Germline mutation
A germline mutation, or germinal mutation, is any detectable variation within germ cells (cells that, when fully developed, become sperm and ova).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Mutations in these cells are the only mutations that can be passed on to offspring, when either a mutated sperm or oocyte come together to form a zygote.<ref name="Griffiths 2000">Template:Cite journal</ref> After this fertilization event occurs, germ cells divide rapidly to produce all of the cells in the body, causing this mutation to be present in every somatic and germline cell in the offspring; this is also known as a constitutional mutation.<ref name="Griffiths 2000"/> Germline mutation is distinct from somatic mutation.
Germline mutations can be caused by a variety of endogenous (internal) and exogenous (external) factors, and can occur throughout zygote development.<ref name=":0">Template:Cite journal</ref> A mutation that arises only in germ cells can result in offspring with a genetic condition that is not present in either parent; this is because the mutation is not present in the rest of the parents' body, only the germline.<ref name=":0" />
When mutagenesis occursEdit
Germline mutations can occur before fertilization and during various stages of zygote development.<ref name=":0" /> When the mutation arises will determine the effect it has on offspring. If the mutation arises in either the sperm or the oocyte before development, then the mutation will be present in every cell in the individual's body.<ref name=":9">Template:Cite journal</ref> A mutation that arises soon after fertilization, but before germline and somatic cells are determined, then the mutation will be present in a large proportion of the individual's cell with no bias towards germline or somatic cells, this is also called a gonosomal mutation.<ref name=":9" /> A mutation that arises later in zygote development will be present in a small subset of either somatic or germline cells, but not both.<ref name=":0" /><ref name=":9" />
CausesEdit
Endogenous factorsEdit
A germline mutation often arises due to endogenous factors, like errors in cellular replication and oxidative damage.<ref name=":1" /> This damage is rarely repaired imperfectly, but due to the high rate of germ cell division, can occur frequently.<ref name=":1">Template:Cite journal</ref>
Endogenous mutations are more prominent in sperm than in ova.<ref name=":2">Template:Cite journal</ref> This is because spermatocytes go through a larger number of cell divisions throughout a male's life, resulting in more replication cycles that could result in a DNA mutation.<ref name=":1" /> Errors in maternal ovum also occur, but at a lower rate than in paternal sperm.<ref name=":1" /> The types of mutations that occur also tend to vary between the sexes.<ref name=":12" /> A mother's eggs, after production, remain in stasis until each is utilized in ovulation. This long stasis period has been shown to result in a higher number of chromosomal and large sequence deletions, duplications, insertions, and transversions.<ref name=":12">Template:Cite journal</ref> The father's sperm, on the other hand, undergoes continuous replication throughout his lifetime, resulting in many small point mutations that result from errors in replication. These mutations commonly include single base pair substitutions, deletions, and insertions.<ref name=":2" />
Oxidative damage is another endogenous factor that can cause germline mutations. This type of damage is caused by reactive oxygen species that build up in the cell as a by-product of cellular respiration.<ref name=":13" /> These reactive oxygen species are missing an electron, and because they are highly electronegative (have a strong electron pull) they will rip an electron away from another molecule.<ref name=":13">Template:Cite journal</ref> This can initiate DNA damage because it causes the nucleic acid guanine to shift to 8-oxoguanine (8-oxoG). This 8-oxoG molecule is then mistaken for a thymine by DNA polymerase during replication, causing a G>T transversion on one DNA strand, and a C>A transversion on the other.<ref>Template:Cite journal</ref>
Male germlineEdit
