Non-state types of inheritance

A fairly large number of hereditary diseases due to DNA changes are known, but they do not have the Mendelian character of inheritance. Below we will consider mitochondrial inheritance of and mitochondrial diseases, as well as imprinting.

Mitochondrial inheritance and mitochondrial diseases

Mitochondria are cellular organelles. Mitochondria have two highly specialized membranes - the outer and inner, the ring DNA molecule, as well as their own systems of transcription and translation. Each cell contains several hundred mitochondria. They carry out a number of important biochemical reaction chains, of which the reactions of the energy metabolism of the cell are of particular importance.

As already noted, mitochondria have their own DNA, each mitochondria contains 10 or more DNA molecules. The genome of mitochondrial DNA( mgDNA) is completely deciphered.

Disturbance of the interaction between the mitochondrial and nuclear genomes causes a variety of mitochondrial pathologies.

Since mtDNA is contained in the cytoplasm of cells, it is inherited only on the maternal line. In the cytoplasm of an egg, there are thousands of mitochondria and, consequently, tens of thousands of mtDNA molecules. At the same time in the spermatozoon There are only a few mtDNA molecules that do not fall into the fertilized egg. Therefore, men inherit mtDNA from their mothers, but do not pass it on to their descendants. This type of inheritance is called maternal inheritance or maternal inheritance.

Normally, all copies of mtDNA are identical, and this condition is called homoplasmy. Sometimes mutations occur in mtDNA.Due to the not very perfect work of mitochondrial DNA polymerase and repair systems, mutations in mtDNA appear 10 times more often than in nuclear DNA.The appearance of a mutation in one of the mtDNA molecules can lead to the emergence of two populations of mtDNA in the cell, which is called heteroplasia. As a result of cell division, the mutant mtDNA enters other cells, where it continues to multiply.

The energy needs of different tissues of the body are different. The most energy consuming is the nervous system. That is why this system is primarily affected by mitochondrial diseases.

The classification of mitochondrial diseases is based on two principles:

1) the involvement of a mutant protein in the energy reactions of oxidative phosphorylation;

2) whether the mutant mtDNA or nuclear DNA is encoded.

Class I of mitochondrial diseases includes atrophy of Leber's discs of the optic nerves. The disease manifests itself as acute or subacute loss of central vision due to atrophy of the optic nerves. The disease can begin both in childhood and in old age. In some patients, the atrophy of the optic nerves is combined with the symptoms of encephalomyopathy. Leber's optic atrophy caused by mutations in mtDNA genes encoding subunits of complex I.

This class refers Leigh disease( subacute necrotizing encephalomyelopathy).Leia syndrome occurs only when the mutant mtDNA is at least 90% of the total mtDNA.If the percentage of mutant DNA is lower, then a syndrome of neuropathy, ataxia and retinitis pigmentosa appears.

The syndrome of neuropathy, ataxia and retinal pigmentary dystrophy( NARP) can manifest itself both in infancy and later, up to the 2nd decade of life. In addition to the pathology included in the name of the syndrome, patients may have dementia, seizures, motor-sensory neuropathy, and hearing loss.

Syndrome myoclonus epilepsy and ragged red muscle fibers( MERRF), which is manifested epilepsy, dementia, ataxia, and myopathy occurs in the case of a mutation in the gene for the tRNA.The syndrome can be manifested in childhood and adulthood. In addition to these symptoms, in patients with MERRF syndrome, neurosensory hearing loss, dementia, atrophy of the optic nerves, spastic diplegia are sometimes observed in patients. Usually, this syndrome reveals pronounced heteroplasmy, so the expressiveness of the syndrome varies dramatically.

Another syndrome caused by point replacement gene tRNA - a syndrome and mitochondrial encephalomyopathies stroke-episodes( MELAS).He also has heteroplasmy, and as a result, the expressiveness of the syndrome varies quite a lot. Major clinical manifestations include encephalomyopathies, stroke-state, usually transient, with restoration functions, seizures, ataxia, myoclonus, epilepsy, migraine headaches.

Mitochondrial diseases due to fission or duplication include Cairns-Saira syndrome( myopathy, cerebellar disorders and heart failure), Pearson's syndrome( pancytopenia, lactic acidosis and pancreatic insufficiency), and chronic progressive external ophthalmoplegia, which is manifested by omissioncentury.

