Genetic background of neurological diseases

Dr Szekeres Márta
Genetic background of neurological diseases.
Doktori értekezés, Szegedi Tudományegyetem.

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4.1 The MTHFR A1298C variant in leukoaraiosis Previous studies demonstrated that an elevated serum homocysteine level may be associated with LA. Also, an elevated serum homocysteine level is presumed to be associated with an endothelial dysfunction or microangiopathy. The genetic studies relating to the MTHFR C677T variant suggest that, although the 677T variant is unfavourable because it raises the serum homocysteine level, it does not increase the risk of contracting LA if it is present on its own. We also demonstrated earlier that the 677T variant in combination with other genetic variants increases the risk of contracting LA. The present study reveals that a person carrying either a heterozygous A1298C or a homozygous 1298CC variant is at a higher risk of contracting LA than one carrying neither of them. This genetic variant has not been examined so far in any LA case–control study. Our findings therefore suggest for the first time that the A1298C variant may be more important than the C677T variant in the evolution of LA. We also found that the heterozygous C677T and A1298C variants do 8 not pose a risk of contracting LA if they are present by themselves. However, their combination in the same person leads to a marked risk of contracting LA. Here, although the number of patients displaying the combination of the two heterozygous MTHFR variants was low, the significance level relating to the approximately tenfold increase in the unfavourable combination in the LA group compared with that in the control group suggests that there is a definite link. At present the exact cause of this synergistic effect is not known. However, two possible explanations readily emerge. They are: (1) the two variants can potentially increase the serum homocysteine level in an additive manner and (2) the co-occurrence of the two unfavourable MTHFR genetic variants may influence the regulation of the enzyme. A properly balanced regulation of the MTHFR may be a key factor that can define the daily shifts in the serum homocysteine level. The clustering of the A1298C and C677T variants might give rise to an unfavourable regulatory nature in the dynamically changing activity of the MTHFR. Then the presence of the two heterozygous variants might result in a significantly unfavourable phenotype of the conformation of the enzyme protein. 4.2 The absolute number of mitochondria per cell in leukocyte cells in leukoaraiosis The basic contents for mDNA and dmDNA were found to be statistically the same in the LA group and control group, and the K value was significantly lower for the LA group than that for the control group. This suggested that there was a larger proportion of dmDNA present. Having dmDNA may possibly lead to a mitochondrial malfunction in the following way: a, lower energy production; b, a lower free radical scavenging capacity; c, a lower rate of adaptation to the prevailing demand for energy production; d, a narrower range in the adjustment to the prevailing energy demand; e, a lower metabolic function capacity in general; f, a greater extent of free radical production; and g, a general malfunction of the mitochondrial genetic regulation. No genetic or biochemical data is available to suggest which of these postulated mechanisms actually exist, but a lower and narrower energy capacity appears probable as the main pathomechanism behind LA. It was demonstrated earlier at the molecular level that LA can arise from a very slight, but chronic level of hypoxia, which may be caused by various environmental and genetic susceptibility factors. Our present findings agree with the earlier ones that uncoupling protein genetic variants play a role in the development of LA. The uncoupling proteins govern the electro-chemical gradient between the inner and outer spaces of the mitochondria, this gradient being essential for the energy production of the mitochondria. If dmDNA is associated with any kind of biochemical malfunction, an uncompensated and larger proportion of dmDNA in the cells may be unfavourable from an energetic viewpoint. Our results appear to indicate that the lower the difference between the contents of mDNA (which compensates for malfunctions of the dmDNA) and dmDNA, the larger the risk of contracting LA in a given individual. 9 Overall, the results of our study suggest that the ratio of the dmDNA content and mDNA content may play a significant role in the pathogenesis of LA. These results also point to the need for new approaches for the examination of mitochondrial contents in other common brain disorders. Limitations of the study 1. The numbers of mitochondria in the affected brain tissues could not be examined, as this study was a clinical one in a human patient population. Brain biopsies would not have been ethical; hence we were unable to identify associations between the numbers of mitochondria in different human tissues. 