X fragile syndrome
Fragile X syndrome (FXS), also known as Martin-Bell syndrome, is a hereditary disorder that causes intellectual disability to varying degrees, being the second leading cause its genetics, second only to Down syndrome. FXS is the leading cause of hereditary intellectual disability. It is also the most frequent cause of intellectual disability in men.
It affects both men and women, although there are differences in its manifestations and incidence. In men, the incidence is 1 in 1,250, while in women it is 1 in 2,500, this difference between the sexes being closely related to the genetic cause of the syndrome.
The genetic cause of the syndrome is a type of mutation known as trinucleotide repeat expansion, which involves an increase in the number of three-base repeats in DNA in offspring. This type of mutation is associated with the phenomenon of anticipation, which manifests as an increase in the severity of symptoms in successive generations.
The mutation that causes the syndrome affects a region of the X chromosome in which the FMR-1 gene is located. The expansion of the trinucleotide takes place in the regulatory region of the gene, this being CGG trinucleotide (Citosine-Guanine-Guanine). When the number of repetitions exceeds the threshold value of 230 repetitions, methylation of the gene occurs and, therefore, it loses its function, thus producing fragile X syndrome.
The product of this gene, the protein FMRP (acronym for Fragile X syndrome Mental Retardation Protein), can be found both in the nucleus and in the cytoplasm, and despite the fact that its function is still poor known, it has been seen that it has the ability to bind to certain messenger RNAs, so this protein could be involved in transporting these from the nucleus to the cytoplasm for translation.
History
In 1943, J. P. Martin and Julia Bell discovered a type of hereditary mental deficiency linked to the X chromosome, now known as fragile X syndrome. They already noted certain peculiarities of the patients' facial features and mentioned that one of the patients had a long face and prominent eyebrows.
In 1969, Lubs studied a family in which four males from three different generations were mentally retarded. Cytogenetic studies of samples from these patients revealed an unusual constriction on the long arm of the X chromosome in 10-33% of cells in culture.
In a subsequent study of the same family, Lubs et al., in 1984, described unusual facial features in members of this family with the condition: elongated faces, long ears set lower than usual, facial features asymmetrical and prominent eyebrows.
Also in 1969, Opitz et al. used the term "Martin-Bell syndrome" to refer to a case of familial mental deficiency with characteristics of said syndrome. Back then, no one had linked Martin-Bell syndrome to Lub's fragile X syndrome.
In 1981, Richards et al. showed that both syndromes were actually the same disorder. To do this, they studied the same family that Martin and Bell had described and using the culture technique used by Lubs, they observed that all affected males had the fragile site of the X chromosome in 5-17% of their cultured cells.
In 1991, Verkerk et al. described a gene associated with the disorder: the FMR-1 1 gene (acronym for Fragile X linked Mental Retardation type 1; X-linked mental deficiency type 1). This discovery has brought about great improvements in prenatal diagnosis and in the identification of affected individuals and in the premutation range.
Origin of name
The name of the syndrome can be misleading at first. There is no break in the X chromosome on the chromosomes of patients with this disorder, nor is there even an actual fragile site on it.
Fragile X refers to a structural chromosomal abnormality that is detected in the long arm of the X chromosome in some cells from the patient under certain culture conditions and that, due to sample manipulation, can break at this level. anomaly, giving rise to two chromosome fragments. That is, the fragile site is the result of the technique and is not found in vivo, but only in vitro.
Therefore, it cannot be the cause of the disease. However, this culture technique, which makes it possible to observe the secondary constriction of fragile X, has been the classic criterion for diagnosis of the disorder, since thanks to it we can distinguish affected from unaffected.
Genetics
The discovery of the fragile sites contributed to the discovery of a new type of mutation: the expansion of trinucleotide repeats; although not all mutations of this type produce fragile sites.
The inheritance of this mutation is X-linked dominant, although it does not respond to the usual rules of such inheritance, since there are normal male carriers and unaffected female carriers who will leave their imprint (necessary for amplification) and individuals affected by the syndrome (mostly males) among the progeny of the latter.
