Chimerism vs. Mosaicism Chimerism ◦ Different cell types are derived from two separate and external sources (i.e. originally separate conceptuses) ◦ Individuals with cells from two separate fertilized eggs ◦ Post-zygotic fusion of dizygotic twin zygotes ◦ Not derived from mitotic error but rather a fusion or absorption event Confined Chimerism ◦ Only specific tissue possesses the two unique cell lines/populations Can explain the presence of two cell lines in a single individual where no apparent error is observed Most likely underlying mechanism for 46,XY/46,XX hermaphroditism and can explain a 45,X/69,XXY fetus identified Can also explain diploid and triploid mosaics (though dispermy may also be a cause in these cases) LO4, Patterns of Cutaneous LO6 Mosaicism Biesecker and Spinner 2013 Additional considerations for chromosome content and aneuploidies UNIPARENTAL DISOMY LO7 Epigenetics and Genomic Imprinting and Aneuploidy Unaltered DNA sequence but altered ability of gene expression Imprinting = epigenetic effects set during germline transmission Functional Genetic Defect ◦ UPD with over (biallelic) or no expression (loss of both) ◦ Deletion with no expression (must be loss in copy not silenced by imprinting) ◦ Relaxation of imprinting with overexpression (alteration of regulatory elements) LO7 Uniparental Disomy Reminder: Each autosome is intended to be inherited with 1 copy from the maternal parent and one copy from the paternal parent Uniparental disomy is when both copies are derived from a single parent (2 copies of the chromosome or region of chromosome from maternal source or paternal source exclusively) Most appear without any phenotypic consequence but can have significant implications in differentially methylated regions Can also result in homozygosity for autosomal recessive genes Distinctions ◦ Both chromosomes are identical = isodisomy ◦ Both chromosomes are different = heterodisomy LO7 Clinical Conditions with link to Uniparental Disomy Beckwith-Wiedemann syndrome (BWS) Prader-Willi syndrome (PWS) Angelman syndrome (AS) Silver-Russell syndrome (SRS) Transient neonatal diabetes (TNDM) Pseudohypoparathyroidism type 1b (PHP1b) Kagami syndrome Temple syndrome Maternal UPD 20 LO8 UPD Mechanisms Each of the scenarios requires two separate abnormal events The errors can be meiotic or mitotic Original event will almost always be sporadic with no increased risk of recurrence in a family if parents are normal Root cause is nondisjunction ◦ Therefore known risk factor is advanced maternal age  LO8 UPD Mechanisms: Whole Chromosomes 3 Monosomic rescue Step 1: Monosom 1 Fertilization ic Gametic Complementation Conceptu s Step 2: Postzygotic Mitosis Replicatio n 2 Trisomic Rescue ◦ Mechanism behind most UPD Trisomic Step 1: Conceptu 4 Mitotic Error and Rescue Fertilization s Disomic Conceptus Error Trisomic Embryo Step 2: Postzygotic Mitosis Subsequent Mitosis LO7 Segmental UPD Segmental UPD = UPD of only parts of a chromosome instead of the entire chromosome Greatest consequences observed for chromosomes subject to imprinting Can impact specific tissues/exhibit mosaicism Mechanisms ◦ Postzygotic somatic recombination between maternal and paternal homologs ◦ Meiotic nondisjunction producing a disomic gamete followed by a trisomic conception with crossing-over between maternal and paternal homologs and chromosome loss ◦ Repair of double strand break via break-induced replication Significant Human Chromosomal Aneuploidies A FEW EXAMPLES TRISOMY 21, 18, 13, AND 16 T U R N E R S Y N D R O M E A N D K L I N E F E LT E R S Y N D R O M E Overall Objective: Identify the chromosomes involved in a particular condition and explain how that chromosomal abnormality/aneuploidy could develop A N E U P LO I DY E X A M P L E S 11. Compare and contrast trisomy 21, 18, 13, 16, Turner and Klinefelter syndromes in terms of causes, phenotypic consequences, and viability 12. Explain why there is variable viability across autosomal and sex chromosome aneuplodies Summary Classification of Aneuploidies and Karyotypic Abnormalities Errors in meiosis (gametogenesis) ◦ Monosomies ◦ Trisomies ◦ Sex chromosome aneuploidies Errors in mitosis leading to mosaicism ◦ Can include “rescue” of a trisomic cell (with or without uniparental disomy) ◦ Chimerism Errors in fertilization ◦ Triploidy ◦ All chromosomes from one parent ◦ Partial and complete moral pregnancies Structural abnormalities and rearrangements  Trisomy 21 ◦ John Langdon Down 1866 = described clinical phenotype ◦ Lejeune et. al. 1969 = chromosome abnormality ◦ 47,XY,+21 or 47,XX,+21 ◦ Trisomy 21 typically refers to the presence of an entire additional copy of chromosome 21; however, partial trisomies are also clinically relevant and fit the classification ◦ Trisomy 21 represents the most common trisomy in viable offspring Trisomy 21 Most common single