Repetitive Elements in Humans Liehr, Thomas
International journal of molecular sciences,
02/2021, Letnik:
22, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Repetitive DNA in humans is still widely considered to be meaningless, and variations within this part of the genome are generally considered to be harmless to the carrier. In contrast, for ...euchromatic variation, one becomes more careful in classifying inter-individual differences as meaningless and rather tends to see them as possible influencers of the so-called 'genetic background', being able to at least potentially influence disease susceptibilities. Here, the known 'bad boys' among repetitive DNAs are reviewed. Variable numbers of tandem repeats (VNTRs = micro- and minisatellites), small-scale repetitive elements (SSREs) and even chromosomal heteromorphisms (CHs) may therefore have direct or indirect influences on human diseases and susceptibilities. Summarizing this specific aspect here for the first time should contribute to stimulating more research on human repetitive DNA. It should also become clear that these kinds of studies must be done at all available levels of resolution, i.e., from the base pair to chromosomal level and, importantly, the epigenetic level, as well.
Platforms for “non-invasive prenatal testing” (NIPT), or also referred to as “non-invasive prenatal screening” (NIPS) have been available for over 10 years, and are the most recent tools available to ...obtain information about genetic condition(s) of an unborn child. The highly praised advantage of NIPT-screening is that results can provide early hints on the detection of fetal trisomies and gonosomal numerical aberrations as early as the 10th week of gestation onward, without any need for invasive procedures, such as amniocenteses or alternatives. Understandably, the public along with gynecologists and obstetricians eagerly await these early test results. Their general hope for normal (=negative) test results is also justified, as in >95% of the tested cases such an outcome is to be expected. However, pregnant women can be disappointed and confused, particularly regarding the genetic information and proposed care when the results are positive, and these emotions are also common with false-positive and false-negative NIPT results. Finally, such concerns in understanding the advantages and limitations of this routinely ordered screening tool end up at Clinical Geneticists and Genetic counselors. In this review, general background on NIPT, differences of NIPT platforms, advantages and limitations of NIPT, as well as consequences of insufficient counseling before and after NIPT are summarized. To provide comprehensive care in all pregnancies situations, professionals need a careful attitude toward offering NIPT along with specially training and qualifications in counseling for these procedures. Often it is gynecologists and obstetricians who discuss the use of NIPT with patients; however, although these physicians have a highly qualified background and knowledge in their respective specialty area(s), they may lack specific training on the interpretation of NIPT-screening results. These potential knowledge gaps must be closed quickly and comprehensively by the corresponding scientific societies to ensure optimal patient care.
Abstract
Background
Non-invasive prenatal testing (NIPT) has had an incomparable triumph in prenatal diagnostics in the last decade. Over 1400 research articles have been published, predominantly ...praising the advantages of this test.
Methods
The present study identified among the 1400 papers 24 original and one review paper, which were suited to re-evaluate the efficacy of > 750,000 published NIPT-results. Special attention was given to false-positive and false-negative result-rates. Those were discussed under different aspects—mainly from a patient-perspective.
Results
A 27: 1 rate of false-positive compared to false-negative NIPT results was found. Besides, according to all reported, real-positive, chromosomally aberrant NIPT cases, 90% of those would have been aborted spontaneously before birth. These findings are here discussed under aspects like (i) How efficient is NIPT compared to first trimester screening? (ii) What are the differences in expectations towards NIPT from specialists and the public? and (iii) There should also be children born suffering from not by NIPT tested chromosomal aberrations; why are those never reported in all available NIPT studies?
Conclusions
Even though much research has been published on NIPT, unbiased figures concerning NIPT and first trimester screening efficacy are yet not available. While false positive rates of different NIPT tests maybe halfway accurate, reported false-negative rates are most likely too low. The latter is as NIPT-cases with negative results for tested conditions are yet not in detail followed up for cases with other genetic or teratogenic caused disorders. This promotes an image in public, that NIPT is suited to replace all invasive tests, and also to solve the problem of inborn errors in humans, if not now then in near future. Overall, it is worth discussing the usefulness of NIPT in practical clinical application. Particularly, asking for unbiased figures concerning the efficacy of first trimester-screening compared to NIPT, and for really comprehensive data on false-positive and false-negative NIPT results.
The concept of "chromosomics" was introduced by Prof. Uwe Claussen in 2005. Herein, the growing insights into human chromosome structure finally lead to a "chromosomic view" of the three-dimensional ...constitution and plasticity of genes in interphase nuclei are discussed. This review is dedicated to the memory of Prof. Uwe Claussen (30 April 1945⁻20 July 2008).
Chromosomics is the study of chromosomes, their three-dimensional positioning in the interphase nucleus, the consequences from plasticity of chromosomal subregions and gene interactions, the influence of chromatin-modification-mediated events on cells, and even individuals, evolution, and disease. Progress achieved in recent years is summarized, including the detection of chromosome-chromosome-interactions which, if damaged, lead to malfunction and disease. However, chromosomics in the Human Genetics field is not progressing presently, as research interest has shifted from single cell to high throughput, genomic approaches.
Chromosomics and its impact were predicted correctly in 2005 by Prof. Claussen. Although some progress was achieved, present reconsiderations of the role of the chromosome and the single cell in Human Genetic research are urgently necessary.
