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  1. 1. PRENATAL DIAGNOSIS Dr.Ankur Puri 25/10/12 1
  2. 2. INTRODUCTIONThe frequency of inherited malformations as well as genetic disorders in newborns account for around 3- 5%.These frequency is much higher in early stages of pregnancy, because serious malformations & genetic disorders usually lead to spontaneous abortion. Thereafter, taking into account the severity of the disorders the decision should be taken in regard of subsequent course of the pregnancy , possibilities of treatment, parents acceptation of a handicapped child but also, in some cases the possibility of termination of the pregnancy according to MTP act 1971. 2
  3. 3. • Definition:‘Prenatal diagnosis is defined as the detection of abnormalities in the fetus, before birth’ 3
  4. 4. The purpose of prenatal diagnosis is not simply to detectabnormalities in fetal life and allow termination.It rather have following goals : Provide a range of informed choice to the couples at risk of having a child with abnormality. Provide reassurance & remove anxiety, especially among high risk groups. Allow couples at high risk to know that the presence or absence of the disorder can be confirmed by testing. Allow the couples the option of appropriate management ( psychological, pregnancy/delivery, postnatal) To enable prenatal treatment of the fetus. 4
  5. 5. Some Disorders for which PRENATAL DIAGNOSIS is available:1. Congenital malformations2. Chromosomal disorders3. Non genetic Fetal disorders *Fetal infections, Immune hydrops, DM,Fetal effects of maternal drugs e.g valproic acid4. Single gene disorders -Multiple malformation synd *Holt oram, Craniosynostosis, Orofacial digital synd -Hematological disorders *Thalassemias, Hemoglobinopathies, Hemophilia -Metabolic Disorders *Tay sach, Wilson, MPS, CAH. -Neuromuscular disorders 5 *Huntington chorea, Myotonic dystrophy, DMD, Fragile X
  6. 6. -Renal Disoders *AD/AR polycystic kidney disease-Connective tissue dis / Skeletal dysplasia * Osteogenesis imperfecta, Ehlers Danlos, Achondroplasia, Marfan.-Skin disorders *Epidermolysis bullosa, Ichthyosis, Ectodermal dysplasia 6
  7. 7. INDICATIONS OF PRENATAL DIAGNOSIS1. Advanced maternal age.2. Previous child with a chromosomal abnormality.3. Family history of a chromosomal abnormality.4. Family history of a single gene disorder.5. Family history Neural Tube Defect.6. Family history of other congenital structural abnormality.7. Abnormalities identified in pregnancy.8. Other risk factors(consanguinity,poor obs. History,maternal history) 7
  8. 8. 1. Advanced maternal age• It is the common indication for prenatal diagnosis.• As a woman’s age increases, so does the risk for chromosome aneuploidy in the fetus.• Most centers offer Amniocentesis or CVS to a women aged >35yrs,although no standard criterion exists at what age women should be investigated. 8
  9. 9. 2. Previous child with a chromosomal abnormality• Previous child with Down’s So due to non dysjunction or unbalanced translocation will give a risk in subsequent pregnancy as, of mother’s age related risk is plus 5%.• If one of the parents have balanced chromosomal rearrangement (translocation, inversion) causing a serious problem for a previous child due to unbalanced rearrangement, then recurrence risk is between 1-2% & 15- 20 %.This risk will depend on nature of rearrangement & nature of segment involved. 9
  10. 10. 3. Family history of a chromosomal abnormality• Usually no increase in risk compared to general population since most chromosomal disorders will arise as a result of disjunction than familial rearrangement.• A history of Down’s So.• However each situation should be confirmed by nature of chromosome abnormality in affected individual or urgent chromosomal analysis from blood of related parents if normal,no invasive tests. 10