This syndrome presents a phenomenon of anticipation: the penetrance and expressiveness of the disorder increases as generations pass. This is due to the increased number of CGG trinucleotide repeats in the FMR-1 gene.
The origin of fragile X syndrome lies in the inactivation of the transcription of said gene. This inactivation is due to methylation of the gene and occurs when the number of repeats exceeds a threshold value from which the methylating enzymes can carry out their function on said gene.
When DNA from fragile X patients was analyzed using molecular techniques, it was observed that they had long sequences with hundreds and even thousands of CGG trinucleotide repeats. These repeats are located in a non-translated region (SANT) before the first exon 1 of the FMR-1 gene, located in the FRAXA site, in the Xq27.3 region. In unaffected individuals, the number of repeats in this region constitutes a polymorphism, with values between 5 and 55 repeats being common. The mutation consists, therefore, in the amplification of the number of CGG triplet repeats.
This mutation is not expressed immediately but evolves over generations, increasing the number of repeats. At first it goes through a stage called "premutation" (between 55 and 230 repetitions). Premutation carriers have completely different symptoms (see Fragile X Carriers or FMR Premutation) than individuals with the full mutation.
When the number of repeats exceeds the threshold of 230 repeats, fragile X syndrome manifests itself, with values between 230 and 1000 or even higher being frequent.
Women who carry a premutation run the risk of having children with the syndrome, the higher the number of repeats being more likely. When calculating the probability of having an affected descendant, it is important to consider interruptions of the CGG repeats by other sequences, since these are considered to prevent expansion.
For this reason, it is important to analyze the sequence of the Xq27.3 region in families in which a case has been diagnosed. In families where there has been a case of fragile X, about 10% of normal males carry the premutation. All the daughters of these carriers will inherit the premutation, which will be normal, but their male descendants have a high probability of suffering from the syndrome, because during oogenesis the mother leaves her genetic imprint in this chromosomal region, which facilitates amplification. during the early embryonic development of their offspring, between the 5th and 20th day of life, although this is a hotly debated topic and it has been suggested that amplification may occur during female meiosis.
When the number of repeats exceeds the threshold, the sequence is methylated by enzymes, extending this methylation to the CpG island in the regulatory region of the FMR-1 gene. Transcription is inhibited and as a consequence the syndrome is originated.
It has been proven that the inhibition of this gene is responsible for the disorder, since studying other types of gene mutations in it, it has been observed that these also produce the syndrome, although it should be noted that they are much more infrequent than the amplification.
Individuals who are cytogenetically positive for the fragile site at Xq27.3, but negative for CGG expansion (which is associated with the FRAXA mutation), may have a more distal mutation, including FRAXE or FRAXF.
The discovery of the FMR-1 gene was an international effort involving the laboratories of Stephen Warren in Atlanta, David Nelson in Baylor, and Ben Oostra in the Netherlands. It was described by Verkerk et al. in 1991. It is actively expressed in spermatogonia, in hippocampal and cerebellar neurons, and in many other cell types. Its product, the FMRP protein, is located in the cytoplasm and its function is poorly understood, although it has been proven that it has the ability to bind to RNA, regulating the translation of approximately 4% of these. It is thought that this protein may be key in the regulation of neuronal structural changes and maturation through environmental stimulation, particularly in the selection of neuronal connections.
Genetic counseling
As with any other inherited disease, genetic counseling is important in families with fragile X syndrome. Individuals should be counseled to make free but risk-aware choices, trying to estimate the probability that a couple will have a child affected by the disorder. They should also be given information about the disease and its treatment.
Phenotype and clinical symptoms
Traits and symptoms Rasges and symptoms: elongated face, prominent front, pronounced chin, big ears.
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The main characteristics of this syndrome, although individually they are not exclusive to this disorder, must be taken into account in people with autism, mental deficiency or learning problems. The possession of several of these traits and symptoms by a person can lead to suspicion of the presence of the syndrome and an opportune diagnosis should be chosen, since it is a family disease.
These features are mental deficiency to a highly variable degree, generally more accentuated in males, an increase in testicular volume above 30 mL (macroorchidism), and facial and connective tissue peculiarities.
