Essential Genetics for Obstetricians
 

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Clinical genetics is one of the most rapidly advancing fields in medicine. Spectacular progress has been achieved in this century with unravelling of the entire draft sequence of the human genome. A ...

Clinical genetics is one of the most rapidly advancing fields in medicine. Spectacular progress has been achieved in this century with unravelling of the entire draft sequence of the human genome. A major contribution of these advances has been in diagnosis, management and prenatal diagnosis of genetic disorders as treatment in most cases is difficult or impossible and where available beyond the means of most families. Genetic technology is advancing rapidly, bringing new, safer and more sensitive ways to diagnose genetic conditions pre- and postnatally. These advances will bring about profound changes in the way we deliver obstetric services to women and their families. Diagnosing a genetic disorder not only allows for disease-specific management options but also has implications for the affected individual's entire family. Hence, a working understanding of the underlying concepts of genetic disease is important for all practicing clinicians. Although it is impossible to know all aspects of clinical and molecular genetics, basic knowledge of certain topics is a must for all practicing obstetrician/gynecologists.

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Essential Genetics for Obstetricians Document Transcript

  • 1.                                                                                                                                        ESSENTIA        AL GENETTICS FOR O      OBSTETRICCIANS
  • 2. Review Article BACKGROUND Genetic diseases are not as rare as once believed. In fact, genetic disease is a major cause of illness and death. Approximately 2% to 3% of all pregnancies result in a neonate with a serious genetic disease or a birth defect that can cause disabilities, mental retardation, and in some cases early death. Genetic factors are present from conception, and their expression may vary throughout development, whereas environmental influences are changing constantly. Many conditions previously thought to be nongenetic are now understood to be multifactorial diseases with the contribution of various genetic and environmental determinants being recognized increasingly. A genetic disorder is a disease that is caused by an abnormality in an individual’s DNA. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes. Genetically determined disorders often are subdivided into 3 major groups: single gene, chromosome, and multifactorial (polygenic) diseases. Somatic cell genetic defects play a role in human cancer and constitute a fourth group. Single-gene disorders This type is caused by changes or mutations that occur in the DNA sequence of one gene. Individually single gene disorders are rare (1 in 10,000-15,000) but collectively they can affect upto 1-2% of all births. Some conditions are highly prevalent in selected populations like sickle cell disease in Africans, thalassemia in the geographical belt extending from the Mediterranean countries to South East Asia. Since the late 1970s, the number of disorders classified as single gene has increased from an estimated 2500 to approximately 14,000 (as ofApril 2003). Of these 14,000 single gene disorders, 93.7% are classified as autosomal, 5.6% as X-linked, and 0.7% as other [1]. Considering that the human genome consists of approximately 30,000 genes, the number of diseases classified as monogenic is expected to increase. Compared to general population, the risk of occurrence of genetic diseases in affected families is very high depending on the pattern of inheritance. However, previous occurrence of the disease in the family is not necessary. The defect may arise de novo for the first time in any individual or there may be silent carriers in the family who give birth to an affected child without a positive family history (autosomal recessive disorder). The four most common patterns of mendelian inheritance are based primarily on two factors: on which type of chromosome (autosome or sex chromosome) the gene locus is found and whether the phenotype is expressed only when both chromosomes of a pair carry the abnormal allele (recessive) or whether the phenotype can be expressed when just one chromosome carries the mutant allele (dominant). Single-gene disorders are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked. If a person carries the dominant gene for a disease, he 251 Apollo Medicine, Vol. 6, No. 3, September 2009 ESSENTIAL GENETICS FOR OBSTETRICIANS Neerja Gupta Consultant Clinical Genetics, Department of Genetics, Indraprastha Apollo Hospitals, Sarita Vihar, New Delhi 110 076, India. e-mail- neerjaagarwal@yahoo.co.in Clinical genetics is one of the most rapidly advancing fields in medicine. Spectacular progress has been achieved in this century with unravelling of the entire draft sequence of the human genome. A major contribution of these advances has been in diagnosis, management and prenatal diagnosis of genetic disorders as treatment in most cases is difficult or impossible and where available beyond the means of most families. Genetic technology is advancing rapidly, bringing new, safer and more sensitive ways to diagnose genetic conditions pre- and postnatally. These advances will bring about profound changes in the way we deliver obstetric services to women and their families. Diagnosing a genetic disorder not only allows for disease-specific management options but also has implications for the affected individual’s entire family. Hence, a working understanding of the underlying concepts of genetic disease is important for all practicing clinicians. Although it is impossible to know all aspects of clinical and molecular genetics, basic knowledge of certain topics is a must for all practicing obstetrician/gynecologists. Key Words: Genomics, Prenatal diagnosis, Genetics, Genetic disorders.