2. Although we found no apparent change in the number of mitochondria in a small cohort of study subjects over several weeks (which involved several turnovers of the mitochondria in the white blood cells), insufficient scientific data is available concerning the stability of the absolute numbers of mitochondria in the different tissues. This should be clarified in future studies. 3. In this study, no investigations were carried out to identify the properties of the normal functioning of the mitochondria with deletion DNAs. However, these limitations do not greatly affect the present results, since they are not directly associated with the findings. Moreover, they really should be viewed as open scientific questions, that should be addressed in future investigations. 4. Although the logistic regression statistical method has greatly decreased the confounding effects of the clinical factors such as age, hypertension, and diabetes mellitus, the results need to be confirmed using a larger population group. 4.3 Genetic polymorphisms of human β-defensins in patients with multiple sclerosis In our present study, an association between human β-defensins and multiple sclerosis was found. By investigating three SNPs of DEFB1, the distributions of the C-44G genotypes were found to be different between patients with MS and those in the healthy control group, while the frequency of the GG genotype was significantly higher in the control population. This suggests that the presence of the G allele most likely leads to strengthened HBD1 antimicrobial activity, which is less frequent among patients with MS. The G allele of C-44G SNP generates a putative binding site for nuclear factor B (NF-B) and in all likelihood induces overexpression. The proposed effect of this SNP could partly explain why the GG genotype was considered to be a protective genotype in atopic dermatitis and also a contributory factor in the susceptibility to Candida colonisation in diabetic patients. By contrast, in these studies, subjects carrying the CC genotype at the -44 locus site of the gene were at a greater risk of becoming infected. It was recently suggested that the C allele of DEFB1 C-44G SNP probably abrogates NF-B -dependent DEFB1 up regulation. The above findings are consistent with our present observation that the GG phenotype might also play a protective role in MS, and vice versa, and the higher frequency of the CC genotype might be connected with a lower expression of human defensin β-1. Among the 250 patients with MS, only 9 10 (4%) had GG homozygote and 62% of the patients had CC homozygote, in contrast with 45% of the control group that had CC homozygote. These observations appear to emphasise the importance of DEFB1 polymorphisms in MS. Similarly, the production of the inducible hBD-2 is lower in MS patients. It is suggested that the significantly lower frequency of the copy number of DEFB2 might be one of the reasons for the decreased levels of circulating hBD2 in the blood samples of patients with MS. When the association between the copy numbers and the plasma levels of hBD2 was investigated, a correlation between the ELISA results and copy number genotypes was found in the control group, but not in the groups of patients with MS. Moreover, the low hBD2 levels correlated nicely with the low frequency of copy numbers (i.e. <4 copy) in the control group, but not in the groups of patients with MS. The low hBD2 levels correlated nicely with the low frequency of copy numbers (i.e. <4 copy), but in the patients with copy numbers >4, the plasma levels of hBD2 did not seem to be elevated. We suppose that other factors not yet defined might be responsible for the low levels of hBD2 even in the case of higher copy numbers. We hypothesise that abnormalities in the production and the function of human defensin-β might be connected with an altered microbiome in MS, as suggested in a recent study. While it is unclear whether enteric microbiota affects human MS, a higher proportion of MS patients exhibited antibody responses against gastrointestinal antigens that those in with healthy control subjects. This might indicate an altered gut microbiome and immune status. In addition, as β defensins can be produced not only by epithelial cells, but also by astrocytes, and microglia cells, their importance in the central nervous system (CNS) needs to be taken into account. Human defensin-β might function as an initial line of defence within the CNS either as an antimicrobial, or an immunomodulator, or both. What is more, these defenins may also be neuroprotective through their ability to inhibit cellular apoptosis in the CNS. The extreme low frequency of the GG genotype of the C-44G SNP of DEFB1, the high frequency of the low copy number (<4) of DEFB2, and the significantly decreased plasma levels of hBD2 in patients highlight the importance of human defensin-β levels in MS patients. Further studies are necessary to elucidate the precise way the impaired function of human defensin-β influences the pathomechanism of multiple sclerosis. 