The interaction between nuclear and mitochondrial genomes explains the mtDNA depletion syndrome, as well as the syndrome of multiple mtDNA divisions. Both these conditions are inherited as autosomal dominant signs, therefore the mutations of nuclear genes are probably the cause.

Diseases of the mitochondrial respiratory chain due to mutations of nuclear genes can be combined into two groups: mitochondrial myopathies and mitochondrial encephalomyopathies. These diseases are inherited as Mendelian signs, but due to the inadequacy of enzymes that enter into one of the complexes of the mitochondrial respiratory chain.

Genomic imprinting

To date, there are three classes of exceptions to the Mendelian rule for the identity of hybrids in the 1st generation. The first exception has been known for a long time, and it is associated with X-linked inheritance.

The second, just discussed, concerns the characteristics identified by mtDNA genes that have a so-called maternal inheritance. These two classes of deviations from Mendelian inheritance are based on differences in the genetic contribution of parents to the genotype of the offspring. In X-linked inheritance, the offspring can receive from the mother only the X chromosome, while the father of the chromosome either X or Y. In mitochondrial inheritance, the zygote that results from the fusion of the sex cells receives mitochondria and the mtDNA contained therein only through the oocyte.

Recently, geneticists and embryologists described the third exception - genomic imprinting, when both parents transmit completely identical genes to the descendants, but these genes carry a specific imprint of the parents' sex, the paternal and maternal genes are activated or suppressed during gametogenesis in different ways. Thus, in some cases it is important from which of the parents the gene is inherited.

The term imprinting( imprint) was first proposed in 1960 by Crouse of Columbia University in the United States.

Genomic imprinting occupies a special place among specific mechanisms regulating gene activity in the early stages of development, leading to differences in the expression of homologous maternal and paternal alleles. Subsequent genetic modifications may lead to the fact that changes in gene expression will be stably transferred during the development of cell generations. Genomic imprinting, for example, can alter the dose of genes that control embryonic growth, cell proliferation, and differentiation.

An example of the imprinting of an entire genome in humans is a true bladder drift that occurs when the ovum is fertilized without a mother chromosome, with two spermatozoa. Despite the presence of a full diploid set, early embryogenesis of such zygotes proceeds abnormally: tissues of the embryo itself are not formed at all. In the case of a double set of maternal chromosomes, a teratoma-embryonic tumor develops. Only the maternal or only paternal genomes are not able to ensure the normal development of the embryo.

At the organism level, the effect of imprinting was detected due to the presence of single( parent or paternal) origin in the chromosomal set of fragments or whole chromosomes - the so-called single-parent diisomy( ORD), namely, a qualitative rather than quantitative chromosomal imbalance is observed.

In recent years, the effect of genomic imprinting has been intensively studied in connection with various pathologies in humans. Examples of diseases, which are based on the disorder of the function of the imprinted areas of the genome, are quite numerous, so we can talk about a special class of human diseases - "imprinting diseases", which number is already more than 30.

The most convincing data are obtained with Prader-Willi syndrome( SPS) andthe Engelman syndrome( SE), which, having significantly different clinical manifestations, basically have similar molecular-cytogenetic changes.

Becquit-Wiedemann syndrome( SBV) is also well studied in terms of imprinting, with the following main symptoms: macrosomia, macroglossia, umbilical hernia, and an increased predisposition to tumors.

The association of genomic imprinting with another human hereditary pathology at the level of chromosomes or individual genes is also clearly traced and is currently being widely studied. So, for example, with Huntington's chorea and spinal cartilage ataxia, the disease occurs earlier and proceeds more severely if the inherited genes are of paternal origin. In neurofibromatosis, myotonic dystrophy, on the contrary, the disease has an earlier onset and a heavy course with the inheritance of mutant genes from the mother. There is no doubt about the implication of genomic imprinting in the etiology of tumor growth.

In recent years, with the help of molecular genetic methods, the phenomenon of genomic imprinting has been observed in multifactorial diseases. For example, clearly expressed paternal imprinting is found in atopic dermatitis, maternal - with bronchial asthma and atopy in children. With insulin-dependent diabetes mellitus, a higher probability of paternal imprinting was found.

GENE ENGINEERING

Methods of molecular genetics have been described above that are used to identify genes of mendicant human hereditary diseases, such methods are included in the international program "Human Genome".Below we will consider the main provisions of genetic engineering and the essence of the project "Human Genome".