known cause of intellectual disability Meiotic considerations ◦ Evidence of increased ratio of males to females (evaluation of spermatogenesis) ◦ Hypothesized preferential segregation of extra 21 with Y ◦ Study showed an increased level of disomic 21 sperm also carried the Y while another study showed excess of males in trisomy 21 conceptuses resulted from paternal meiotic errors ◦ Other explanation is that selection against female trisomy 21 fetuses in utero must also exist Clinical Features of Trisomy 21 Variable/Spectrum Critical Region: 3 Mb at Frequently associated with 21q22 heart defects (atrioventricular ◦ 21q22.1 to q22.2 septal defect-AVSD), malformations of Example Genes: gastrointestinal tract, hearing ◦ MX1 (interferon inducible loss, vision disorders, delayed protein p78- a GTPase) development, behavioral ◦ Linked to morphological features and problems, and intellectual brain development impairment ◦ RCAN1 (DSCR1) ◦ Overexpression leads to loss of Leukemia regulation for key transcription factors including errors in translocation of NFAT ◦ Linked to heart (primarily expressed in skeletal and cardiac tissues)  Trisomy 18/Edwards Syndrome Edwards 1960 47,XY,+18 or 47,XX,+18 1 in 5,000 births (Genetics Home Reference; other sources quote 1 in 6,000 to 10,000) 1:3-4 male to female ratio Viability is variable with significant losses in utero or within the first days of life ◦ 95% of individuals not surviving past 1 year of life ◦ Although mosaicism for this is considered rare, these tend to be the more viable options 90% exhibit congenital heart defects  Trisomy 13 Patau et. al. 1960 ◦ A.K.A Patau Syndrome ◦ 47,XY,+13 or 47,XX,+13 1 in 16,000 (Genetics Home Reference; 1 in 10,000 to 16,000 other sources) Slightly more common in females than males There is an acknowledged recurrence risk for future pregnancies of approximately 1% Variable consequences though 80% will not survive past the first month of life ◦ More positive outcomes in mosaic form  Chromosome 16 Chromosome 16 contains several key genes Full trisomy of chromosome 16 is most frequently embryonic lethal ◦ Identified in 30% of tested spontaneous early pregnancy losses Mosaicism, Partial Trisomy, Uniparental Disomy, and Partial Deletions appear to be only survivable forms of chromosome 16 alterations ◦ Mixed evidence regarding the health of infants with mosaic trisomy ◦ IF a mosaic trisomy baby is born “healthy” at birth the outlook for survival past newborn is better than if any health issues are observed  Chromosome 16 Clinical Features of mosaic trisomy 16 include: ◦ Average gestational age of 35.7 weeks ◦ Significantly smaller than average birth weight (up to 2 standard deviations below average) Clinical Features of mosaic trisomy 16 can include: ◦ Cardiac malformations (ventricular septal defect most common) ◦ Hypospadias (opening of urethra not located at end of penis) ◦ Pulmonary hypoplasia (incomplete development of lungs) Turner Syndrome Female 45, X – Missing all or part of the second X chromosome Approximately 1 in 2,000 to 1 in 2,500 live female births Considered the most viable human monosomy Symptoms in Infants – Swollen hands and feet – Wide and webbed neck Symptoms in Older Females – Absent or incomplete development at puberty, including sparse pubic hair and small breasts – Short height  More than one way to get to Turner Syndrome (45, X) Up to approximately 50% of cases involve other than traditional (45,X) Mosaicism ◦ Presence in mosaic form may modify phenotypic features ◦ Studies have found mosaicism in ~67% of tested patients diagnosed with Turner Syndrome ◦ 45,X/46,XY Mosaicism ◦ Ranges in physical appearance and the associated Y is often structurally abnormal ◦ 45,X/47,XXX Mosaicism ◦ More mildly affected in terms of clinical phenotype Isochromosome X (~18% of Turner patients) ◦ Two copies of long arm (missing all or most of short arm) ◦ Can be observed in mosaicism ◦ Phenotypically indistinguishable from pure 45,X karyotypes Ring X ◦ Most often as mosaic ◦ Size varies ◦ Many lack many of the classic somatic features of Turner and can have severe phenotypes ◦ More severe phenotypes may be associated with lack of XIST and therefore failures in inactivation ◦ Can have tissue limited expression Klinefelter Syndrome One or more additional copies of all or part of the X chromosome in male cells ◦ 47, XXY Affects an approximate 1 in 500 to 1 in 1,000 live male births Extra genetic material on the X negatively impacts development of male sexual characteristics (reduces testosterone) ◦ Amount of additional X as it relates to gene content and types of genes correlate with severity of phenotype Delayed or reduced puberty due to loss of testosterone Some cognitive impairment (severity correlated with number of X chromosomes) Different Nondisjunction Events Capable of Producing Klinefelter Syndrome