  • 3. Apollo Medicine, Vol. 6, No. 3, September 2009 252 Review Article or she will usually have the disease and each of the person’s children will have a 1 in 2 (50%) chance of inheriting the gene and getting the disease. Diseases caused by a dominant gene include achondroplasia (a form of dwarfism), Marfan syndrome (a connective tissue disorder), and Huntington disease (a degenerative disease of the nervous system). People who have one recessive gene for a disease are called carriers, and they don’t usually have the disease but they are at risk of producing children with autosomal recessive diseases (Fig. 1) such as Cystic fibrosis, Sickle cell anemia , and thalassemia are caused by recessive disease genes from both parents coming together in a child. Some recessive genetic variants are carried only on the X chromosome, which means that usually only boys can develop the disease because they have only one X chromosome. Girls have two X chromosomes, so they would need to inherit two copies of the recessive gene to get the disease. Examples are hemophilia and Duchenne muscular dystrophy. In X-linked dominant inheritance, both male and female children have a 1 in 2 risk of inheriting the mutant allele from the affected mother and thus being affected as well. Sons of an affected male do not inherit the condition, whereas all daughters are affected clinically. Chromosomal Chromosomes are carriers of genetic material. Any abnormality in chromosome structure as missing or extra copies or gross breaks and rejoining (translocations) can result in disease. Some chromosome anomalies are “balanced” and include the full complement of genetic material in a rearranged form. Although balanced rearrangements have been associated with infertility and medical complications (perhaps because of breakpoints in important genes or secondary to positional effects of gene expression), most people with balanced chromosome rearrangements are healthy. Statistically, numerical chromosome abnormalities are the most common type of chromosome disorder. Chromosome aneuploidy occurs when there is other than a multiple of the typical haploid set. About 60% of the chromosomal abnormalities are spontaneously aborted in the first trimester. This prevalence goes down approximately 5% in the late abortions and stillbirths. At birth, only 0.6% of the newborns have been found to have a chromosomal abnormality.Although trisomy 16 is the most common autosomal trisomy in miscarriages, trisomies 21 (Down syndrome), 18 (Edwards syndrome), and 13 (Patau syndrome) are seen at considerable frequencies in newborns. Of these, Down syndrome or trisomy 21 is the commonest one. Notably, the risk of having a newborn with any of these chromosome trisomies increases with maternal age, although not all chromosome aneuploidy is associated with maternal age. Turner syndrome (45, X) is most often caused by loss of the paternal X chromosome and is present in 1% of all conceptions; however, 98% result in miscarriage. Chromosome polyploidy occurs when the number of chromosome sets is other than two. The most common type of chromosome polyploidy is triploidy (69 chromosomes), present in 1% to 3% of all conceptions. Triploidy is a sporadic occurrence and most commonly happens when 1 haploid egg is fertilized by 2 haploid sperm. Another group of chromosome disorders includes those resulting in genetic imbalance despite retention of the normal number of 46 chromosomes. This group includes chromosome translocations in their unbalanced form, deletions, and duplications. In these situations, there is some net loss or gain of genetic material. Chromosomal disorders are often suspected by the presence of mental retardation, facial dysmorphism, multiple congenital abnormalities, and failure to thrive.Fig.1. Autosomal recessive inheritance