4.4 Association between human defensin β-2 and AD, and between human defensin-α (HNP 1-3) and AD Higher concentrations of the inducible hBD-2 were found in the cerebrospinal fluid and in the sera of AD patients. It is suggested that the significantly higher frequency of the copy number of DEFB4, encoding hBD2, might be one of the reasons for the increased levels of circulating hBD2 in the blood samples and in the CF of patients with MS. 11 As β defensins can be produced not only by epithelial cells, but also by astrocytes, and microglia cells their importance in the CNS should be taken into consideration in future studies. Human defensin-β might function as an initial line of defence within the CNS either as an antimicrobial, or an immunomodulator, or both. In addition, the contribution of the microbiota to AD pathogenesis was recently investigated and it supports the hypothesis of a microbiome-brain axis. Here, microbiome means the collective genomes of total microbiota. Recent studies have also begun to clarify the degree of involvement of microbiome in AD pathogenesis. It was found that the composition of the human microbiome and exposure to pathogens varies with age, diet, lifestyle and biological environment. Studies indicate that incidence of AD and microbiome exposure and complexity vary greatly in different human populations. Here, the participation of defensins in AD should not be neglected. An elevated hBD expression in the CF suggests that brain synthesises antimicrobial peptides and they circulate throughout the ventricular system and protect the CNS against microbial infection. Choroid plexus regulates immune functions between peripheral and brain circulation, and it is well documented in AD neuropathology (epithelial cell atrophy, impaired secretory and transporter functions, reduced amyloid β clearance). The dysfunction of the choroid plexus may give rise to neuropathological and inflammatory processes. In vitro results suggest that astrocytes may be responsible for local hBD-2 synthesis in the brain. Astrocytes and microglia are important in the cerebral neuroinflammatory response, and in vitro they express hBD-1 and -2; and they may modulate adaptive immunity. Williams et al. found a significant elevation of the hBD-1 mRNA level in the chorioid plexus, and increased protein level in hippocampal neurons of an AD brain. The increased hBD-1 expression within an AD brain may be a protective response to inflammatory stimuli and potential modulator of the host’s innate immune response within the CNS. It has been suggested that chronic infections might be initial events in AD pathogenesis, which can lead to persistent inflammatory stimuli. The inflammatory response thereafter may indirectly lead to the upregulation of amyloid β production. It may well be that the induction of defensins is also involved in the amyloid development. Many antimicrobial peptides exhibit structural characteristics including β–sheet conformation similar to amyloid β that contribute to olygomerization. It should also be mentioned here that the existence of the oligomerization of monomeric hBD-2 has now been demonstrated. Not only were the inducible hBD-2 elevated in the cerebrospinal fluid and in the sera of AD patients, but the levels of HNP1-3 were also higher both in the sera and in the CF. Our findings are in accordance with recent data by Watt et al. in 2015, who reported that peripheral α-defensins are elevated in Alzheimer’s disease. However, no measurements of the defensins in the CF were included in their study. While copy number polymorphism of the DEFB4 gene has been reported to influence 12 the production of hBD2, the secretion of HNP 1-3 however seems to be independent of the copy number of the DEFA gene. Hence we did not investigate it here. The present study supports the view of the potential role for antimicrobial peptides like human α and β defensins in AD pathology; they are pathogen targeting agents in brain infections involving AD. Whether the elevated levels of defensins are a consequence of inflammation, or they themselves induce neurodegeneration and amyloid formation is currently unknown. Further investigations are therefore necessary to elucidate the regulatory functions of defensins in the pathomechanism of AD.

Mű típusa: Disszertáció (Doktori értekezés)
Doktori iskola: Interdiszciplináris Orvostudományok Doktori Iskola
Tudományterület / tudományág: orvostudományok > elméleti orvostudományok
Magyar cím: Neurológiai kórképek genetikai háttere
Idegen nyelvű cím: Genetic background of neurological diseases
Témavezető neveBeosztás, tudományos fokozat, intézményEmail
Dr Somogyvári Ferencegyetemi docens, habil, Ph.D., SZTE ÁOK Orvosi Mikrobiológiai és Immunbiológiai Inté
Dr Szolnoki ZoltánPh.D., SZTE ÁOK Orvosi Mikrobiológiai és Immunbiológiai Inté
EPrint azonosító (ID): 3113
Publikációban használt név : Dr Szekeres Márta
A mû MTMT azonosítója: 3195419
doi: 10.14232/phd.3113
A feltöltés ideje: 2016. szept. 26. 08:14
Utolsó módosítás: 2017. márc. 03. 09:24
Egyebek (raktári szám): B 6091
Védés állapota: védett

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