Here the role of molecular cytogenetics in the context of yet available all other cytogenomic approaches is discussed. A short introduction how cytogenetics and molecular cytogenetics were ...established is followed by technical aspects of fluorescence
hybridization (FISH). The latter contains the methodology itself, the types of probe- and target-DNA, as well as probe sets. The main part deals with examples of modern FISH-applications, highlighting unique possibilities of the approach, like the possibility to study individual cells and even individual chromosomes. Different variants of FISH can be used to retrieve information on genomes from (almost) base pair to whole genomic level, as besides only second and third generation sequencing approaches can do. Here especially highlighted variations of FISH are molecular combing, chromosome orientation-FISH (CO-FISH), telomere-FISH, parental origin determination FISH (POD-FISH), FISH to resolve the nuclear architecture, multicolor-FISH (mFISH) approaches, among other applied in chromoanagenesis studies, Comet-FISH, and CRISPR-mediated FISH-applications. Overall, molecular cytogenetics is far from being outdated and actively involved in up-to-date diagnostics and research.
Uniparental disomy (UPD) is well-known to be closely intermingled with imprinting disorders. Besides, UPD can lead to a disease by 'activation' of a recessive gene mutation or due to incomplete ...(cryptic) trisomic rescue. Corresponding to all common theories how UPD forms, it takes place as a consequence of a "chromosomic problem", like an aneuploidy or a chromosomal rearrangement. Nonetheless, UPD is rarely considered as a cytogenetic, but most often as a molecular genetic problem.
Here a review on the ~ 4900 published UPD-cases is provided, and even though being biased as discussed in the paper, the following insights have been given from that analysis: (1) the rate of maternal to paternal UPD is 2~3 to 1; (2) at most only ~ 0.03% of the available UPD cases are grasped scientifically, yet; (3) frequencies of single whole-chromosome UPDs are non-random, with UPD(16) and UPD(15) being most frequent in clinically healthy and diseased people, respectively; (4) there is a direct correlation of UPD frequency and known frequent first trimester trisomies, except for chromosomes 1, 5, 11 and 18 (which can be explained); (5) heterodisomy is under- and UPD-mosaicism is over-represented in recent reports; and (6) cytogenetics is not considered enough when a UPD is identified.
As UPD is diagnosed using molecular genetic approaches, and thus by specialists considering chromosomes at best as a whim of nature, most UPD reports lack the chromosomal aspect. Here it is affirmed and substantiated by corresponding data that UPD is a chromosomic disorder in the first place and cytogenetic analyses is indicated in each diagnosed UPD-case.
The formation of small supernumerary marker chromosomes (sSMC) still remains enigmatic. However, it is suggested that majority of all kinds of de novo sSMC (inverted duplication-, ring-, and ...centric-minute-shaped ones) are products of incomplete trisomic rescue during early embryogenesis. Recent work, based on molecular cytogenetics, suggests that these trisomic rescue events are going together with chromothripsis, directed against the supernumerary chromosome to be degraded. Here we present a protocol how to characterize so-called discontinuous sSMC by means of fluorescence in situ hybridization (FISH).
Chromosomal heteromorphisms (CHs) are a part of genetic variation in man. The past literature largely posited whether CHs could be correlated with the development of malignancies. While this ...possibility seemed closed by end of the 1990s, recent data have raised the question again on the potential influences of repetitive DNA elements, the main components of CHs, in cancer susceptibility. Such new evidence for a potential role of CHs in cancer can be found in the following observations: (i) amplification and/or epigenetic alterations of CHs are routinely reported in tumors; (ii) the expression of CH-derived RNA in embryonal and other cells under stress, including cancer cells; (iii) the expression of parts of CH-DNA as long noncoding RNAs; plus (iv) theories that suggest a possible application of the "two-hit model" for euchromatic copy number variants (CNVs). Herein, these points are discussed in detail, which leads to the conclusion that CHs are by far not given sufficient consideration in routine cytogenetic analysis, e.g., leukemias and lymphomas, and need more attention in future research settings including solid tumors. This heightened focus may only be achieved by approaches other than standard sequencing or chromosomal microarrays, as these techniques are at a minimum impaired in their ability to detect, if not blind to, (highly) repetitive DNA sequences.
The Genome Reference Consortium (GRC) has according to its own statement the "mission to improve the human reference genome assembly, correcting errors and adding sequence to ensure it provides the ...best representation of the human genome to meet basic and clinical research needs". Data from GRC is included in genome browsers like UCSC (University of California, Santa Cruz), Ensembl or NCBI (National Center for Biotechnology Information) and are thereby bases for scientific and diagnostically working human genetic community.
Here long standing knowledge deriving from classical molecular genetic, cytogenetic and molecular cytogenetic data, not being considered yet by GRC was revisited.
There were three major points identified: (1) GRC missed to including three chromosomal subbands, each, for 1q32.1, 2p21, 5q13.2, 6p22.3 and 6q21, which were defined by International System for Human Cytogenetic Nomenclature (ISCN) already back in 1980s; instead GRC included additional 6 subbands not ever recognized by ISCN. (2) GRC defined 34 chromosomal subbands of 0.1 to 0.9 Mb in size, while it is general agreement of cytogeneticists that it unlikely to detect chromosomal aberrations below 1-2 Mb in size by GTG-banding. And (3): still all sequences used in molecular cytogenetic routine diagnostics to detect heterochromatic and/ or pericentromeric satellite DNA sequences within the human genome are not included yet into human reference genome. For those sequences, localization and approximate sizes have been determined in the 1970s to 1990, and if included at least ~ 100 Mb of the human genome sequence could be added to the genome browsers.
Overall, taking into account the here mentioned points and correcting and including the data will definitely provide to the still not being completely finished mapping of the human genome.
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