  • 4. Review Article 253 Apollo Medicine, Vol. 6, No. 3, September 2009 Standard chromosome analysis using G-banding allows only the detection of relatively large structural rearrange- ments (3-4 megabases) and depends on the band resolution. Multifactorial Many genetic disorders appear familial but do not follow a single gene pattern of inheritance. These multifactorial (polygenic, complex) disorders are the result of a combination of alterations in multiple genes with varying degrees of effect that act in concert with environmental factors, thus producing a clinical phenotype when a developmental threshold is reached [2]. These disorder occur with high frequency in close relatives as compared to the general population. Examples include heart disease, hypertension, Alzheimer’s disease, arthritis, diabetes, and obesity. Another important group of multifactorial disorders is congenital malformation. Recent advances in various molecular techniques like array CGH etc have opened the possibility of identifying major genes that can predispose to these disorders. Genetic Counseling An accurate diagnosis of the disorder is very essential for any genetic counseling [3-5]. It is defined as “the process by which patients or relatives at risk of a disorder are advised of the consequences of the disorder, the probability of developing and transmitting it, and ways in which this can be ameliorated” [1]. It also helps the individual or family to choose a course of action which seems to them appropriate in view of their risk, their family goals, and their ethical and religious standards and act in accordance with the decision and also to make the best possible adjustments to the disorder in an affected family member and/ or to the risk of recurrence of that disorder. There are certain situations which can be identified before or after conception in which genetic counseling and prenatal diagnosis may be required. These indications are • Advanced maternal age (>35 years) • Recurrent miscarriages (3 or more) / Infertility / primary ammennorhea • Previous child with – dysmorphism /single or multiple malformations like cardiac renal, brain defects/short stature/ neuromuscular disorder/neurogenetic disorder/ Metabolic disorder/Unexplained MR/Cerebral Palsy / autism / Chromosomal abnormality / Deafness/ thalassemia/Hemophilia • Previous unexplained still birth/s, neonatal or infantile deaths with or without congenital malformations • Family history of a genetic disorder like any chromosomal abnormality like Down syndrome, thalassemia, spinal muscular atrophy, hemophilia, congenital deafness or Gaucher disease • Consanguinity especially with a history of suspected genetic disorder • Maternal disease like diabetes, hypothyroidism to identify high risk fetuses through level II ultrasound • Positive maternal serum screen either first or second trimester/Abnormal fetal ultrasound • Exposure to known or suspected teratogen during pregnancy • Amniotic fluid abnormalities in second/third trimester especially in association with growth retardation • Maternal Infection (TORCH infection) Steps in an antenatal case Management [3,4] The skills required to make a genetic diagnosis are similar to those used for more common health problems, including history taking, physical examination, and proper laboratory testing. History- The pregnancy history of the patient’s mother might disclose maternal disease potentially causative of or related to the fetal condition, as seen in certain metabolic disorders such as untreated maternal PKU or fatty acid oxidation disorders. Sometimes, maternal disorders (diabetes) environmental or drug exposures (valproate, warfarin etc) during pregnancy can cause multiple malformations such as in fetal valproate syndrome or warfarin embryopathy. Medical history of maternal disorders like SLE, hypothyroidism is also important for better fetal outcome. Family History - A thorough family history includes detailed information on relatives’ ages, current and past medical health (including developmental or learning problems), birth defects, obvious dysmorphism, and surgeries. Specifically, questions about miscarriages, stillbirths, and infant deaths, as well as infertility, should be asked. For deceased family members, age and cause of death should be documented. The racial and ethnic background is of importance in identifying higher risk groups. In addition, the possibility of consanguinity in the family history should be explored when clinically relevant. Drawing a family tree (pedigree) that symbolically (Fig.2)