  11. 11. 4. Family history of a single gene disorder.• A previous affected child• Affection of one of the parents• Postive family history.• Have a 25-50 % recurrence and prenatal diagnosis should be offered as many can be diagnosed by DNA analysis and Biochemical testing(achondroplasia, huntington disease, neurofibromatosis) 11
  12. 12. 5. Family history Neural Tube Defect.• In the 1st & 2nd degree relatives the risk should be determined• High risks were diagnosed by Amniocentesis & AFP assessment.• Ultrasound with MSAFP is method of choice now a days 12
  13. 13. 6. Family history of other congenital structural abnormality.• Evaluation of family pedigree• Calculation of the risk.• If increased risk-detailed ultrasound can be offered between 16-18 weeks of pregnancy, it will detect most serious defects (cranial, cardiac, renal & limb deformity) 13
  14. 14. 7. Abnormalities identified in pregnancy• Uncertainty of maternal serum screening & fetal anomaly scanning can make invasive procedure for the diagnosis more necessary.• Poor fetal growth can be indication for prenatal chromosome analysis as well as for confirmation of a serious & non viable abnormality. 14
  15. 15. 8. Other risk factors(consaguinity,poor obs. History,maternal history)• Parental consanguinity leading to hereditary disorder or congenital anomalies(offer a detailed USG)• Poor obst history as recurrent miscarriage or still birth indicating high risk in future pregnancy(offer USG of fetus & chromosomal analysis of parents)• Maternal illness as poorly controlled DM or maternal epilepsy treated with some drugs such as sodium valproate(offer a detailed USG) 15
  16. 16. METHODS OF PRENATAL DIAGNOSISNON INVASIVE INVASIVE TECHNIQUES TECHNIQUES Fetal visualizationFetal visualization Fetal tissue samplingMaternal serum screening CytogeneticsSeparation of fetal cells Molecular genetics from the mothers blood 16
  17. 17. NON INVASIVE TECHNIQUESFETAL VISUALISATION1. ULTRASONOGRAPHY2. FETAL ECHOCARDIOGRAPHY3. MAGNETIC RESONANCE IMAGING (MRI) 17
  18. 18. FETAL VISUALISATION1. ULTRASONOGRAPHY : -It is a noninvasive procedure for imaging fetal anatomy & is harmless to both the fetus and the mother. -The developing embryo can first be visualized at about 6 weeks gestation. Recognition of the major internal organs & extremities to determine if any are abnormal can best be accomplished between 16 to 20 weeks gestation. - Thus USG is used in the 2nd trimester to identify major fetal structural anomalies & fetal anatomical markers. -Ultrasound also is used to guide invasive sampling, such as amniocentesis, CVS, cordocentesis, & various fetal biopsies 18
  19. 19. US markers of fetal congenital abnormalities or genetic syndromes found in first trimester scanning [at 11-13weeks gestation] 19
  20. 20. 2D US 20
  21. 21. 2D US 21
  22. 22. 2D US 22
  23. 23. 2D US 23
  24. 24. 2D US 24
  25. 25. 3D & 4D US• In recent years three-dimensional ultrasound (3D) & four-dimensional ultrasound (4D) have started to play an increasing role in prenatal diagnosis. They can be applied in assessing facial features, central nervous system abnormalities and skeletal defects 25
  26. 26. ‹#›
  27. 27. Ultrasonography cont…• Although an ultrasound examination can be quite useful to determine the size & position of the fetus, the size & position of the placenta, the amount of amniotic fluid, & the appearance of fetal anatomy, there are limitations to this procedure as findings are based upon views of the fetus, the estimated gestational age, sonographer experience, & the degree of anomaly severity. 27
  28. 28. FETAL VISUALISATION2. FETAL ECHOCARDIOGRAPHY -Fetal echocardiography is capable of diagnosing most significant congenital heart lesions as early as 17-19 wk of gestation. -When this technique is used with duplex or color flow Doppler, it can identify a number of major structural cardiac defects & rhythm. -Fetal echocardiography is recommended in cases where cardiac defects are suspected. 28
  29. 29. FETAL ECHOCARDIOGRAPHY : cont… 29
