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Neural Tube Defects
Neural Tube Defects RH Finnell, The University of Texas at Austin, Austin, TX, USA; and Dell Children’s Medical Center, Austin, TX, USA TM George, Dell Children’s Medical Center, Austin, TX, USA LE Mitchell, The University of Texas School of Public Health, Houston, TX, USA ã 2014 Elsevier Inc. All rights reserved.
Embryology Definition Risk Factors Diagnosis, Treatment, and Outcome References
1 1 1 2 3
Embryology Neurulation is the critical morphogenetic event occurring during the fourth week of human gestation, converting the previously developed neural plate into the ectoderm covered neural tube that will eventually differentiate into the brain and spinal cord. Genetic regulation of the events involved in mammalian neural tube morphogenesis is a complicated process that involves a multitude of genes. These genes have vital functions in a range of biological activities that are thought to include signaling molecules, transcription proteins and factors, cytoskeletal and gap junction proteins, growth factors, and tumor suppressor genes (Copp et al., 2003; Ewart et al., 1997; Sah et al., 1995; Zhang et al., 1996). The neural tube, formed by the cell shape changes and movements, ultimately fuses at several discrete points.
Definition Abnormalities of neural tube closure may result from alterations in any of the processes that are involved in the formation of the primary neural tube (e.g. elevation and bending, apical constriction, convergent extension). As these processes are under the control of a multitude of genes, the molecular basis of neural tube closure defects is likely to be quite heterogeneous. Indeed, in the mouse, defects of neural tube closure have been shown to result from genetic mutations affecting cell proliferation and apoptosis, convergent extension, elevation/apposition of the neural folds, and neural tube fusion (Blom et al., 2006; Copp and Greene, 2010; Lee et al., 2005; Massa et al., 2009). Despite the complexity of the processes that are required for normal neural tube closure, the gross phenotypic consequences of abnormalities in these processes are relatively homogenous. In general, failure of neural tube closure is associated with defects in the overlying bony structures (i.e. cranial vault and neural arches) such that the underlying neural tissue is exposed to the body surface. Consequently, defects of primary neural tube closure are often referred to as ‘open’ neural tube defects (NTDs). Further classifications of the open NTDs are based on the location and extent of the defect. In addition to the open NTDs, there are also a number of ‘closed’ or skin covered conditions that involve the neural tube, including encephalocele, iniencephaly and lesions often referred to as occult spinal dysraphisms such as meningocele (that may be partially skin covered), spinal lipomas (lipomyelomeningocele or lipomeningocele), myelocystocele, spilt cord malformations, and various forms of sacral agenesis. In general, these conditions are not thought to result from defects in primary neural tube closure, but may result from defects in secondary neural tube development. Nonetheless, there is evidence that open and closed NTDs may have some common etiological underpinnings.
Risk Factors The open NTDs are recognized as being etiologically heterogeneous. A small proportion of affected individuals have an associated chromosomal or Mendelian malformation syndrome (Lynch, 2005) and there are rare families in which NTDs segregate in patterns consistent with X-linked or autosomal recessive inheritance. In addition, a small proportion of cases can be linked to an established risk factor. The most notable of these factors are maternal pregestational, insulin-dependent diabetes and maternal use of medications, in particular, specific anticonvulsant drugs. While many environmental agents are suspected of possessing a teratogenic potential based on data from studies in animals, relatively few have been confirmed as human teratogens. This may be due to several factors, including differences in dose; laboratory studies performed on mice generally utilize doses that are much higher than encountered by humans; exposure assessment, environmental exposures are more difficult to quantify in studies involving humans than in mice kept under laboratory conditions; and degree of experimental control, genetic and environmental differences that may influence disease susceptibility are more easily controlled in studies of laboratory animals than in studies of human populations. However, several environmental exposures have emerged as potential risk factors for NTDs.
