Akhondrogenez – group of the hereditary diseases belonging to the class of the heavy skeletal displaziya which are often leading to antenatalny death or a lethal outcome at early age. The main display of these pathologies is sharp violation of processes of ossification which is externally shown by shortening of extremities, an underdevelopment of edges, a thorax, backbone. Diagnostics is made on the basis of radiological data, and also genetic researches – search of mutations in the genes associated with akhondrogenezy. Specific treatment is absent, only the supporting therapy and resuscitation actions is possible.
Akhondrogenez – several genetic diseases which are characterized by an underdevelopment of bone and cartilaginous elements of a skeleton. The term "akhondrogenez" was offered in 1952 by the Italian doctor Marko Frakkaro, in his honor one of forms of this pathology is called. On the basis of radiological data now allocate three types of an akhondrogenez (1a, 1b, 2) which are characterized by a different way of inheritance and have a difference in a clinical current. Methods of genetics found out that to each form of an akhondrogenez there corresponds the mutation of a certain gene – the specified genes are coded by the squirrels directly participating in processes of ossification and formation of cartilaginous and bone fabrics. The disease with an equal share of probability affects both boys, and girls, occurrence is defined only for an akhondrogenez of type 2 and makes about 1:50000 newborns. The mechanism of inheritance both autosomno-prepotent, and autosomno-recessive, however, considering weight of an akhondrogenez and that fact that most of patients does not live up to reproductive age, the large role in development of a disease is played by spontaneous somatic mutations. In total about 100 cases of pathology are authentically described.
Reasons of an akhondrogenez
Violations of formation of elements of a skeleton at an akhondrogeneza are caused by mutations of genes which code the proteins participating in processes of formation of connecting fabrics. So, 1a type (Huston-Harris's syndrome) violations in structure of a gene of TRIP1 localized in the 17th chromosome are the reason of an akhondrogenez – it codes the protein 210 associated with Golgi's complex and beta receptors to tireoidny hormones. The mutation of this kind is inherited on the autosomno-recessive mechanism, that is at healthy parents heterozygotes with probability of 25% the patient akhondrogenezy the child can be born. The defects in structure of protein 210 caused by a mutation lead to numerous violations of formation not only a skeleton, but also heart, and lungs that it becomes most frequent a cause of death of the child soon after the birth.
Akhondrogenez 1b type (Frakkaro's syndrome) is called by a mutation of a gene of SLC26A2 localized on the 5th chromosome. It codes protein carrier sulfate ions which is directly involved in sulfonation of proteoglikan of connecting fabrics, generally cartilages. Violations of this process because of defect of a carrier lead to delay of formation of an intercellular matriks in cartilaginous tissue, and also stop formation of endokhondralny bones, as leads to an akhondrogenez. In a neonatology there are also others hereditary a dysplasia, caused by SLC26A2 gene mutation – in particular, . The mutations of this gene leading to an akhondrogenez are inherited on the autosomno-recessive mechanism.
Akhondrogenez 2 types (Langer-Saldino's syndrome) is the most often found pathology from this group. It is caused by a mutation of a gene of COL2A1 located on the 12th chromosome, it codes protein collagen of the 2nd type of a class alfa-1. This protein is a structural element of cartilaginous and bone fabrics therefore defects in its structure make impossible normal formation of a skeleton and conduct to an akhondrogenez. Mutations of a gene of COL2A1 can be inherited on the autosomno-prepotent mechanism, but 2 types play the greatest role in emergence of an akhondrogenez spontaneous mutations of de novo. This version is considered the easiest form of a skeletal dysplasia and on observations of neonatolog is characterized by bigger life expectancy, than at other options of an akhondrogenez.
Classification of an akhondrogenez
At the moment allocate three clinical forms of an akhondrogenez, at the same time divide type 1 (Parenti-Frakkaro's syndrome) into two versions – 1a and 1b.
Akhondrogenez type 1a – is caused by a mutation of a gene of TRIP1 localized on the 17th chromosome, is inherited on the autosomno-recessive mechanism. This disease has such features as fractures of edges, presence of shipoobrazny shoots at a proximal metafiz of femurs and a frequent underdevelopment of internals, mainly heart and lungs.
