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Down Syndrome Update 2013

Down Syndrome Update 2013

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Down Syndrome Update 2013

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  1. Down Syndrome Update2013 Tamison Jewett, MD Professor, Department of Pediatrics Section on Medical Genetics Wake Forest Baptist health

  2. Down syndrome (as we know it today) has existed since the beginning of humankind.

  3. Down syndrome history • Pottery artifacts from the Tumaco-La Tolita culture from 2500 years ago depict individuals with features of Down syndrome (DS). • DS features are evident in a terra cotta sculpture carved by the Toltecs in Mexico (500-1200 AD). • Esquirol, a French psychiatrist, wrote a description of the DS facial appearance in 1838.

  4. Down syndrome history (cont.) • First described in the medical literature by Dr. John Langdon Down in England in 1866 who coined the term “Mongoloid.” • In 1956, scientists discovered that the typical human cell has 46 chromosomes. • In 1958, Lejeune discovered that the cells from an individual with DS had an extra chromosome 21. • In 1959, 9 people with DS were found to have an extra chromosome 21.

  5. Syndrome: a recognizable pattern of features, usually owing to a specific cause, e.g., Down syndrome, wherein the cause is extra chromosome 21 material

  6. Down syndrome features • Brachycephaly (short skull front-to back) • Excess nuchal (back of neck) skin • Hypoplastic (underdeveloped) midface • Upslanting palpebral fissures (eyelids) • Small ears w/ over-folded helices

  7. Down syndrome features (cont.) • 5th finger clinodactyly (incurving) • Wide gap between 1st and 2ndtoes • Single transverse palmar crease(s) (40%) • Heart defect (45%) • Fine, soft hair

  8. It is important to remember that individuals with DS look mainly like their families; it is the characteristic pattern of features that causes them to resemble one another.

  9. Typical Female Chromosomes

  10. Typical Male Chromosomes

  11. p-arm=> <= centromere q-arm=> autosomes Acrocentric=> Sex chromosomes Ideogram of the human chromosomes

  12. Trisomy 21 Female Chromosomes

  13. Trisomy 21 Male Chromosomes

  14. Nondisjunction

  15. Abnormal cell division leading to abnormal chromosome distribution can occur in EITHER PARENT. As women age, our risk for this to occur increases, as follows: Maternal AgeIncidence of DS at delivery 15-29 1 in 1500 30-34 1 in 800 35-39 1 in 270 40-44 1 in 100 45 and over 1 in 50

  16. Chromosomal translocations… …can result in Down syndrome.

  17. Down syndrome • Occurs in ~1:650 live births • The underlying causes: • 94% due to nondisjunction (unequal cell division) resulting in 47 chromosomes with an extra #21 • 3.3% due to an unbalanced translocation • ~2.4% are mosaic (some cells have the typical 46 chromosomes, and some have 47) • <1% have a duplication of a portion of chromosome 21

  18. Extra chromosomes mean extra genes.

  19. DNA and chromosomes are related, but are not the same.

  20. The genes of chromosome 21: what do we know? • There are more than 400 genes on chromosome 21. • Of these, ~170 code for proteins that are also encoded by genes in mice and other animals. • When genes are conserved across species, it is typically because they are important.

  21. Chromosome 21 Not all genes on chromosome 21 cause problems when occurring in a triple dose. • 2/3 of these genes are “compensated,” meaning that the amount of their product is similar to that seen in a typical population

  22. Chromosome 21 (cont.) 1/3 of the genes on chromosome 21 are over-expressed in people with DS and are referred to as being “dosage sensitive.” • Most of these are expressed at 1.5 x the typical amount (as expected). • Some of these actually have amplified expression (more than 1.5 x the typical amount).

  23. Chromosome 21 (cont.) Examples of genes on chromosome21 that are dosage sensitive and may cause problems: • COL6A1 heart defects • CRYA1 cataracts • DYRK1A intellectual disability; Alzheimer disease • ETS2 leukemia; skeletal anomalies • IFNAR immune dysfunction • SOD1 premature aging • APP Alzheimer disease

  24. What’s New?

  25. Non-invasive Prenatal Testing Using Cell-free Fetal DNA A new method of screening the fetus for abnormal chromosome number using the mother’s blood

  26. Traditional Prenatal Testing • Maternal serum screening • Amniocentesis • Chorionic villus sampling (CVS)

  27. Definition of Screening • Test that either increases or lowers a patient’s prior risk for certain fetal conditions • Does NOT give a diagnosis • Identifies pregnancies that may need further evaluation • Should be presented as an optional test to all patients

  28. Maternal serum screening • Specific chemicals that are made by the fetus and cross the placenta to enter the mother’s blood are measured on a maternal blood sample • Based on the amounts of the chemicals measured as well as other factors (maternal weight, smoking status, race, age, and diabetes status), a risk is determined regarding the likelihood that the fetus has a particular condition, e.g., Down syndrome