Reference Module in Biomedical Research
http://dx.doi.org/10.1016/B978-0-12-801238-3.05586-0
1
2
Neural Tube Defects
Studies of the recurrence patterns in families ascertained through an individual affected with anencephaly or spina bifida indicate that these two conditions tend to co-segregate within families and are, therefore, likely to share a common etiology. In addition, family studies have demonstrated that the relatives of an affected individual are at increased risk of having an neural tube defect (NTD) compared with the general population. The relative risk ratio (i.e. risk to relative vs. risk in general population) for the sib of an affected individual is 30–50. This is much lower than the relative risks exhibited by Mendelian conditions, and higher than expected for conditions that are determined solely by environmental factors (Khoury et al., 1988). Hence, it is commonly believed that the risk of NTDs is determined by multiple risk factors, including both genes and environmental agents. Epidemiological studies of potential environmental risk factors for NTDs have a relatively long history, and have identified several factors (reviewed previously) that appear to be related to NTD risk. In contrast, the study of genetic risk factors for NTDs has a relative short history. Although traditional genetic linkage approaches (e.g. LOD score and affected relative pair analyses) have been applied to the study of other conditions since the 1980s, the data required by such approaches (i.e. DNA from multiple affected individuals within families) are largely unavailable for NTDs. Consequently, studies designed to identify genes that influence the risk of NTDs did not become feasible until the 1990s, when the Human Genome Project began to provide new information regarding the variability within the human genome. Such advances in our understanding of the human genome have provided new opportunities for studying the genetic contribution to conditions such as NTDs (e.g. case–control and family-based genetic association studies of non-Mendelian conditions). The relative lack of success in identifying genetic variants that are related to the risk of NTDs in humans is somewhat surprising given the large number of mouse models that provide strong evidence of the genetic contribution of NTD. To date, over 240 gene mutations have been associated with NTD in mouse lines (Harris and Juriloff, 2010), yet none of the 155 genes implicated by these models has been identified as a determinant of NTD risk in humans. In general, the lack of success in past studies in humans is likely to reflect a number of factors including issues of sample size and power, etiologic heterogeneity, and failure to adequately capture the etiological complexities that are likely to underlie human NTDs. These complexities are likely to include gene–gene and gene–environment interactions; maternal and embryonic genotypic effects; the involvement of both rare and common variants; and consideration of pathways (as opposed to individual genes).
Diagnosis, Treatment, and Outcome Screening to identify pregnant women who are at an increased risk of carrying an NTD affected fetus can be achieved by the evaluation of maternal serum alpha-fetoprotein (MSAFP) levels and/or ultrasound imaging (Drugan et al., 2001; Krantz et al., 2010; Wald, 2010). Follow-up studies for women with a positive result on either screening test include amniocentesis and/or detailed ultrasound evaluation. When amniocentesis is performed, evaluation of amniotic fluid alpha-fetoprotein and acetyl cholinesterase levels can be used to confirm the presence of an open fetal malformation and differentiate between open ventral wall and open NTDs. In addition, the fetal karyotype can be evaluated to rule out chromosomal anomalies. Detailed ultrasound can also be used to differentiate between open defects of the ventral wall and the neural tube, and to identify the presence of other associated anomalies (Sepulveda et al., 2004). When a diagnosis of spina bifida is confirmed, ultrasound and prenatal magnetic resonance imaging (using ultrafast T2-weighted sequences) can be used to identify spontaneous leg and foot motion, leg and spine deformities, and the presence of a Chiari II malformation (Mangels et al., 2000). The majority of individuals with spina bifida have their spinal lesion closed postnatally, usually within 72 h of birth (Bowman et al., 2001; Carmichael et al., 2002). However, a small proportion of fetuses