Neuronal Migration

An Overview of the Role of Genetic Mutations in Microcephaly

To what extent does gene expression and mutation affect neuronal migration, and what is neuronal migration’s consequent influence on the risk of developing microcephaly during the gestation period?

Abstract

One of the most critical periods in a person’s life is their fetal development. In this stage, a range of complex processes interact to turn a bundle of undifferentiated cells into a fully functioning human brain. However, this process does not always proceed smoothly. A range of genetic mutations can drive the brain to develop abnormally. In this paper I review the process of gene expression and how it is altered by a range of genetic mutations, especially in the MCPH family of genes. I also cover the process of neuronal migration and neuronal migration’s influence on microcephaly. Finally, I link gene expression and mutation to their consequent effect on neuronal migration and microcephaly. The paper also covers potential areas of study to gain a better understanding of abnormal developments and find cures.

Introduction

For a foetal brain to develop during gestation, a complex process must occur, in which new neurons need to develop and migrate to their necessary positions to form neural circuits. After neurogenesis, the formation of new neurons, neuronal precursor cells move from their initial positions to their target positions in neural circuits. This process is known as neuronal migration. Neuronal migration distributes neurons across the nervous system. The process is controlled by chemical signals. The absence of these chemical signals, or dysregulation as a result of factors such as genetics or exposure to toxins, can result in abnormal developments of the brain. Microcephaly is one such abnormal development where the circumference of the head at birth is smaller than the average head circumference1. These patients have smaller brain volumes due to lack of proper development. There are two major types of microcephaly: primary and secondary. In primary microcephaly, the brain is smaller at the time of birth of the child. In secondary microcephaly, the brain is the average size at birth but growth halts soon afterwards. Microcephaly is rare (approximately 2-12 infants per 10,000 births)2 and untreatable. Symptoms include intellectual and muscular disability, and in severe cases, seizures too. Understanding microcephaly better will enable the development of treatments to increase the quality of life of patients. Prior work suggests gene expression and mutations play a significant role in neuronal migration. Gene expression is when the genetic information from DNA is transcribed to short-lived instructions for the cell, in the form of messenger RNA (mRNA), which is translated by the cell to make proteins, used in various processes in the human body, for example, repair of cells, energy, and hormones3. Genetic mutations occur when there is an alteration in the basic units of DNA, nucleotides. A nucleotide consists of a phosphate group, a pentose sugar (deoxyribose in DNA or ribose in RNA), and a nitrogenous base pair. The bases in the DNA are adenine (A), thymine (T), cytosine (C) and guanine (G). Sequences of these four bases code for every protein our cells make. The bases in RNA are A, C, G, and uracil (U) instead of T. Since specific genes code for proteins, an alteration in a base pair will cause a genetic mutation, resulting in either a malfunction or incorrectly formed protein. MCPH genes are a family of primary microcephaly genes. They comprise 7 loci and influence cell division and spindle orientation. This paper synthesizes the relevant work in this field, more specifically, gene expression, and mutations especially in the MCPH family - and the effect on proteins like the spindle-associated microcephaly protein (WDR62) - to provide a perspective of the field’s current understanding of microcephaly and proposes potential solutions.

Gene Expression

The Central Dogma6 was first proposed by Francis Crick, a British molecular biologist most known for his work in discovering the DNA helix, in 1958. The purpose of this development was to understand the transfer of genetic information from DNA to mRNA, translating into proteins. The DNA molecule is packed into chromosomes, thread-like structures, by tightly coiling around histone proteins7. Chromosomes are present in the nuclei of cells and contain all genetic information in the DNA. Like the name suggests, the mRNA or messenger RNA, is a messenger responsible for carrying genetic information to the ribosomes which then synthesize amino acid into proteins using transfer RNA (tRNA). This process of converting the genetic code to proteins is known as gene expression and has two major stages – transcription (DNA to mRNA) and translation (mRNA to protein). The final products, proteins, composed of repeating units of amino acids, play different roles in the human body. Proteins can be described to dictate cell function and hence directly control observable characteristics in the body, or phenotype. This forms a relationship between the genotype and phenotype – genotype influences the phenotype since genes control protein synthesis and proteins manipulate the phenotype8. Gene expression is a very strictly regulated process as the amount of proteins produced can be used to manipulate the synthesis of a ‘gene-product’, i.e., a final protein which affects the targeted phenotype. Gene expression is stated to have an ‘on/off’ switch and when expressed, a gene is said to be ‘on’.

Gene expression can be influenced by both internal and external factors. Internal factors, for example hormones, cannot be controlled definitively by individuals. However, they may have control over external or environmental factors. Exposure to mutagens, oxygen levels, diet, temperature fluctuations, humidity and circadian rhythms are few examples of environmental factors that can be controlled to a certain extent9. Additionally, gene expression can be altered due to genetic mutations.

