Spina Bifida: Causes and Consequences

Introduction

There are four stages of early embryonic development that takes place as soon as fertilisation takes place. These stages are morula, blastula, gastrula and neurula stages, where important morphogenic changes take place, in creating all the essential layouts and plans for later embryonic development. Such as body plan axis, germ layers and CNS (Central Nervous System). Failure can occur within any of these stages that lead to very serious consequences, such as Spina bifida (SB), which is caused by the failure of neural tube closure in neurulation. It is estimated that 5%-10% of the population may have spina bifida without realising, and the most severe form of spina bifida is very rare, in affecting around 1 out 2,000 pregnancies. The neural tube is the precursor of the brain and spinal cord. It is formed from the ectoderm region of the germ layer, which consists of endoderm, mesoderm and ectoderm. In normal embryogenesis, the neural tube is made by the bilateral folding of the neural plate, where folds elevate and come into contact in the midline and fuse to create the neural tube. This process consists of cellular events and precise molecular control, to create the initial structures for the brain and spinal cord. Errors that occur in these events and controls lead to neural tube defects (NTD). NTDs include, Anencephaly, which is where the head end of the neural tube fails to close, leading to stillborn due to absence of main part of forebrain, Encephaloceles, Hydranencephaly, Iniencephaly and spina bifida, which is the failure of closure at the lower back region which usually leads to problems with walking, amongst other issues. Spina bifida can range from mild, where it’s usually accidentally identified, to severe as it affects the nerves and therefore leads to complications. There are two types of Spina bifida; Spina bifida cystica (SBC) and spina bifida occulta (SBO). Spina bifida cystica has two subsets, which are Meningocele and Myelomeningocele. SBO is the mildest form of SB, the spinal cord does not protrude so the skin appears normal, but might be characterised by hair growth there, a dimple or birthmark. This is diagnosed from spinal x-rays, to identify the splits in the vertebrae as it hadn’t closed properly during neurulation. Meningocele SBC is the least common form of spina bifida, where the meninges herniate between the vertebrae but doesn’t damage the nervous system. Myelomeningocele SBC results in the most severe complications as it affects the meninges and nerves. Unfused part of the neural tube protrudes through an opening. A sac is present at the site containing spinal parts such as cerebrospinal fluid and parts of the spinal cord. 68% of children who have spina bifida, also presents a latex allergy. This essay with be going into the details of processes that occurs, that leads to spina bifida and its functional consequences, also what clinical therapies can be proposed to prevent or treat spina bifida.

Cellular and genetic processes

There are multiple cellular events and molecular processes that take place in controlling the morphogenesis of the embryo in creating the neural tube. Cellular events include, convergent extension, apical constriction and interkinetic nuclear migration, and the precise molecular control that takes place are, non-canonical Wnt pathway (planar cell polarity pathway), SHH/BMP signalling and the control of transcription factors like Grhl2/3, Pax3, Slug and Zic2. (Nikolopoulou et al, 2017) These are very important early embryonic developmental mechanisms required to, later on, create the brain and spinal cord. These processes are part of the neurulation stage which is the coordinated morphogenetic movements within the primitive streak. This consists of a primary neurulation, which takes place during weeks 3 and 4 of gestation, and secondary neurulation, which occurs in weeks 5 and 6 of gestation. Primary neurulation is when the neural tube forms by the shaping, folding and midline fusion of the neural plate. This is done by the induction of the neural plate to differentiate, it undergoes bending to create the neural folds which lift into the dorsal midline, and finally, the tips fuse to have a complete closure of the neural tube. This becomes roofed by surface ectoderm and creates the brain and most of the spinal cord. This process differs between species, but all species go through similar secondary neurulation process, in making the cordial part of the spinal cord. (Copp et al, 2003) This is when the tail-bud cells condense in the midline to form the medullary cord, which then undergoes epithelialisation around a lumen, forming a neuroepithelium which goes to make the rest of the spinal cord. The cellular events of the tissue movements in neurulation involve changes in the behaviour of the cells, such as changes in cell number, position, shape, size and adhesion. The mechanisms involved are found in the neuroepithelium, non-neural ectoderm, mesoderm and notochord, take part in regulating the neural tube closure. (Colas and Schoenwolf, 2001) The failure in this led to NTD development. Convergent extension, a cellular event that occurs, which is where the tissues narrow along its mediolateral axis and elongate along the anteroposterior axis. It is responsible for shaping the neural plate before closure and is linked to the non-canonical Wnt pathway, which is precisely controlled.  Mutations in this Wnt signalling has proven to cause failure of neural tube closure as it prevents the neural plates from elevating to the midline and fusing. This results in the most severe case of NTD (Craniorachischisis) which essentially leads to death. (Coop et al, 2003) The heterozygote mutation of this Wnt signalling results in the development of spina bifida. Coordinated morphogenetic movements within the primitive streak and nascent primary germ layer, signalling occur between them causing the initiation of neurulation, through induction of the neural plate. Majority of the molecular basis of neurulation remains unknown. Some of the genes involved in neurulation, which when mutated causes a development of spina bifida is, Pax3, which is known to be a haplotype associated with an increased risk for NTD, as it is a transcription factor important for specification of neural crest-derived structures during patterning of the neural tube. (Lu et al, 2007) Another gene, when mutated is Zic2, which is a zinc finger transcriptional regulator of sonic hedgehog activity. (klootwijk et al, 2004) Sonic hedgehog is an important signalling molecule needed for embryonic development. Mutation in Slug, zinc finger transcription factor, causes a defect in the dorsalisation of the neural tube. (Stegmann et al, 1999) Aldehyde dehydrogenase (ALDH1A2) and cytochrome P450 enzyme are needed for synthesis and metabolism of retinoic acid during the patterning of the neural tube. (Deak et al, 2005) Mutations in genes involved in this result in the increased risk of developing NTD. This and other genetic mutation lead to a defective development of the neural tube during neurulation, leading to conditions like spina bifida. 