have had their lesion closed in utero. A randomized clinical trial, designed to compare the outcome of infants treated postnatally with those treated in utero was initiated in 2003 and stopped in December 2010 based on the evidence for efficacy of prenatal surgery (Adzick et al., 2011). Based on data from 158 patients who underwent a 12-month evaluation, the primary outcome (death or need for cerebrospinal fluid shunt by 12 months) was significantly less common in infants undergoing prenatal as compared to postnatal surgery (relative risk 0.70; 97.7% CI 0.58, 0.84). The prenatal surgery group also had lower rates of hindbrain herniation, brain-stem kinking and syringomyelia and, at 30 months, the difference between the functional and anatomical level of the lesion was significantly better as compared to the postnatal surgery group. In the United States and many other areas of the world, it is recommended that all women of childbearing age consume a daily, multivitamin supplement containing at least 0.4 mg of folic acid in order to reduce their risk of having a child with an NTD. Women who have had a child affected with an NTD may be counseled to take higher doses of folic acid as per the initial recommendations from the Centers for Disease Control Centers for Disease Control (1991). Specific recommendations for other categories of high-risk women (e.g. sisters of women who have had an affected pregnancy, women take antiepileptic drugs such as valproic acid, and diabetics) are not available. As folic acid supplementation does not prevent 100% of all NTDs, women who are at increased risk for having an affected child (whether or not they are taking folic acid supplements) should be informed of the availability of MSAFP screening/ultrasound/ amniocentesis for the prenatal identification of affected fetuses. At this time, there are no recommended genetic screening or testing procedures for nonsyndromic NTDs. Evaluation of genetic variants, such as the MTHFR C677T polymorphism, is not currently recommended due to the lack of consistent associations across studies, the relatively low proportion of cases that are likely to be attributable to any single genetic factor, and the fact that it would not change preconceptional advice regarding the utility of vitamin supplements or pregnancy management (Finnell et al., 2002).
Neural Tube Defects
3
References Adzick NS, Thom EA, Spong CY, Brock JW III, Burrows PK, Johnson MP, Howell LJ, Farrell JA, Dabrowiak ME, Sutton LN, et al. (2011) A randomized trial of prenatal versus postnatal repair of myelomeningocele. The New England Journal of Medicine 364: 993–1004. Blom HJ, Shaw GM, den Heijer M, and Finnell RH (2006) Neural tube defects and folate: Case far from closed. Nature Reviews. Neuroscience 7(9): 724–731, PMID: 16924261. Bowman RM, McLone DG, Grant JA, Tomita T, and Ito JA (2001) Spina bifida outcome: A 25-year prospective. Pediatric Neurosurgery 34: 114–120. Carmichael SL, Shaw GM, Neri E, Schaffer DM, and Selvin S (2002) Physical activity and risk of neural tube defects. Maternal and Child Health Journal 6: 151–157. Centers for Disease Control (1991) Use of folic acid for prevention of spina bifida and other neural tube defects—1983–1991. MMWR 40: 513–516. Copp AJ, Green NDE, and Murdoch JN (2003) The genetic basis of mammalian neurulation. Nature Reviews. Genetics 4: 784–793. Copp AJ and Greene ND (2010) Genetics and development of neural tube defects. The Journal of Pathology 220: 217–230. Drugan A, Weissman A, and Evans MI (2001) Screening for neural tube defects. Clinics in Perinatology 28: 279–287. Ewart JL, Cohen MF, and Meyer RA (1997) Heart and neural tube defects in transgenic mice overexpressing the Cx43 gap junction gene. Development 124: 1281–1292. Finnell RH, Shaw GM, Lammer EJ, and Volcik KA (2002) Does prenatal screening for 5,10-methylenetetrahydrofolate reductase (MTHFR) mutations in high-risk neural tube defect pregnancies make sense? Genetic Testing 6: 47–52. Harris MJ and Juriloff DM (2010) An update to the list of mouse mutants with neural tube closure defects and advances toward a complete genetic perspective of neural tube closure. Birth Defects Research. Part A, Clinical and Molecular Teratology 88: 653–669. Khoury MJ, Beaty TH, and Liang KY (1988) Can familial aggregation of disease be explained by familial aggregation of environmental factors? American Journal of Epidemiology 127: 674–683. Krantz DA, Hallahan TW, and Sherwin JE (2010) Screening for open neural tube