  30. 30. FETAL VISUALISATION3. MAGNETIC RESONANCE IMAGING (MRI)• MRI is used in combination with ultrasound, usually at or after 18 weeks‘ gestation. MRI provides a tool for examination of fetuses with large or complex anomalies, and visualization of the abnormality in relation to the entire body of the fetus. Apparently MRI is a risk-free method 30
  31. 31. MAGNETIC RESONANCE IMAGING (MRI) 31
  32. 32. MATERNAL SERUM SCREENING Maternal serum screening is used to identify women at increased risk of having a child with trisomies 18 or 21 or an open neural tube defect (NTD), while posing no risk to the pregnancy. Screening in the first trimester involves the measurement of PAPP-A (pregnancy associated plasma protein A) & free b HCG (beta human chorionic gonadotropin) levels in maternal serum. These measurements used in conjunction with USG scanning that includes assessment of USG markers such as nuchal translucency (NT) thickness & absence/presence of the nasal bone (NB) gives a detection rate of abt. 85% while alone the detection rate with PAPP-A & bHCG was around 65 % 32
  33. 33. MATERNAL SERUM SCREENING Levels of MSAFP ( alpha Feto protein ), human chorionic gonadotrophin (HCG) & unconjugated oestriol (UE3) are measured between 15 & 18 weeks gestation. These substances are of fetal origin & cross from the amniotic fluid into maternal circulation via the placenta. Low maternal serum AFP, low UE3 and/or elevated HCG levels are associated with increased risks of fetal Down syndrome, whereas low levels of all three substances suggests increased risks for trisomy 18 or triploidy. High levels of AFP are associated with increased risk of neural tube & abdominal wall defects; while high levels of HCG can be associated with increased risk for pregnancy complications. 33
  34. 34. MATERNAL SERUM SCREENING Down So :1st Trimester Screening Tests• Maternal Serum Markers -Preg. asso. Placental Protein A (PAPP-A) -Free ß hCG• Fetal Marker- Nuchal thickness2nd Trimester Screening Tests• Maternal Serum Markers -AFP -E3 Triple test 70% -hCG Quadruple -Inhibin A test 34
  35. 35. MATERNAL SERUM SCREENING Trisomy 18 or Triploidy : The level of all three substances ( MSAFP, UE3 and HCG ) is low in trisomy 18 35
  36. 36. MATERNAL SERUM SCREENING Neural Tube Defects & Abdominal Wall Defects :• AFP is produced by the yolk sac & later by the liver; it enters the amniotic fluid & then the maternal serum via fetal urine. Therefore MSAFP level can be used to determine the AFP levels from the fetus.• In the condition of an open NTD (eg, anencephaly, spina bifida) & abdominal wall defects in the fetus, AFP diffuses rapidly from exposed fetal tissues into amniotic fluid, and the MSAFP level rises.• Also, a NTD can be distinguished from other fetal defects, such as abdominal wall defects, by the use of an Acetylcholinesterase test carried out on amniotic fluid. If the level of acetylcholinesterase rises along with AFP, it is suspected as a condition of a NTD. 36
  37. 37. MATERNAL SERUM SCREENING• AFP levels are also elevated when the fetus has congenital nephrosis, or intestinal atresias.• However, the MSAFP levels also increase with gestational age, gestational diabetes, twins, pregnancies complicated by bleeding, & in association with intrauterine growth retardation. 37
  38. 38. Separation of fetal cells from the mothers blood A technique currently being developed for clinical use involves isolating fetal cells from maternal blood to analyse fetal chromosomes and/or DNA. Ordinarily, only a very small number of fetal cells enter the maternal circulation; but once they enter,can be readily identified, they will be accessible for analysis by a variety of techniques, without the risks of complications or miscarriage associated with invasive procedures (CVS & amniocentesis). These cells can be collected safely from approximately 12- 18 weeks gestation onward. Nucleated fetal red blood cells (erythroblasts) are currently the ideal candidates for analysis, although leucocytes & trophoblast cells may also be identified 38