  • 5. Apollo Medicine, Vol. 6, No. 3, September 2009 254 Review Article represents the family and demonstrates relationships between affected family members is an efficient and highly informative exercise. History of any genetic disorders in the family- Family photographs or medical records may be of help, particularly if other family members are suspected to have the same genetic disorder. Examination of the couple is required especially when there is a family history of a particular genetic disorder like neurofibromatosis, tuberous sclerosis, or incontinentia pigmenti. Investigations are done based on the indications apart from routine antenatal screening. Specialized investigation The importance of precise diagnosis for genetic counseling cannot be over emphasized. However, specialized tests like chromosomal analysis, enzyme analysis, and DNA analysis are required to arrive at a final diagnosis. Before these tests are ordered, information should be obtained on the type and volume of the specimen required (blood, urine, fibroblasts, amniocytes), type of tube in which the specimen should be kept, and conditions under which the specimen should be sent (Appendix- A). DNA based tests (Molecular tests) DNA testing investigates alterations in a gene that result in disease. Confirmed molecular diagnosis in index case would also help in carrying out prenatal diagnosis (by amniocentesis or chorionic villi sampling) for the respective disorder. Unless the type of mutation/s in the proband or carrier parents is identified, prenatal diagnosis is not feasible. It should preferably be identified before next pregnancy. Examples of widely available molecular genetic tests include thalassemias , muscular dystrophies, spinal muscle atrophy, Fragile X syndrome, hemophilia A and B, cystic fibrosis, albinism, achondroplasia etc. Chromosomal analysis (Cytogenetics) Chromosomal abnormalities can be diagnosed after birth using a blood test, or before birth using prenatal tests (amniocentesis or chorionic villi sampling). Tissues most commonly used are lymphocytes and amniocytes. Any abnormal finding has its own implication and management. Cytogenetic analysis on bone marrow also helps in diagnosis and prognosticating, especially in cancers. It takes on average 1 to 3 weeks to obtain a definitive result, the time depending on the method. Newer diagnostic techniques include: (i) Rapid-FISH (rapid fluorescence in situ hybridization); (ii) MLPA (multiple ligation PCR amplification), and (iii) QF-PCR (Quantitative Fluorescent Polymerase Chain Reaction). These methods of analysis do not require culturing, the amount of the sample material may be very small and the result is obtained in just few days. In comparison, classical cytogenetic analysis (karyotyping) after amniocentesis requires 15-20 mL of amniotic fluid, culturing of fetal cells (amniocytes) and takes around 10 to 21 days to produce the result. Biochemical testing Biochemical testing refers to analyses of metabolites that are either the substrates or the products of a deficient enzyme. Thus, increases or decreases of metabolite concentrations are indirect indicators of metabolic disorders caused by an enzyme deficiency.If abnormal metabolites are identified, the disease may then be confirmed by enzyme analysis when available like in mucopolysacharidoses, Gaucher disease, Tay Sachs disease, etc. Enzyme analysis often requires a fibroblast culture or a fresh liver biopsy. Some enzyme tests can be done on serum, red blood cells, or leukocytes. PRENATAL SCREENING AND DIAGNOSIS Prenatal diagnosis must be considered in the contest of Fig.2. Commonly used pedigree symbols
  • 6. Review Article 255 Apollo Medicine, Vol. 6, No. 3, September 2009 the entire situation – the risk of disease in the baby, confirmed diagnosis in affected child/carrier status in parents, availability of treatment for the disease in question and above all the wish of the couple. Ideally the discussion and planning should start pre-pregnancy which is invariably not the case in our situation. Prenatal diagnostic techniques are shown in Table 1 & 2. Prenatal screening for common chromosomal disorders has good sensitivity using maternal serum biochemical markers and ultrasonography. Maternal serum screening should be a routine prenatal test to determine the risk of anenploidies and certain malformations like neural tube defects. Earlier, maternal serum screening was classically done in the