In mice and humans the spontaneous mutation rate in the male germ line is significantly lower than in somatic cells.<ref name = Aitken2023>Aitken RJ, Lewis SEM. DNA damage in testicular germ cells and spermatozoa. When and how is it induced? How should we measure it? What does it mean? Andrology. 2023 Jan 5. doi: 10.1111/andr.13375. Epub ahead of print. PMID 36604857</ref> Furthermore, although the spontaneous mutation rate in the male germ line increases with age, the rate of increase is lower than in somatic tissues. Within the testicular spermatogonial stem cell population the integrity of DNA appears to be maintained by highly effective DNA damage surveillance and protective DNA repair processes.<ref name = Aitken2023/> The progressive increase in the mutation rate with age in the male germ line may be a result of a decline in the accuracy of the repair of DNA damages, or of an increase in DNA replication errors. Once spermatogenesis is complete, the differentiated spermatozoa that are formed no longer have the capability for DNA repair, and are thus vulnerable to attack by prevalent oxidative free radicals that cause oxidative DNA damage. Such damaged spermatozoa may undergo programmed cell death (apoptosis).<ref name = Aitken2023/>
Exogenous factorsEdit
A germline mutation can also occur due to exogenous factors. Similar to somatic mutations, germline mutations can be caused by exposure to harmful substances, which damage the DNA of germ cells. This damage can then either be repaired perfectly, and no mutations will be present, or repaired imperfectly, resulting in a variety of mutations.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Exogenous mutagens include harmful chemicals and ionizing radiation; the major difference between germline mutations and somatic mutations is that germ cells are not exposed to UV radiation, and thus not often directly mutated in this manner.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Clinical implicationsEdit
Different germline mutations can affect an individual differently depending on the rest of their genome. A dominant mutation only requires a single mutated gene to produce the disease phenotype, while a recessive mutation requires both alleles to be mutated to produce the disease phenotype.<ref name=":11">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> For example, if the embryo inherits an already mutated allele from the father, and the same allele from the mother underwent an endogenous mutation, then the child will display the disease related to that mutated gene, even though only one parent carries the mutant allele.<ref name=":11" /> This is only one example of how a child can display a recessive disease while a mutant gene is only carried by one parent.<ref name=":11" /> Detection of chromosomal abnormalities can be found in utero for certain diseases by means of blood samples or ultrasound, as well as invasive procedures such as an amniocentesis. Later detection can be found by genome screening.
CancerEdit
Mutations in tumour suppressor genes or proto-oncogenes can predispose an individual to developing tumors.<ref name=":10">Template:Cite news</ref> It is estimated that inherited genetic mutations are involved in 5-10% of cancers.<ref name="genetics">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> These mutations make a person susceptible to tumor development if the other copy of the oncogene is randomly mutated. These mutations can occur in germ cells, allowing them to be heritable.<ref name=":10" /> Individuals who inherit germline mutations in TP53 are predisposed to certain cancer variants because the protein produced by this gene suppresses tumors. Patients with this mutation are also at a risk for Li–Fraumeni syndrome.<ref name="genetics" /> Other examples include mutations in the BRCA1 and BRCA2 genes which predispose to breast and ovarian cancer, or mutations in MLH1 which predispose to hereditary non-polyposis colorectal cancer.
Huntington's diseaseEdit
Huntington's disease is an autosomal dominant mutation in the HTT gene. The disorder causes degradation in the brain, resulting in uncontrollable movements and behavior.<ref name=":14">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The mutation involves an expansion of repeats in the Huntington protein, causing it to increase in size. Patients who have more than 40 repeats will most likely be affected. The onset of the disease is determined by the amount of repeats present in the mutation; the greater the number of repeats, the earlier symptoms of the disease will appear.<ref name=":14" /><ref>Template:Cite book</ref> Because of the dominant nature of the mutation, only one mutated allele is needed for the disease to be in effect. This means that if one parent is affected, the child will have a 50% chance of inheriting the disease.<ref name=":15">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> This disease does not have carriers because if a patient has one mutation, they will (most likely) be affected. The disease typically has a late onset, so many parents have children before they know they have the mutation. The HTT mutation can be detected through genome screening.
