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Introduction: Brief Overview on the Major Histocompatibility Complex
Published in Gérard Chaouat, The Immunology of the Fetus, 2020
Jean-Pierre Abastado, P. Kourilsky
The immune system of the host is, therefore, capable of discriminating between self and nonself. Genetic and biochemical studies have shown that the structure recognized on the graft by the immune system is a set of glycoproteins expressed at the surface of its cells and called the transplantation antigens. Some of these antigens cause a more rapid graft rejection (within 12 d in the mouse) and are, therefore, called major transplantation antigens (H-2 in the mouse and HLA in the man). They are expressed on almost all tissues except for the cells of the germinal line and those of the early embryo. They are encoded in several loci. For each locus, a large number of different alleles exists in the populations.1 Since a difference at a single locus is sufficient to trigger the graft rejection, two randomly chosen individuals are generally histo-incompatible. Mutants have been isolated by the virtue of the offspring to reject skin grafts from their parents. The rate of mutation was found very high in some loci (2 × 10−4 per gamete for H-2Kb). Special mechanisms (gene conversion and equal and unequal crossing-over) have been put forward to explain this high rate of mutation and the high level of polymorphism.2
B Cells and Humoral Immunity
Published in Constantin A. Bona, Francisco A. Bonilla, Textbook of Immunology, 2019
Constantin A. Bona, Francisco A. Bonilla
Two additional mechanisms may also result in class-switching, although they are not thought to occur with significant frequency, if at all. One hypothesis is identical to the differential splicing of a transcript containing μ and δ. If we imagine that RNA polymerase may continue along the DNA transcribing all of the C region genes, then VDJH joined to any isotype can be generated by differential splicing. Another model for class-switching invokes unequal sister chromatid exchange. During mitosis, sister chromatids may exchange some portions of themselves. If the positions of the joints between chromatids are identical, one has homologous recombination, or equal crossing over. The amount of genetic information in each chromatid remains unchanged. If the breaks occur in different positions in the chromatids, one has non-homologous recombination, or unequal crossing over. This results in deletion of genetic information from one chromatid, and its duplication in the other.
Repeated DNA Sequences and Polyploidy in Cereal Crops
Published in S. K. Dutta, DNA Systematics, 2019
During the course of evolution, unequal crossing over is believed to be responsible for the generation, maintenance, and variation of the tandemly repeated sequences.77 The origin of the dispersed sequences is different. At least some of them have originated from transposable elements.78,79 Members of a diverse array of repeated sequence families often show a high degree of homogeneity within a species, but show substantial differences between related species. Dover80 has suggested that the “concerted” evolution of such sequences can be brought about by some process of “molecular drive”. It is visualized that certain members of a repeated sequence family preferentially convert other members of the family to their own sequence repeats. Sequences with the strongest drive become the predominant family in a relatively homogeneous array. The tandem array of telomeric heterochromatin in rye chromosomes is an example of homogenization of sequences and subsequent fixation.71 The evolutionary advantage of these events remains unclear.
Copy-number variation of the NPHP1 gene in patients with juvenile Nephronophthisis
Published in Acta Clinica Belgica, 2021
Mayssa Abdelwahed, Ines Maaloul, Valerie Benoit, Pascale Hilbert, Mongia Hachicha, Hassen Kamoun, Leila Keskes-Ammar, Neila Belguith
The NPHP1, responsible for Juvenile nephronophthisis, has been located on chromosome 2 (2q13) [15]. Chromosome 2 appeared after the telomere-telomere fusion of two ancestral chromosomes located in band 2q13 [15]. The telomeric regions contain several repetition sequences which make this region susceptible to rearrangements (deletions or duplications). In addition, the NPHP1 has been shown to be located between two large inverted repeats of 330 kb and a second sequence of 45 kb. These repeats are extremely unstable and fragile, which could have a negative effect on the stability of the NPHP1 region, and lead to the deletion of the whole gene. In order to better understand the mechanisms of CNV formation in juvenile nephronopthisis, Saunier et al. made two hypotheses. They suggest that CNVs could result either in interchromic rearrangement, leading to the replacement of a short genomic sequence containing STS 804H10R by a repeat sequence of 45 or by intra-chromosomal rearrangement by unequal crossing-over, leading to the inversion of the entire NPHP1 kb region [15]. Nevertheless, no study has validated any of these hypotheses.
How many copies of GSTM1 and GSTT1 are associated with head and neck cancer risk?
Published in Biomarkers, 2019
Isabela Firigato, Fernanda de Toledo Gonçalves, Juliana De Antonio, Otávio Alberto Curioni, Mariana Rocha Silva, Gilka Jorge Fígaro Gattás
Because these enzymes participate in several cellular homeostasis maintenance processes, it is reasonable to expect a high frequency of two or more copies of GSTM1 and GSTT1. However, our findings indicate that one copy from each gene presented higher frequencies than two or more copies, considering only the gene carriers among HNC patients and control group. A unique copy of GSTM1 was found in 41.7% of HNC patients and in 47.4% of the subjects with no malignancies, whereas two or more copies were observed in 10.7 and 11.2% of the cases and control, respectively. One copy of GSTT1 was determined in 49.4 of cases and in 49.6% of control, and two or more copies were carried by 28.6 and 29.7% of the HNC patients and control, respectively. The high frequency of one copy of the genes was also found by Emeville et al. (2014) and appeared to guarantee the cellular homeostasis. The predominance of one copy of GSTM1 and GSTT1 may be attributed to the CNV formation mechanism, which arises from mutations during meiosis or mitosis (Conrad et al.2010). For GSTM1 and GSTT1, the CNV is thought to derive from an unequal crossing-over event in chromosomal misalignment between two homologous sequences that flank the genes, resulting in their duplication in tandem or total gene deletion (McLellan et al.1997, Sprenger et al.2000). Their duplication in tandem might be a rare event, which explains the low frequency of two copies and few individuals that carried three copies of the genes.
Protein evolution revisited
Published in Systems Biology in Reproductive Medicine, 2018
Peter L. Davies, Laurie A. Graham
Most fish AFPs have only moderate TH activity (when compared to hyperactive AFPs from insects) and need to be present at 10–35 mg/mL of blood to completely protect the fish from freezing in icy seawater (Raymond and DeVries 1977; Scotter et al. 2006). The mechanism that most fish have employed to attain adequate AFP concentrations is to amplify the AFP gene copy number. The type I AFPs in winter flounder that are produced in the liver for circulation around the body come from ~30 copies of the gene, many of which are present in 7.8-kb tandem repeats (Scott et al. 1985). Once a gene has been duplicated, the opportunity for gene copy number expansion exists through unequal crossing over coupled with selection pressure for higher concentrations of the AFP in the body of the fish (Figure 3D). Type III AFP in the wolffish is also encoded in tandem repeats, the signature of rapid expansion, but in this case with two genes in opposite orientations in each repeat (Scott et al. 1988). The variability of AFP gene organization is striking. In the ocean pout, a related type III AFP-producing species from a different zoarcoid family, the genes are numerous and linked, though not in obvious tandem arrays (Hew et al. 1988).