defects. Clinics in Laboratory Medicine 30(3): 721–725, Epub 2010 Jun 15. PMID: 20638584. Lee S, Jurata LW, Nowak R, Lettieri K, Kenny DA, Pfaff SL, and Gill GN (2005) The LIM domain-only protein LM04 is required for neural tube closure. Molecular and Cellular Neurosciences 28: 205–214. Lynch SA (2005) Non-multifactorial neural tube defects. American Journal of Medical Genetics. Part C, Seminars in Medical Genetics 135: 69–76. Mangels K, Tulipan N, Tsao L, Alarcon J, and Bruner JP (2000) Fetal MRI in the evaluation of intrauterine myelomeningocele. Pediatric Neurosurgery 32: 124–131. Massa V, Savery D, Ybot-Gonzalez P, Ferraro E, Rongvaux A, Cecconi F, Flavell R, Greene ND, and Copp AJ (2009) Apoptosis is not required for mammalian neural tube closure. Proceedings of the National academy of Sciences of the United States of America 106(20): 8233–8238, Epub 2009 May 6. PMID: 19420217. Sah VP, Attardi LD, Mulligan GJ, Williams BO, Bronson RT, and Jacks T (1995) A subset of p53-deficient embryos exhibit exencephaly. Nature Genetics 10: 175–180. Sepulveda W, Corral E, Ayala C, Be C, Gutierrez J, and Vasquez P (2004) Chromosomal abnormalities in fetuses with open neural tube defects: Prenatal identification with ultrasound. Ultrasound in Obstetrics & Gynecology 23: 352–356. Wald NJ (2010) Prenatal screening for open neural tube defects and down syndrome: Three decades of progress. Prenatal Diagnosis 30: 619–621. Zhang J, Hagopian-Donaldson S, and Serbedzija G (1996) Neural tube, skeletal and body wall defects in mice lacking transcription factor AP-2. Nature 381: 238–241.
Embryology Definition Risk Factors Diagnosis, Treatment, and Outcome References
1 1 1 2 3
Embryology Neurulation is the critical morphogenetic event occurring during the fourth week of human gestation, converting the previously developed neural plate into the ectoderm covered neural tube that will eventually differentiate into the brain and spinal cord. Genetic regulation of the events involved in mammalian neural tube morphogenesis is a complicated process that involves a multitude of genes. These genes have vital functions in a range of biological activities that are thought to include signaling molecules, transcription proteins and factors, cytoskeletal and gap junction proteins, growth factors, and tumor suppressor genes (Copp et al., 2003; Ewart et al., 1997; Sah et al., 1995; Zhang et al., 1996). The neural tube, formed by the cell shape changes and movements, ultimately fuses at several discrete points.
Definition Abnormalities of neural tube closure may result from alterations in any of the processes that are involved in the formation of the primary neural tube (e.g. elevation and bending, apical constriction, convergent extension). As these processes are under the control of a multitude of genes, the molecular basis of neural tube closure defects is likely to be quite heterogeneous. Indeed, in the mouse, defects of neural tube closure have been shown to result from genetic mutations affecting cell proliferation and apoptosis, convergent extension, elevation/apposition of the neural folds, and neural tube fusion (Blom et al., 2006; Copp and Greene, 2010; Lee et al., 2005; Massa et al., 2009). Despite the complexity of the processes that are required for normal neural tube closure, the gross phenotypic consequences of abnormalities in these processes are relatively homogenous. In general, failure of neural tube closure is associated with defects in the overlying bony structures (i.e. cranial vault and neural arches) such that the underlying neural tissue is exposed to the body surface. Consequently, defects of primary neural tube closure are often referred to as ‘open’ neural tube defects (NTDs). Further classifications of the open NTDs are based on the location and extent of the defect. In addition to the open NTDs, there are also a number of ‘closed’ or skin covered conditions that involve the neural tube, including encephalocele, iniencephaly and lesions often referred to as occult spinal dysraphisms such as meningocele (that may be partially skin covered), spinal lipomas (lipomyelomeningocele or lipomeningocele), myelocystocele, spilt cord malformations, and various forms of sacral agenesis. In general, these conditions are not thought to result from defects in primary neural tube closure, but may result from defects in secondary neural tube development. Nonetheless, there is evidence that open and closed NTDs may have some common etiological underpinnings.