Trisomy 21Edit
Trisomy 21 (also known as Down syndrome) results from a child having three copies of chromosome 21.<ref name=":3" /> This chromosome duplication occurs during germ cell formation, when both copies of chromosome 21 end up in the same daughter cell in either the mother or father, and this mutant germ cell participates in fertilization of the zygote.<ref name=":3">Template:Cite journal</ref> Another, more common way this can occur is during the first cell division event after the formation of the zygote.<ref name=":3" /> The risk of Trisomy 21 increases with maternal age with the risk being 1/2000 (0.05%) at age 20 increasing to 1/100 (1%) at age 40.<ref>Template:Cite journal</ref> This disease can be detected by non-invasive as well as invasive procedures prenatally. Non-invasive procedures include scanning for fetal DNA through maternal plasma via a blood sample.<ref>Template:Cite journal</ref>
Cystic fibrosisEdit
Cystic fibrosis is an autosomal recessive disorder that causes a variety of symptoms and complications, the most common of which is a thick mucous lining in lung epithelial tissue due to improper salt exchange, but can also affect the pancreas, intestines, liver, and kidneys.<ref name=":4">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref> Many bodily processes can be affected due to the hereditary nature of this disease; if the disease is present in the DNA of both the sperm and the egg, then it will be present in essentially every cell and organ in the body; these mutations can occur initially in the germline cells, or be present in all parental cells.<ref name=":4" /> The most common mutation seen in this disease is ΔF508, which means a deletion of the amino acid at the 508 position.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> If both parents have a mutated CFTR (cystic fibrosis transmembrane conductance regulator) protein, then their children have a 25% of inheriting the disease.<ref name=":4" /> If a child has one mutated copy of CFTR, they will not develop the disease, but will become a carrier of the disease.<ref name=":4" /> The mutation can be detected before birth through amniocentesis, or after birth via prenatal genetic screening.<ref name=":16">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Current therapiesEdit
Many Mendelian disorders stem from dominant point mutations within genes, including cystic fibrosis, beta-thalassemia, sickle-cell anemia, and Tay–Sachs disease.<ref name=":11"/> By inducing a double stranded break in sequences surrounding the disease-causing point mutation, a dividing cell can use the non-mutated strand as a template to repair the newly broken DNA strand, getting rid of the disease-causing mutation.<ref name=":6" /> Many different genome editing techniques have been used for genome editing, and especially germline mutation editing in germ cells and developing zygotes; however, while these therapies have been extensively studied, their use in human germline editing is limited.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
CRISPR/Cas9 editingEdit
This editing system induces a double stranded break in the DNA, using a guide RNA and effector protein Cas9 to break the DNA backbones at specific target sequences.<ref name=":6" /> This system has shown a higher specificity than TALENs or ZFNs due to the Cas9 protein containing homologous (complementary) sequences to the sections of DNA surrounding the site to be cleaved.<ref name=":6">Template:Cite journal</ref> This broken strand can be repaired in 2 main ways: homologous directed repair (HDR) if a DNA strand is present to be used as a template (either homologous or donor), and if one is not, then the sequence will undergo non-homologous end joining (NHEJ).<ref name=":6" /> NHEJ often results in insertions or deletions within the gene of interest, due to the processing of the blunt strand ends, and is a way to study gene knockouts in a lab setting.<ref name=":5">Template:Cite journal</ref> This method can be used to repair a point mutation by using the sister chromosome as a template, or by providing a double stranded DNA template with the CRISPR/Cas9 machinery to be used as the repair template.<ref name=":6" />
This method has been used in both human and animal models (Drosophila, Mus musculus, and Arabidopsis), and current research is being focused on making this system more specific to minimize off-target cleavage sites.<ref>Template:Cite journal</ref>
TALEN editingEdit
The TALEN (transcription activator-like effector nucleases) genome editing system is used to induce a double-stranded DNA break at a specific locus in the genome, which can then be used to mutate or repair the DNA sequence.<ref name=":7">Template:Cite journal</ref> It functions by using a specific repeated sequence of an amino acid that is 33-34 amino acids in length.<ref name=":7" /> The specificity of the DNA binding site is determined by the specific amino acids at positions 12 and 13 (also called the Repeat Variable Diresidue (RVD)) of this tandem repeat, with some RVDs showing a higher specificity for specific amino acids over others.<ref>Template:Cite journal</ref> Once the DNA break is initiated, the ends can either be joined with NHEJ that induces mutations, or by HDR that can fix mutations.<ref name=":6" />
ZFN editingEdit
Similar to TALENs, zinc finger nucleases (ZFNs) are used to create a double stranded break in the DNA at a specific locus in the genome.<ref name=":7" /> The ZFN editing complex consists of a zinc finger protein (ZFP) and a restriction enzyme cleavage domain.<ref name=":8">Template:Cite journal</ref> The ZNP domain can be altered to change the DNA sequence that the restriction enzyme cuts, and this cleavage event initiates cellular repair processes, similar to that of CRISPR/Cas9 DNA editing.<ref name=":8" />
Compared to CRISPR/Cas9, the therapeutic applications of this technology are limited, due to the extensive engineering required to make each ZFN specific to the desired sequence.<ref name=":8" />