second trimester but now first trimester screen has been found to be more effective and useful. Second trimester screen can be done between 15-22 weeks of pregnancy, however, it is best preferred at 16 -18 Table 1. Prenatal Screening/ Diagnostic Techniques (i) Noninvasive techniques : Timing (a) Maternal serum screening – First trimester (PAPP-A& free BetaHCG) 11-13+6wk – Second Trimester Triple test (AFP, HCG, unconjugated estriol) 16-18weeks Quadruple screen (AFP, HCG, unconjugated estriol &inhibinA) 16-18weeks (b) Fetal inspection by – Fetal ultrasonography (USG) First trimester (NT& Newer markers) 11-13+6wk Second Trimester (Anomaly scan) 18-20weeks – Fetal echocardiography 18-20weeks – Fetal MRI >26weeks (ii) Invasive techniques – Chorionic villus sampling (CVS) 11-13weeks – Amniocentesis 16-18weeks – Cord blood sampling >18weeks – Fetal skin, liver or muscle biopsy 18-20weeks Table 2 Compares various invasive techniques Procedure Risk Timings Indications* Amniocentesis Fetal loss 0.5-1% 16-18 weeks Cytogenetic Amniotic fluid leak Molecular Respiratory problem Biochemical TORCH infections CVS Fetal loss 1.5-2% 11-13 weeks Molecular Fetomaternal hemorrhage Biochemical Cytogenetic Cordocentsis Fetal loss 2-3% after 18 weeks Hematological Infections Cytogenetics Molecular * Any of the samples obtained by fetal sampling can be used for cytogenetic, molecular or biochemical tests but CVS sample is a desired sample for DNAbased tests where as amniotic fluid is preferred for cytogenetic analysis.
  • 7. Apollo Medicine, Vol. 6, No. 3, September 2009 256 Review Article weeks. Commonly used 2nd trimester markers are maternal serum alphafetoprotein (MSAFP), human Chorionic gonadotropin (hCG) and estriol (the triple screen). MSAFP and estriol is low and hCG is high in the maternal serum if the mother is carrying a Down syndrome fetus. A positive screen is when the risk of Down syndrome is 1 in 250 (which means if 250 women have the given screen parameters one would have a baby with Down syndrome). High MSAFP (>2.5 MoM) can identify over 95% of anencephaly and 75% of open spina bifida cases. Use of three markers for Down syndrome screening will give a maximum detection rate of around 70%.This approach will also result in detection of approx. 50 % of al cases of trisomy 18. Quadruple Test Addition of a 4th biochemical marker, Inhibin-A, (increased in DS pregnancies) in the second trimester screen, increases the sensitivity of screening for DS from 60% to 75%. First Trimester Screening Although many markers have been studied in the first trimester two robust markers suggested are free hCG and pregnancy associated plasma protein A (PAPP-A). Free hCG has been found to be elevated with the median MoM values of 2.15, almost similar to the second trimester. PAPP-A values are low with the median MoM of around 0.45-0.51, this alteration is not seen in the second trimester .Based on the available data the detection rates using these two markers varies between 60-67% with a false positive rate of 5%. Detection of trisomy 18 and 13 has also been reported by first trimester screening with good detection rates. Values of these biochemical tests are to be interpreted in multiples of median (MoM). Each lab has its own cut offs and the risk is calculated based on previous history, age, gestation, number of fetuses, smoking, weight, ethnicity, gravidity and parity, previous screening results assisted reproduction, pregnancy complications and diabetes (lower levels). When first trimester biochemical screen is combined with nuchal translucency (combined test), detection rate for trisomy 21 increases to 85% at a false positive rate of 5%. Inclusion of various other newer markers such as nasal bone, tricuspid regurgitation and ductus venosus further raises the sensitivity to 92% at a false positive rate of 5%. So first trimester combined screen is clearly has more sensitivity and specificity and provides early reassurance [6]. While offering any screening method, one should offer both pretest as well as posttest counseling. One should remember that it is not a diagnostic test but is a screening test to pick up high risk pregnancies and is certainly not a substitute for fetal karyotyping. Definitive diagnosis can be provided by chromosomal studies on amniotic fluid, chorionic villus biopsy or cord blood sample. Ultrasound scanning in prenatal diagnosis [7] Fetal anomaly scan is done at 11-14 weeks and 18-20 weeks to look at the major malformations and soft markers. 