In February 2001, simultaneously in two journals, "Nature" and "Science", the results of the draft of the whole human genome, obtained independently from each other by an international consortium, the project "Human Genome" and private company "Celera", for which the genome projectperson is a commercial enterprise. These publications, despite the incompleteness of the project, are a significant achievement of all biological science and medicine.

Recombinant DNA technology

Indeed, by the time the "Human Genome" program was launched, a whole trend in molecular genetics had been formed, which was called "genetic engineering", or "recombinant DNA technology".The latter can be divided into two large areas: DNA cloning techniques and DNA analysis methods, primarily the determination of the nucleotide sequence in the DNA molecule.

Cloning of DNA

Cloning of DNA in vivo( in a living organism) involves 6 steps:

1) obtaining DNA fragments, including genes or parts thereof, using restriction enzymes;

2) recombination of fragments;

3) Inserting a DNA fragment into a vector;

4) transformation with the vector of the host organism;

5) screening for the recombinant vector;

6) selection of interesting clone researchers.

The concept of restriction enzymes

In each human chromosome, there is only one continuous strand of DNA.It is difficult to pack in order to fit in the chromosome. It is practically impossible to manipulate with a DNA molecule of this length. Therefore, the discovery in the 70's. XX century.special bacterial enzymes that cut DNA into separate fragments, was very relevant. Enzymes have been termed restriction enzymes or endonucleases. In bacteria, these enzymes serve to protect against the entry into the cell of foreign DNA.

Recombination of DNA fragments

Restrictases cut both strands of DNA, which as a result form either blunt or sticky ends. The DNA of one organism is cut by a specific restriction enzyme in strictly defined places, therefore such a DNA after restriction( which is also called digestion) will always give the same set of fragments. If one type of restriction enzyme is used to cut DNA from different organisms, then the set of fragments will be different, but the sequence of nucleotides at the cutting sites will be the same for all fragments and, consequently, complementary to each other when fragments of sticky ends form. The latter are called sticky, because because of their complementarity they can be combined with other fragments formed by the same restriction enzyme or other restriction endonuclease, which forms the same ends. The combination of fragments with sticky complementary ends is accelerated and stabilized by a special enzyme called ligase. Thus, if a single restriction enzyme is cut into DNA of two different species and mixed fragments, then a completely new recombinant DNA molecule, which does not exist under natural conditions, can form.

In order to explore a DNA fragment of interest to a researcher, it must be multiplied. This can be done by two different methods, moving it to the host cell or multiplying it in vitro( in vitro).

Introduction of DNA fragments into a host cell using

vectors To move a DNA fragment to a host cell, special constructs are commonly used, which are called vectors. The most frequently used vectors are bacterial substances, bacteriophages, bacterial and yeast artificial chromosomes. Recently, it has been proposed to use human artificial chromosomes as vectors.

Creation of genomic libraries of

The restriction of genomic DNA into fragments and cloning of fragments with the help of various vectors created the basis for the formation of genomic libraries. To do this, genomic DNA is cut or, as they say, digested with a certain restriction enzyme, and the fragments formed are cloned by means of different vectors, for which recombinant DNA methods are used. The genomic library should contain not only genes, but also all non-coding DNA located between the genes. Since digestion with a restriction enzyme is not complete, so that DNA fragments with partially overlapping nucleotide sequences are formed. This facilitates the subsequent restoration of the pattern of the location of fragments in native DNA( DNA in the living body).In addition to genomic libraries, there are cDNA libraries.

Cloning of DNA sequences by polymerase chain reaction( PCR)

In addition to the described method for cloning DNA sequences in vivo, there is also an in vitro cloning method which has been termed polymerase chain reaction( PCR).

A prerequisite for conducting PCR is knowing the sequence of nucleotides that determine the cloned sequence. For carrying out PCR, it is necessary to synthesize a pair of so-called primers, which are short sequences of nucleotides that are complementary to the sequences of the DNA fragment being replicated.

After separation into two strands of the studied DNA fragment, substances are added to the reaction mixture which are complementarily associated with the corresponding sections of these strands. Then follows the separation of the newly formed DNA chains by means of a temperature treatment. To the newly formed strands of the DNA fragment, complementary strands are again completed using the DNA polymerase enzyme.

This can be repeated indefinitely or until the free nucleotides are exhausted in the reaction mixture, but usually 20-30 cycles are sufficient to obtain a sufficient amount of the DNA of the fragment being studied for any subsequent manipulation of this fragment.