  39. 39. Separation of fetal cells from the mothers blood Fetal blood cells can then be analyzed for the diagnosis of genetic disorders using FISH, PCR etc. Fetal cells separated from a mothers blood have been successfully used in the diagnosis of cystic fibrosis, sickle cell anemia, and thalassemia in a fetus. A. Maternal RBCs B. Fetal RBCs (nucleated) 39
  40. 40. INVASIVE TECHNIQUESFetal visualizationFetal tissue samplingCytogeneticsMolecular genetics 40
  41. 41. INVASIVE TECHNIQUESFetal visualization -Embryoscopy -Fetoscopy 41
  42. 42. Fetal visualization-Embryoscopy Embryoscopy is performed in the first trimester. In this technique, a rigid endoscope is inserted via the cervix in the space between the amnion and the chorion, under sterile conditions and ultrasound guidance, to visualize the embryo for the diagnosis of structural malformations. 42
  43. 43. Fetal visualization-Embryoscopy 43
  44. 44. Fetal visualization-Fetoscopy Fetoscopy is performed during the second trimester (after 16 weeks’ gestation). In this technique, a fine-caliber endoscope is inserted into the amniotic cavity through a small maternal abdominal incision, under sterile conditions and ultrasound guidance, for the visualization of the embryo to detect the presence of subtle structural abnormalities. It also is used for fetal blood and tissue sampling. Fetoscopy is associated with a 3-5% risk of miscarriage; 44
  45. 45. Fetal visualization-Fetoscopy 45
  46. 46. Fetal Tissue Sampling Amniocentesis Chorionic villus sampling (CVS) Percutaneous umbilical blood sampling (PUBS) Percutaneous skin biopsy Other organ biopsies, including muscle & liver biopsy 46
  47. 47. Fetal Tissue Sampling Amniocentesis : Amniocentesis is an invasive, well-established, safe, reliable, & accurate procedure & can be performed at 10- 14 weeks of gestation (early amniocentesis) but usually done at 16-18 weeks of gestation. Although early amniocentesis is ass. with a pregnancy loss rate of 1 – 2 % & an increased incidence of clubfoot. It is performed under ultrasound guidance. A 22-gauge needle is passed through the mothers lower abdomen into the amniotic cavity inside the uterus, & 10- 20 mL of amniotic fluid ( that is replaced by fetus within 24hrs ) that contains cells from amnion, fetal skin, fetal lungs, and urinary tract epithelium are collected. 47
  48. 48. Fetal Tissue Sampling Amniocentesis : 48
  49. 49. Fetal Tissue Sampling Amniocentesis :1. The Cells are grown in culture for chromosomal, biochemical, & molecular biologic analyses.2. The Supernatant amniotic fluid is used for the measurement of substances such as AFP, AChE,bilirubin & pulmonary surfactant3. In the third trimester of pregnancy, the amniotic fluid can be analyzed for determination of fetal lung maturity. The results of cytogenetic and biochemical studies on amniotic cell cultures are more than 90% accurate. Risks with amniocentesis are rare but include 0.5-1.0% fetal loss and maternal Rh sensitization. 49
  50. 50. Fetal Tissue Sampling Chorionic villus sampling (CVS) : Under USG guidance, a sample of placental tissue is obtained through a catheter places either transcervically or transabdominally. Performed at or after 10 wks’ gestation,CVS provides the earliest possible detection of a genetically abnormal fetus through analysis of trophoblast cells. Transabdominal CVS can also be used as late as the 3rd trimester when amniotic fluid is not available or when fetal blood sampling cannot be performed. CVS, if preformed before 10 wks’ gestation , can be ass. with an increased risk of fetal limb reduction defects & oromandibular malformations. 50
  51. 51. Fetal Tissue Sampling Chorionic villus sampling (CVS) : Transabdominal 51
  52. 52. Fetal Tissue Sampling Chorionic villus sampling (CVS) : Transcervical 52