Genetic Mutations

Mutations, or changes in the DNA base pair, occur in genes causing the final, translated proteins to malfunction. Mutations can be categorised into four major groups – germline mutations, somatic mutations, chromosomal alterations and point mutations10. Germline mutations occur in gametes and can thus be inherited by an offspring. Somatic mutations occur in other cells of the body and cannot be passed on to the next generation. Chromosomal alterations change the structure of the chromosome as sections of the chromosome undergo either deletion, duplication, insertion, inversion or translocation. Point mutations occur when single nucleotides experience changes, i.e., a change in base pairs. These can be of 3 types – silent (same amino acid coded), missense (different amino acid) and nonsense (stop codon, which when reached halts the process of transcription).

Recent studies have shown a link between genetic mutations and the onset of microcephaly. Primary microcephaly genes, MCPH genes, form proteins that are responsible for cell division, more specifically spindle orientation (the process of spindles aligning in their correct angles and positions, using the spindle motor and encoder)11, and hence play a significant role in brain development as early as in the gestation period. Mutations in MCPH genes have been shown to cause abnormal developments, leading to microcephaly. Studies state a minimum of 7 MCPH loci, identifying five genes: the MCPH1 locus encodes Microcephalin which controls brain size. The MCPH2 locus contains the WDR62 expressed in neural precursors and progenitors and is responsible for spindle positioning. MCPH3 encodes CDK5RAP212 or Cyclin dependent kinase 5 regulatory subunit-associated protein 2, a centrosome protein. MCPH 6 - Centrosome protein J encoded by the CENPJ gene, which plays a role in maintaining centrosomes and spindles. Lastly, MCPH 5 with the ASPM gene encoding protein Abnormal Spindle Protein Homolog7. The ASPM protein is present in most cells of the human body, but is most useful in the brain during cell division of neurons in the early development period. It controls the process, maintaining appropriate division. Previous studies have shown a link between ASPM gene mutations and primary microcephaly, through the formation of truncated proteins13. The mutations discussed above have been found to play a role in inducing primary microcephaly through their influence on neuronal migration.

Neuronal Migration

Neuronal migration occurs when neurons migrate to their positions in the body. There are two main types of neuronal migration in the brain – tangential (movement parallel to the pial surface) and radial (perpendicular to the neuroepithelial surface) migration14. The early cerebral cortex comprises the ventricular zone, subventricular zone, cortical plate and marginal zone15. The WDR62 gene at the MCPH2 locus plays an important role during cortical development - as discussed earlier, it is responsible for spindle positioning. It is expressed at spindle poles in neural precursors and progenitor cells. Mutations in WDR62 could cause defects in cell division and migration of these cells to their correct positions in the early cerebral cortex. It has also been found that a smaller brain size (like in microcephaly) has a correlation with fewer neural progenitor cells. Mutation in WDR62 causes fewer neural stem cells and intermediate neural progenitors (INPs) that would later develop into neural progenitor cells, as the mutation either halts or delays the process of mitosis leading to cell death16. Fish et al (2006) examined the role of ASPM in detail, by studying mice neuroepithelial (NE) cells. They found that ASPM is primarily responsible for maintaining symmetrical proliferative divisions of NE cells17. Their study demonstrated that maximum ASPM concentration is present at mitotic spindle poles of NE cells. When RNA interferes, the ASPM concentration decreases at the poles. Consequently, NE cells’ cleavage plane is altered (the perpendicular position is lost), decreasing the likelihood of symmetric cell division. This causes cell fate determinants to distribute evenly amongst daughter cells, causing them to function appropriately. Asymmetric cell division can cause cellular diversity, leading to abnormal developments (microcephaly). Further manipulations of ASPM concentrations confirmed their initial findings, allowing them to form a link between ASPM and the onset of primary microcephaly17.

Future Directions

In the past few years, scientists have furthered their understanding of neuronal migration, microcephaly and the genes involved. However, more significant research is needed to completely grasp the root cause of the disorder. For example, epigenetics, in the form of environmental factors, may be involved in causing gene mutations. Robinson et al.19 studied centrosome regulation in neural stem cells of Drosophila melanogaster to understand their functioning in microcephaly. Drosophila melanogaster shares genes and a few metabolic pathways with humans. Thus, studying the Drosophila gave researchers insight into centrosomes. However, no similar study has been carried out on humans to fully gauge the functioning of the centrosome. As a result, there isn’t enough research to confidently reach conclusions as of now. In the coming few years, with new technological advancements, it is likely that the areas mentioned above will be studied in greater depth. In the meantime, treatments like gene therapy should be explored, because no cure seems plausible as of now. Even though the genes involved in microcephaly are quantitatively few, the types of mutations are vast. Hence, advancements in gene therapy are of extreme importance to improve the quality of life of patients of following generations. The MCPH gene family should be studied in the further depth as well, focussing especially on the unidentified loci. The MCPH gene group is vast in itself, but other gene groups could also be examined to rule out potential correlation to microcephaly. Overall, this field of abnormal developments, more specifically microcephaly, requires more qualitative research fueled by advancements in technology in perhaps the near future.

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