Risk Factors The open NTDs are recognized as being etiologically heterogeneous. A small proportion of affected individuals have an associated chromosomal or Mendelian malformation syndrome (Lynch, 2005) and there are rare families in which NTDs segregate in patterns consistent with X-linked or autosomal recessive inheritance. In addition, a small proportion of cases can be linked to an established risk factor. The most notable of these factors are maternal pregestational, insulin-dependent diabetes and maternal use of medications, in particular, specific anticonvulsant drugs. While many environmental agents are suspected of possessing a teratogenic potential based on data from studies in animals, relatively few have been confirmed as human teratogens. This may be due to several factors, including differences in dose; laboratory studies performed on mice generally utilize doses that are much higher than encountered by humans; exposure assessment, environmental exposures are more difficult to quantify in studies involving humans than in mice kept under laboratory conditions; and degree of experimental control, genetic and environmental differences that may influence disease susceptibility are more easily controlled in studies of laboratory animals than in studies of human populations. However, several environmental exposures have emerged as potential risk factors for NTDs.
Reference Module in Biomedical Research
http://dx.doi.org/10.1016/B978-0-12-801238-3.05586-0
1
2
Neural Tube Defects
Studies of the recurrence patterns in families ascertained through an individual affected with anencephaly or spina bifida indicate that these two conditions tend to co-segregate within families and are, therefore, likely to share a common etiology. In addition, family studies have demonstrated that the relatives of an affected individual are at increased risk of having an neural tube defect (NTD) compared with the general population. The relative risk ratio (i.e. risk to relative vs. risk in general population) for the sib of an affected individual is 30–50. This is much lower than the relative risks exhibited by Mendelian conditions, and higher than expected for conditions that are determined solely by environmental factors (Khoury et al., 1988). Hence, it is commonly believed that the risk of NTDs is determined by multiple risk factors, including both genes and environmental agents. Epidemiological studies of potential environmental risk factors for NTDs have a relatively long history, and have identified several factors (reviewed previously) that appear to be related to NTD risk. In contrast, the study of genetic risk factors for NTDs has a relative short history. Although traditional genetic linkage approaches (e.g. LOD score and affected relative pair analyses) have been applied to the study of other conditions since the 1980s, the data required by such approaches (i.e. DNA from multiple affected individuals within families) are largely unavailable for NTDs. Consequently, studies designed to identify genes that influence the risk of NTDs did not become feasible until the 1990s, when the Human Genome Project began to provide new information regarding the variability within the human genome. Such advances in our understanding of the human genome have provided new opportunities for studying the genetic contribution to conditions such as NTDs (e.g. case–control and family-based genetic association studies of non-Mendelian conditions). The relative lack of success in identifying genetic variants that are related to the risk of NTDs in humans is somewhat surprising given the large number of mouse models that provide strong evidence of the genetic contribution of NTD. To date, over 240 gene mutations have been associated with NTD in mouse lines (Harris and Juriloff, 2010), yet none of the 155 genes implicated by these models has been identified as a determinant of NTD risk in humans. In general, the lack of success in past studies in humans is likely to reflect a number of factors including issues of sample size and power, etiologic heterogeneity, and failure to adequately capture the etiological complexities that are likely to underlie human NTDs. These complexities are likely to include gene–gene and gene–environment interactions; maternal and embryonic genotypic effects; the involvement of both rare and common variants; and consideration of pathways (as opposed to individual genes).