  53. 53. Fetal Tissue Sampling Chorionic villus sampling (CVS) : Direct preparations of rapidly dividing cytotrophoblasts can be prepared, making a full karyotype analysis available in 2 days. Although direct preparation minimize maternal cell contamination, most centers also analyse cultured trophoblast cells, which are embryologically closer to the fetus.This procedure takes 8 – 12 days. In approximately 2% of CVS samples, both karyotypically normal & abnormal cells are identified.B’coz CVS acquired cells reflect placental constitution, in these cases, amniocentesis is typically performed as a followup study to analyze fetal cells.Approximatally 1/3rd of CVS mosaicisms are confirmed in the fetus through amniocentesis. 53
  54. 54. Fetal Tissue Sampling Percutaneous umbilical blood sampling (PUBS) (cordocentesis) PUBS is preformed under USG guidance from the 2nd trimester until term. PUBS can provide diagnostic samples for cytogenetic, hematologic, immunologic, or DNA studies: it can also provide access for treatment in utero. An anterior placenta facilitates obtaining a sample close to the cord insertion site at the placenta. Fetal sedation is usually not needed. PUBS has a 1% - 2% risk of fetal loss, along with complication that can lead to a preterm delivery in another 5%. 54
  55. 55. Fetal Tissue Sampling Percutaneous umbilical blood sampling (PUBS) (cordocentesis) 55
  56. 56. Fetal Tissue Sampling Percutaneous skin biopsy (Preimplantation Biopsy or Preimplantation Genetic Diagnosis) : The most frequent candidates are parents with family histories of serious monogenic disorders & translocations, who are therefore at increased risk for transmitting these conditions to future generations. Polar body & blastomere testing are the two primary methods of PGD. In Polar Body Testing, positive test results in two polar bodies ensure that the egg itself is unaffected – therefore, the mutation has segregated to the polar body, not to the developing ovum. Once an egg is found to be unaffected, it is fertilized via traditional in vitro fertilization (IVF) & implanted into the uterus. 56
  57. 57. Fetal Tissue Sampling Percutaneous skin biopsy (Preimplantation Biopsy or Preimplantation Genetic Diagnosis) : Blastomere PGD first requires traditional in vitro fertilization, after which cells are grown to the 8-cell stage. One or two cells are harvested & analysed, & an unaffected blastocyst is implanted into the uterus. An advantage to preconception testing over traditional postconception prenatal diagnosis is that it allows parents to avoid the possibility of receiving abnormal prenatal diagnosis results, & thus the difficult decisions associated with pregnancy management and/or maintenance. PGD can be laborious, time-consuming & expensive. Complicating factors include a high rate of polyspermia, & a small amount of DNA in polar bodies (making it difficult to amplify) which can produce less definitive test results. 57
  58. 58. Fetal Tissue Sampling Other organ biopsies, including muscle & liver biopsy : Fetal liver biopsy is best performed between 17-20 weeks gestation under ultrasound guidance. Fetal liver biopsy is needed to diagnose inborn errors of metabolism, such as glucose-6-phosphatase deficiency , glycogen storage disease type IA & nonketotic hyperglycemia. Fetal muscle biopsy is carried out under ultrasound guidance at about 18 weeks gestation to analyze the muscle fibers histochemically for prenatal diagnosis of Becker-Duchenne muscular dystrophy. 58
  59. 59. Cytogenetic Investigations Chromosome Analysis ( Karyotype Analysis ) Fluorescence in situ Hybridization (FISH) 59
  60. 60. Cytogenetic Investigations Chromosome Analysis : Chromosome analysis is a technique used to identify aneuploidy, microdeletions, microduplications & major structural aberrations. The most common method of detecting aneuploidy is karyotype analysis, wherein metaphase cells are examined microscopically & the number of chromosomes counted. Typically 10–15 cells are analysed to rule aneuploidy in or out. 60