  29. Maternal Serum Marker AFP (alpha feto-protein) hCG (human chorionic gonadotropin) uE3 (unconjugated estriol) DIA (dimeric inhibin A) Fetal Tissue of Origin Liver Placenta Adrenals, liver, placenta Placenta Conc. in 2nd trimester Increases 15%/wk Decreases exponentially Increases 20%/wk Minor fluctuations Ranges in 2ndtrimester 20-100 ng/ml 5-70 IU/ml 0.2-5 ng/ml ~100-400 pg/ml

  30. Down syndrome Screening • Markers used: Maternal Serum AFP, hCG, uE3 and DIA in combination with maternal age • Detects: 75-80% of Down syndrome (using these 4 markers) • General Trend: ↓AFP, ↑hCG, ↓uE3, ↑DIA • If there is an increased Down syndrome risk, the patient should be offered options of genetic counseling, detailed ultrasound, and amniocentesis

  31. Amniocentesis (“Amnio”) • Typically performed between 15-18 weeks of pregnancy • Fetal cells floating in the amniotic fluid are collected • Risk for complication of ANY sort is about 1/270 • Allows for DIRECT analysis of fetal chromosomes

  32. Chorionic Villus Sampling (CVS) • Typically performed between 10-12 weeks of pregnancy • Using a catheter inserted through the cervix with ultrasound guidance, a sample of the placenta is obtained for study • Risk for complication of any sort is 1-2% • Allows for DIRECT analysis of fetal chromosomes

  33. Non-invasive Prenatal Testing Using Cell-free Fetal DNA A new method of screening the fetus for abnormal chromosome number using the mother’s blood

  34. What is cell-free fetal DNA? • DNA that is thought to be mainly from the placenta during a pregnancy and is floating free in maternal blood along with some of mother’s own cell-free DNA • Fetal cell-free DNA is thought to be cleared from the mother’s blood within hours after childbirth

  35. Cell-free fetal DNA

  36. Cell-free fetal DNA testing… • Can detect trisomies 13, 18 , and 21 • Can be performed as early as 10 weeks of pregnancy • Can produce results by one week after maternal blood draw

  37. For women with increased risk of fetal aneuploidy, large studies show… • Detection rates for trisomies 13, 18, and 21 of greater than 98% • Low false positive rates (less than 0.5%) • Low false negative rates

  38. American College of Obstetrics and Gynecology (ACOG) recommends considering the option of cell-free DNA screening for… • Maternal age of 35 or older at delivery • Fetal ultrasound findings suggestive of trisomy 13, 18, or 21 • History of a prior pregnancy with trisomy • Positive maternal serum screen for trisomy • Parental balanced Robertsonian translocation with increased risk for trisomy 13 or 21 Noninvasive prenatal testing for fetal aneuploidy. Committee Opinion No. 545. American College of Obstetricians and Gynecologists. ObstetGynecol 2012;120:1532–4.

  39. NOTE! • Cell-free screening is not recommended for low-risk women or women with multiple gestations because it has not been adequately evaluated for these groups. • This test only screens for the common trisomies and does not rule out other chromosome abnormalities or birth defects.

  40. NOTE! (cont.) • Although the test is quite good, a negative result does not ensure an unaffected pregnancy. • Women with positive results should be referred for genetic counseling and be offered amnio. or CVS for confirmation.

  41. Down Syndrome Researchers Remove Extra Copy of Chromsome 21 • Researchers at Univ. of Washington have successfully removed an extra copy of chromosome 21 from the blood cells of a person with Down s. • The goal is to be able to treat people with Down s. who have leukemia. Cell Stem Cell 11, 615–619, November 2, 2012 ª2012 Elsevier Inc.

  42. Down Syndrome Researchers Remove Extra Copy of Chromsome21 (cont.) Immature cells can be removed from the bone marrow of the patient Doctors remove the extra chromosome 21 from those cells The new cells are grown and transplanted back into the patient.

  43. Down Syndrome Researchers Remove Extra Copy of Chromsome 21 (cont.) • To do this, • A virus was used to deliver an engineered gene (TKNEO) to a targeted spot on chromosome 21. • The cells were grown in conditions that did not allow the chromosome 21 containing the TKNEO to survive. • The cells lost the chromosome 21 with the TKNEO, leaving cells with only two copies of chromosome 21.

  44. Down Syndrome Researchers Remove Extra Copy of Chromsome 21 (cont.) • So far, this seems to be safe; it has not altered genes on other chromosomes. • Dr. Russell, co-author of the paper describing the breakthrough says, “We are certainly not proposing that the method we describe would lead to a treatment for Down syndrome,” said study co-author Dr. David Russell, from the University of Washington’s Department of Medicine.  “What we are looking at is the possibility that medical scientists could create cell therapies for some of the blood-forming disorders that accompany Down syndrome.”