Diagnosis, Treatment, and Outcome Screening to identify pregnant women who are at an increased risk of carrying an NTD affected fetus can be achieved by the evaluation of maternal serum alpha-fetoprotein (MSAFP) levels and/or ultrasound imaging (Drugan et al., 2001; Krantz et al., 2010; Wald, 2010). Follow-up studies for women with a positive result on either screening test include amniocentesis and/or detailed ultrasound evaluation. When amniocentesis is performed, evaluation of amniotic fluid alpha-fetoprotein and acetyl cholinesterase levels can be used to confirm the presence of an open fetal malformation and differentiate between open ventral wall and open NTDs. In addition, the fetal karyotype can be evaluated to rule out chromosomal anomalies. Detailed ultrasound can also be used to differentiate between open defects of the ventral wall and the neural tube, and to identify the presence of other associated anomalies (Sepulveda et al., 2004). When a diagnosis of spina bifida is confirmed, ultrasound and prenatal magnetic resonance imaging (using ultrafast T2-weighted sequences) can be used to identify spontaneous leg and foot motion, leg and spine deformities, and the presence of a Chiari II malformation (Mangels et al., 2000). The majority of individuals with spina bifida have their spinal lesion closed postnatally, usually within 72 h of birth (Bowman et al., 2001; Carmichael et al., 2002). However, a small proportion of fetuses have had their lesion closed in utero. A randomized clinical trial, designed to compare the outcome of infants treated postnatally with those treated in utero was initiated in 2003 and stopped in December 2010 based on the evidence for efficacy of prenatal surgery (Adzick et al., 2011). Based on data from 158 patients who underwent a 12-month evaluation, the primary outcome (death or need for cerebrospinal fluid shunt by 12 months) was significantly less common in infants undergoing prenatal as compared to postnatal surgery (relative risk 0.70; 97.7% CI 0.58, 0.84). The prenatal surgery group also had lower rates of hindbrain herniation, brain-stem kinking and syringomyelia and, at 30 months, the difference between the functional and anatomical level of the lesion was significantly better as compared to the postnatal surgery group. In the United States and many other areas of the world, it is recommended that all women of childbearing age consume a daily, multivitamin supplement containing at least 0.4 mg of folic acid in order to reduce their risk of having a child with an NTD. Women who have had a child affected with an NTD may be counseled to take higher doses of folic acid as per the initial recommendations from the Centers for Disease Control Centers for Disease Control (1991). Specific recommendations for other categories of high-risk women (e.g. sisters of women who have had an affected pregnancy, women take antiepileptic drugs such as valproic acid, and diabetics) are not available. As folic acid supplementation does not prevent 100% of all NTDs, women who are at increased risk for having an affected child (whether or not they are taking folic acid supplements) should be informed of the availability of MSAFP screening/ultrasound/ amniocentesis for the prenatal identification of affected fetuses. At this time, there are no recommended genetic screening or testing procedures for nonsyndromic NTDs. Evaluation of genetic variants, such as the MTHFR C677T polymorphism, is not currently recommended due to the lack of consistent associations across studies, the relatively low proportion of cases that are likely to be attributable to any single genetic factor, and the fact that it would not change preconceptional advice regarding the utility of vitamin supplements or pregnancy management (Finnell et al., 2002).