  61. 61. Cytogenetic Investigations Chromosome Analysis Karyotype Analysis : Each chromosome pair has a unique banding pattern that can be seen with various stains. The most common method of karyotype analysis is Giemsa (G) banding, wherein chromosomes are denatured (with trypsin), revealing a pattern of light & dark bands. Counting the number of staining chromosomes allows for detection of aneuploidies. Analysing for the absence, presence, rearrangement, etc. of these bands allows for detection of larger deletions, duplications and structural aberrations. Although G banding is typically used first to analyse prenatal specimens, various other banding techniques (including quinacrine (Q), reverse (R), centromeric heterochromatin (C) & high-resolution banding) may be used to analyse different portions of particular chromosomes. 61
  62. 62. Cytogenetic Investigations• Chromosome Analysis ( Karyotype Analysis ) Normal Karyotype 62
  63. 63. Cytogenetic Investigations• Chromosome Analysis ( Karyotype Analysis ) Down Syndrome Karyotpe 63
  64. 64. Cytogenetic Investigations Fluorescence in situ Hybridization (FISH) : 64
  65. 65. Cytogenetic Investigations Fluorescence in situ Hybridization (FISH) : FISH is mainly used to detect the presence or absence of microdeletions, microduplications & aneuploidy without the full effort associated with DNA sequencing or complete karyotype analysis This three-step technique allows specific DNA sequences or chromosomes to be visualized microscopically.1. A specific, single-stranded DNA probe is hybridized to its complementary, target DNA sequence, while the cell is in prophase, metaphase or interphase;2. fluorescent antibodies are then hybridized to the probe DNA sequence;3. finally, the fluorescent signals are examined under the microscope. 65
  66. 66. Cytogenetic Investigations Fluorescence in situ Hybridization (FISH) : 66
  67. 67. Cytogenetic Investigations Fluorescence in situ Hybridization (FISH) :Human chromosome 1 painting probe (green) Terminal band painting probe centromere probe 67
  68. 68. Cytogenetic Investigations Fluorescence in situ Hybridization (FISH) : FISH analysis for common aneuploidies (involving chromosomes 13, 18, 21, X and Y) is often performed by simultaneously applying specific multicoloured centromeric probes. In fetal trisomies, three probes are present for a specific chromosome, while monosomies show only one. (a).A nucleus has been hybridized with probes for chromosomes 18 (aqua), X (green) and one Y(red).(b) A nucleus has been hybridized with probes for chromosomes 13 (green) and 21 68 (red).
  69. 69. Cytogenetic Investigations Fluorescence in situ Hybridization (FISH) :Microdeletions/microduplications detectable by fluorescence in situ hybridization (FISH) DISORDER CHROMOSOMAL BAND FINDING Angelman syndrome 15q12(maternal) Microdeletion Duchenne Muscular Dystrphy Xp21 Microdeletion Prader-Willi Syndrome 15q12(paternal) Microdeletion Retinoblastoma 13q14 Microdeletion α- thalassemia 16p13 Microdeltion WAGR syndrome 11q13 Microdeletion 69
  70. 70. Molecular genetics Direct DNA Analysis Linkage Analysis(indirect DNA analysis) DNA Sequencing 70
  71. 71. Molecular genetics Direct DNA analysis :• Direct mutation analysis involves analysing a target segment of DNA for the presence of a specific mutation. Like FISH, it requires knowledge of the correct sequence for the specific gene or DNA segment before analysis. Once known, the sample sequence may be compared to the known, ‘model’, genomic sequence in a variety of methods, as described below : Mutation analysis with restriction enzymes. Sequencing of restriction enzyme products. Allele-Specific Oligonucleotide (ASO) analysis. 71
  72. 72. Molecular genetics Direct DNA analysis : Mutation analysis with restriction enzymes : If the putative mutation is known to alter the recognition for a splice site, direct analysis by restriction enzyme assay is possible. The presence of a mutation can be detected by digesting control and sample DNA with the same restriction enzymes (known to cut the DNA at a specific splice site) and then analysing resultant DNA fragments (called Restriction Fragment Length Polymorphisms, or RFLPs ) for differences by Southern blotting. Those segments containing mutation(s) at or near a splice site are identifiable because they were not cut by a restriction enzyme, and are therefore longer, appearing higher on the Southern blot gel. (Longer fragments do not migrate as quickly or as far as shorter fragments.) This technique is used in genetic testing for sickle cell anaemia. 72