Akhondrogenez type 1b – is called by a mutation of a gene of SLC26A2 located on the 5th chromosome, is also inherited by the autosomno-recessive principle. At this form of pathology integrity of a thorax is, as a rule, kept, shipoobrazny shoots at a x-ray research are found in disteel metafiz of femurs. At a histologic research of cartilages decrease in volume of an intercellular matriks at abundance of collagenic fibers comes to light.
Akhondrogenez type 2 – is caused by a mutation of a gene of COL2A1 located on the 12th chromosome, the inheritance mechanism – autosomno-prepotent, however the large role is played by spontaneous somatic mutations. Radiological features of this form of a disease is sharp decrease in an ossifikation of a backbone and sacral bone.
Data on identification of the 3rd and 4th type of an akhondrogenez are a subject of scientific disputes today. Doctors geneticists did not confirm now opening of mutations of new genes which would cause such violations.
Symptoms of an akhondrogenez
The main manifestations of an akhondrogenez are the hardest skeletal violations having the nature of a dysplasia, which are often followed by other anomalies of development. Shortenings of extremities concerning a body up to their almost full underdevelopment are observed. The size of the head of patients akhondrogenezy is increased, hydrocephaly, the person flat, the nose bridge which sank down is often observed. The stomach is blown quite often up, the thorax is reduced. Rather frequent phenomenon at an akhondrogeneza is the mikrognatiya – an underdevelopment of the lower jaw. Because of violations of a structure of a thorax, and also because of anomalies of development of lungs and heart (especially at type 1a) there is respiratory and heart failure which also serves as a cause of death of the patient in many cases.
Among other malformations at an akhondrogeneza often reveal a crevice of a soft and hard palate, deformation of ear cartilages, turbidity of a cornea. If the child is born live, then duration of his life makes from several hours to several months then there comes the lethal outcome because of numerous anomalies of development.
Diagnostics of an akhondrogenez
Diagnosis "" is based on assessment of the present status of the patient, radiological data, genetic researches and studying of the hereditary anamnesis and a histologic structure of cartilages. On roentgenograms of bones shortening of long tubular bones, violations of ossification of a spine column, edges is found, often reveal pathological changes of the last (especially at an akhondrogeneza type 1a). Decrease in an ossifikation is also observed in pelvic bones and on a sacrum. On metafiza of femurs at an akhondrogeneza shipoobrazny shoots are found.
In case of an akhondrogenez the type 1a at histologic studying of cartilages comes to light preservation of an intercellular matriks, hondrotsita have a large number of inclusions in a look a vacuole. At type 1b opposite, sharp decrease in volume of intercellular substance is observed, at the same time a large amount of collagenic fibers which quite often surround hondrotsita in the form of a dense cocoon is observed. The histologic picture at an akhondrogeneza of type 2 can be quite various – from total absence of fibers in a matriksa to the picture similar to a disease like 1b.
Genetic diagnostics of an akhondrogenez is made by a sekvenirovaniye of genes of COL2A1 and SLC26A2 for the purpose of identification of mutations. Concerning a gene of SLC26A2 also diagnostics of a carriage at healthy heterozygotes is possible. Studying of the hereditary anamnesis is justified only concerning an akhondrogenez of the types 1a and 1b having the autosomno-recessive mechanism of inheritance. Type 2 in most cases arises because of de novo mutation therefore search of similar violations at relatives does not make sense. Prenatal diagnostics of an akhondrogenez by means of ultrasonographies is also possible and the genetic analysis – material is taken by amniocentesis method.
Treatment and forecast of an akhondrogenez
Specific treatment of an akhondrogenez at the moment does not exist, only the supporting therapy is possible. It comes down to resuscitation actions, support of breath and blood circulation of the patient. The forecast is extremely adverse if the child was born live, then time of his life makes from several hours to several months. Death in most cases comes because of the respiratory insufficiency caused by anomaly of a structure of a thorax and associated with akhondrogenezy defects of internals.