Neural Tube Defects
3
References Adzick NS, Thom EA, Spong CY, Brock JW III, Burrows PK, Johnson MP, Howell LJ, Farrell JA, Dabrowiak ME, Sutton LN, et al. (2011) A randomized trial of prenatal versus postnatal repair of myelomeningocele. The New England Journal of Medicine 364: 993–1004. Blom HJ, Shaw GM, den Heijer M, and Finnell RH (2006) Neural tube defects and folate: Case far from closed. Nature Reviews. Neuroscience 7(9): 724–731, PMID: 16924261. Bowman RM, McLone DG, Grant JA, Tomita T, and Ito JA (2001) Spina bifida outcome: A 25-year prospective. Pediatric Neurosurgery 34: 114–120. Carmichael SL, Shaw GM, Neri E, Schaffer DM, and Selvin S (2002) Physical activity and risk of neural tube defects. Maternal and Child Health Journal 6: 151–157. Centers for Disease Control (1991) Use of folic acid for prevention of spina bifida and other neural tube defects—1983–1991. MMWR 40: 513–516. Copp AJ, Green NDE, and Murdoch JN (2003) The genetic basis of mammalian neurulation. Nature Reviews. Genetics 4: 784–793. Copp AJ and Greene ND (2010) Genetics and development of neural tube defects. The Journal of Pathology 220: 217–230. Drugan A, Weissman A, and Evans MI (2001) Screening for neural tube defects. Clinics in Perinatology 28: 279–287. Ewart JL, Cohen MF, and Meyer RA (1997) Heart and neural tube defects in transgenic mice overexpressing the Cx43 gap junction gene. Development 124: 1281–1292. Finnell RH, Shaw GM, Lammer EJ, and Volcik KA (2002) Does prenatal screening for 5,10-methylenetetrahydrofolate reductase (MTHFR) mutations in high-risk neural tube defect pregnancies make sense? Genetic Testing 6: 47–52. Harris MJ and Juriloff DM (2010) An update to the list of mouse mutants with neural tube closure defects and advances toward a complete genetic perspective of neural tube closure. Birth Defects Research. Part A, Clinical and Molecular Teratology 88: 653–669. Khoury MJ, Beaty TH, and Liang KY (1988) Can familial aggregation of disease be explained by familial aggregation of environmental factors? American Journal of Epidemiology 127: 674–683. Krantz DA, Hallahan TW, and Sherwin JE (2010) Screening for open neural tube defects. Clinics in Laboratory Medicine 30(3): 721–725, Epub 2010 Jun 15. PMID: 20638584. Lee S, Jurata LW, Nowak R, Lettieri K, Kenny DA, Pfaff SL, and Gill GN (2005) The LIM domain-only protein LM04 is required for neural tube closure. Molecular and Cellular Neurosciences 28: 205–214. Lynch SA (2005) Non-multifactorial neural tube defects. American Journal of Medical Genetics. Part C, Seminars in Medical Genetics 135: 69–76. Mangels K, Tulipan N, Tsao L, Alarcon J, and Bruner JP (2000) Fetal MRI in the evaluation of intrauterine myelomeningocele. Pediatric Neurosurgery 32: 124–131. Massa V, Savery D, Ybot-Gonzalez P, Ferraro E, Rongvaux A, Cecconi F, Flavell R, Greene ND, and Copp AJ (2009) Apoptosis is not required for mammalian neural tube closure. Proceedings of the National academy of Sciences of the United States of America 106(20): 8233–8238, Epub 2009 May 6. PMID: 19420217. Sah VP, Attardi LD, Mulligan GJ, Williams BO, Bronson RT, and Jacks T (1995) A subset of p53-deficient embryos exhibit exencephaly. Nature Genetics 10: 175–180. Sepulveda W, Corral E, Ayala C, Be C, Gutierrez J, and Vasquez P (2004) Chromosomal abnormalities in fetuses with open neural tube defects: Prenatal identification with ultrasound. Ultrasound in Obstetrics & Gynecology 23: 352–356. Wald NJ (2010) Prenatal screening for open neural tube defects and down syndrome: Three decades of progress. Prenatal Diagnosis 30: 619–621. Zhang J, Hagopian-Donaldson S, and Serbedzija G (1996) Neural tube, skeletal and body wall defects in mice lacking transcription factor AP-2. Nature 381: 238–241.