  73. 73. Mutation analysis with restriction enzymes : 73
  74. 74. Molecular genetics  Direct DNA analysis :  Sequencing of restriction enzyme products :• Disorders secondary to deletion of DNA ( e. g α thalassemia, DMD, CF & growth harmone deficiency) can be detected by the altered size of DNA segments produced following a Polymerase Chain Reaction(PCR)• DNA sequences that have been cut with restriction enzymes can also be sequenced by a specialized amplification technique. Copies of a particular piece of DNA(cut by restriction enzymes) are placed into four vials & amplified by polymerase chain reaction (PCR).• Fragments from the vials are then allowed to migrate, in parallel, down a Southern blot gel.• The shortest fragments travel furthest, the longer segments remain closer to the top. From top to bottom, the banding pattern produced represents fragments that decrease in size by one nucleotide base. The DNA sequence can therefore be read from the shortest, single- base strand at the bottom of the gel, up to the entire sequence length at the top 74
  75. 75. Molecular genetics Direct DNA analysis : Allele-Specific Oligonucleotide (ASO) analysis :• Direct detection of a DNA mutation can also be accomplished by allele specific oligonucleotide analysis.• If the PCR –amplified DNA is not altered in size by deletion or insertion, recognition of mutated DNA sequence can occur by hybridization with the known mutant allele.• ASO analysis allows direct DNA diagnosis of Tay-Sachs Disease, alpha & beta thalassemia, Cystic fibrosis & phenylketonuria 75
  76. 76. Molecular genetics  Linkage Analysis(indirect DNA analysis) :• Linkage analysis is a means of indirectly detecting a patient’s mutation status, when several family members are known to be affected with the same genetic disorder, & when an exact mutation is not known.• DNA from affected & unaffected family members is analysed for polymorphisms such as microsatellite repeats, restriction fragment length polymorphisms (RFLPs) and variable number tandem repeats (VNTRs) 76
  77. 77. Molecular genetics DNA Sequencing :• DNA sequencing for many disorders has revealed that a multitude of different mutations within a gene can result in same clinical disease.• For Example,Cystic Fibrosis can result from more than 1,000 different mutations.• Therefore, for any specific disease, prenatal diagnosis by DNA testing may require both Direct & Indirect methods. 77
  78. 78. ®PRENATAL TREATMENT• In the most situations the diagnosis of prenatal abnormalities has a subsequent option of termination of the pregnancy.• While this applies in most situations, there is cautious optimism that with the advent of gene therapy prenatal diagnosis will, in time, lead to effective treatment in utero.• Example-Treatment of the autosomal recessive disorder-Congenital Adrenal Hyperplasia (CAH). Affected female are born with virilisation of the external genitalia. There is an evidence that this can be prevented by powerful steroid therapy at early gestational age. 78
  79. 79. ETHICS & LEGAL ASPECTS• Before screening or testing pregnancies for underlying genetic disorders, it is important to consider the ethics of a given situation. Genetic diagnosis may affect decisions about maintaining or ending a pregnancy, counselling should be done in such cases.• Prenatal diagnosis in high risk cases should be done at appropriate gestational age as according to MTP act pregnancies can be terminated only upto 20 weeks of gestation when Substantial risk of physical or mental abnormalities in the fetus as to render it seriously handicapped. 79
  80. 80. SUMMARY 80
  81. 81. 81
  82. 82. THANK YOU 82

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