1st trimester scan It has been shown that around this time there is strong association between chromosomal abnormality and abnormal accumulation of fluid behind baby’s neck, referred to as increased ‘fetal nuchal translucency.’ This applies both to DS and other autosomal trisomy syndromes like trisomy 13 and 18. By combining information on maternal age with results of fetal nuchal translucency and thickness measurements, it is possible to detect approx. 80% of fetuses with trisomy 21, if invasive testing is offered to the 5 % of pregnant women with the highest risk. 2nd trimester scan Significant sonographic findings are seen in nearly all fetuses with trisomy 13, 77-100% of trisomy 18. Current sonographic criteria can identify 65%-75% of fetuses with Down syndrome with a false positive rate of 4-15% in second trimester. Presence of multiple abnormalities raises the risk of any chromosomal abnormality to 35%. With the combined usage of sonographic markers for Down syndrome and maternal serum screening, the vast majority of fetuses with Down syndrome could be potentially detected. Perinatal pathology In case of unexplained fetal deaths or detection of major congenital malformations on antenatal ultrasound, fetal autopsy for complete genetic evaluation is of utmost importance in order to make a specific diagnosis and ascertain the risk of recurrence. CONCLUSION Pediatricians and Gynaecologists are the primary physicians for the diagnosis, and management of children and high risk couples with genetic disorders. Also besides treating the patients, physicians should make the parents or couple aware of the genetic disorder, risk of recurrence, prognosis and prenatal diagnosis. The development of genetic and molecular biology methods has opened up new
  • 8. Review Article 257 Apollo Medicine, Vol. 6, No. 3, September 2009 opportunities in genetic prenatal diagnosis. Genetic counseling in association with modern prenatal diagnostic procedures constitutes a basic element of prevention of congenital anomalies and genetic disorders. REFERENCES 1. National Center for Biotechnology Information. Online Mendelian Inheritance in Man (OMIM). Available at: www.ncbi.nlm.nih.gov/ omim/. Accessibility verified May 21, 2003. 2. Peltonen L, McKusick VA. Genomics and medicine: dissecting human disease in the postgenomic era. Science. 2001; 291:1224-1229. 3. Nussbaum RL, McInnes RR, Williard HF. Genetic APPENDIX-A HOW TO SEND SAMPLES TOAGENETIC LAB Genetic lab/Geneticist should be informed before sending the sample. PRENATALSAMPLES Amniotic fluid sample-About 20 mL of clear amniotic fluid is sent in sterile tubes (tubes are collected from the lab). It is required for chromosomal analysis, DNA and biochemical analysis.# Chorionic villi sample- About 20 -30 mg of chorionic villi should be sent in the transport media. (It can be collected from the genetic lab).It is required for DNA analysis, enzyme analysis and chromosomal analysis.# #in the order of preference POST NATAL sample Blood DNAstudies: Collect 3-6 mLblood in EDTA(purple top vacutainer) Chromosomal analysis: Collect 3 mLof blood in heparin (green top vacutainer) Enzyme analysis: 3-6 mL blood in heparin(green top vacutainer) Product of conception/ Skin for chromosomal analysis or DNA can be sent in the culture media (can be collected from the genetic lab) or normal saline with 2 drops of crystalline penicillin and gentamycin. counseling and risk assessment. In Thompson and Thompson Genetics in Medicine. 6th edition by WB Saunders 2001. 4. Harper PS.Prenatal diagnosis and related reproductive aspects. In Practical Genetic counseling, 6th Edition, Arnold Publishers 2004. 5. Muller R F, Young ID. Genetic counseling. In Emery’s Elements of Medical Genetics. Eleventh edition by Churchill Livingstone in 2002. 6. Malone FD, et al. FASTER research Consortium.N Engl J Med 2005; 353:2001-2011. 7. Shipp TD, Benacerraf BR. Second trimester ultrasound screening for chromosomal abnormalities. Prenatal diagnosis 2002; 22: 296-307.
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