Causes of spina bifida

The specific causes of spina bifida are not fully understood but many studies have shown that there are environmental such as teratogens, and genetic factors, which are heterogeneous, including chromosome abnormalities, single gene disorder that leads to an increased risk of spina bifida. (Mitchell et al, 2004) The main factor seen in patients that give birth to individuals with spina bifida is known to have had lower doses of folic acid during the pregnancy, and many studies have shown that 70% of cases have been prevented by the correct daily intake of folic acid during pregnancy in reducing prevalence spina bifida cases. (Mitchell et al, 2004) Evidence has indicated that the minimal intake of 0.4mg of folic acid reduces the number of NTD cases. (Hewitt et al, 1992) Folic acid doesn’t prevent all NTD cases as there is folic acid non-responsive NTD that can occur, this shows that there is a genetic influence on the increased risk of spina bifida. Research done on mice has shown that mice lacking inositol kinases have a higher prevalence of NTD. This kinase is important for intracellular signalling, interaction with cytoskeletal proteins and regulation of membrane identity in trafficking and cell division. (Greene et al, 2017) Spina bifida has proven to have a genetic component to is as well. Research has shown that two mutations in methylenetetrahydrofolate reductase (MTHFR) gene, leads to a risk of NTDs such as spina bifida, occurring. Mothers with elevated plasma homocysteine have shown to have children with NTDs. Homocysteine is an amino acid that comes from the demethylation of an important amino acid, methionine. This demethylation or homocysteine is remethylated, by the use of MTHFR enzyme and adequate supply of folic acid. The mutations in the MTHFR gene causes dysfunctional enzyme of this and causes an elevated concentration of homocysteine. (van der Put et al, 1998) Spina bifida has shown to have a genetic inheritable component in the family history of spina bifida causes an increased risk of this malformation occurring. Chromosomal abnormalities such as triploidy and trisomy have also shown to have a carried link causes spina bifida in offspring, but the reason for this is not fully understood. Maternal age and health have also shown to be linked to increased risk of spina bifida, as maternal obesity have also shown to be a risk factor for NTDs. The intake of medications such as thalidomide and valproic acid have shown to be teratogen to embryos in causes congenital malformations like spina bifida. (Chen. C. P, 2007) As most cases of spina bifida can be prevented through improved maternal health, nutrition like folic acid intake and avoidance of teratogens, the prevalence of spina bifida has reduced throughout the years, in areas that this can be maintained.

Clinical therapies

Most spina bifida cases present with a development of weakness or paralysis in the legs, urinary and bowel incontinence, lack of sensation in the skin and build up cerebrospinal fluid causing pressure on the brain, which may lead to brain damage. Some cases have their nervous system prone to infections which can be life-threatening. Myelomeningocele has further symptoms that will be present, such as cognitive problems and learning difficulties, as it impacts brain development. Other problems that arise from spina bifida is a higher risk of meningitis, have latex allergies and skin problems. Majority of spina bifida cases, around 90%, are identified during ultrasound scan at 18 weeks of pregnancy (third trimester), other ways of detections are maternal blood test which measures alpha-fetoprotein (AFP) or use of MRI. Although very mild cases of spinal Bifida have no symptoms and may never be detected. 

The main treatments used for spina bifida are surgical intervention and physical and therapy. There are also ways to treat the symptoms that spina bifida individual has. Surgery is done to repair the spine, which is usually done within 2 days of birth. Surgery is also done to remove the build-up of cerebrospinal fluid in the brain (hydrocephalus), usually using a permanent shunt that drains excess fluid away. (Lorber and Salfield, 1981) There are various treatments and tools used for bowel and urinary incontinence such as drugs or further surgery. Studies have shown that infants with untreated open spina bifida, passed away within 6 months of birth. Main causes of mortality in those with spina bifida are hydrocephalus and meningitis. (Smith and Smith, 1973) But with these treatments those with spina bifida can live past 40 years old, with follow up treatments along the years, the majority can live active and productive lives.

Conclusion

To conclude, understanding the precision in the mechanisms and cellular activities that takes place during early embryogenesis will allow us to create better ways in which we can prevent congenital health defects, for spina bifida the current and best suggested to altogether prevent spina bifida from occurring is for the pregnant individual to take folic acid and to maintain a healthy diet, and for those who have spina bifida cases in their family is to take a little more folic acid than what an individual without spina bifida cases in their family. This will allow a normal development of the foetus and prevent surgery or other interventions after birth, and improve the quality of life for both the infant and family and prevent the passing on of spina bifida to further generations. There are future studies looking to expand our understanding on the mechanisms and pathophysiology of spina bifida, to evaluate the long-term impact of in utero intervention and to refine timing and technique of spina bifida surgery using tissue engineering technology. (Adzick. N.S, 2013) With recent spinal surgery technology, even those with severe spina bifida stands a chance at having a long and productive life with minimal complications.

References:

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