Due to a reduction in the interzygomatic distance, the shape of the face is more elongated than usual. Other typical facial and cranial features, although not necessarily found in all patients, are macrocephaly, a rough face, a broad forehead, prominent eyebrows, and long ears, often set low. As for macro-orchidism, in most cases it does not manifest until after puberty, although some cases of congenital macro-orchidism have been detected.
As far as connective tissue is concerned, the patient may have scoliosis, loose joints, and flat feet. Other physical features include a hollowed-out chest, prolapsed mitral valve, mild dilatation of the ascending aorta, and periventricular heterotopia.
Neuroanatomical changes in the brain of individuals with fragile X syndrome include enlargement of the caudate nucleus, hippocampus, and lateral ventricles. The cerebellar vermis is smaller than normal. Cerebellar size is correlated with cognitive level, including executive function.
Regarding psychological traits, the most significant is intellectual disability, being more accentuated in men and generally profound (although in some cases it can be moderate), while in women it is usually mild. The IQ of affected males is between 35 and 45, while in the case of affected women the IQ is less affected, being between 60 and 80. In addition, they present milder somatic signs. This is due to the mosaicism that women present, due to the random heterochromatinization of one of their X chromosomes in each cell during embryonic development.
Approximately 70% of females with the full mutation have cognitive deficits in the borderline or range of mental retardation, while approximately 85% of males with the full mutation are mentally retarded. that present a minor deficiency and even lack it, usually present mosaicism, that is, some cells have premutation and others complete mutation or do not present methylation despite having the complete mutation.
Stereotyped head and hand movements and psychiatric and personality manifestations are also common, as are hyperactivity and autism. Generally, patients with fragile X syndrome have poor or no eye contact and periods of aggressiveness alternated with periods of notable shyness are common. Difficulties in the use of language and learning, especially mathematics, and sensory integration problems due to difficulty understanding stimuli (visual, auditory or tactile), as well as the systematic rejection of new stimuli are also common..
Carriers of Fragile X or FMR gene premutation
In relation to possible pathologies or disorders that may affect carriers of the Premutation, Fragile X-Associated Premature Ovarian Insufficiency (FXPOI) and Fragile X Associated Tremor Ataxia Syndrome (FXTAS). Recently, a series of health disorders linked to Premutation have been described. In this way, individuals carrying the premutation (with 55 to 200 CGG repeats), without showing symptoms or characteristics similar to individuals with the complete mutation (more than 200 CGG repeats), that is, without Fragile X syndrome, seem be exposed to a greater or lesser extent to a series of disorders that are related to their condition as carriers of the premutation. According to bibliographic sources, the clinical manifestations associated with premutation carriers are many and very diverse, carriers may manifest several or none of these, and to a greater or lesser degree. These cover:
- anxiety,
- autistic spectrum disorder (ATE),
- ADHD (hyperactivity disorder or attention deficit),
- compulsive obsessive disorder,
- trends in substance abuse,
- impulsiveness, depression, chronic fatigue,
- autoimmune problems (fibromyalgia, thyroid alterations—mainly hypothyroidism—, etc.),
- hypertension,
- migraine,
- neuropathy,
- Chronic pain,
- sleep problems (such as apnea),
- vestibular involvement,
- Low intellectual coefficient,
- difficulties in the calculation,
- executive dysfunction,
- affectation of memory (verbal, work or visual),
- deterioration in time-space processing,
- deficits in the fluency of language, in organization and in planning,
- etc.
Thus, we began to talk about Neuropsychiatric Disorders Associated with Fragile X (FXAND).
Fragile X Associated Ataxia and Tremor (FXTAS)
It is a late-onset neurodegenerative disorder associated with problems with movement, memory, and the autonomic nervous system. FXTAS can present with many of the symptoms of multiple system atrophy and often includes parkinsonism, dysautonomia, peripheral neuropathy, and dementia.
This disorder is due to an extension of CGG trinucleotides in the FMR-1 gene, in a range of between 55 and 230 repeats, that is, in the premutation range of fragile X-linked intellectual disability. Although it involves this gene, it is a very different clinical disorder.
It occurs more often in men, but it can also occur in women.
There is no cure for FXTAS, but some of its symptoms can be improved with medication. Intensive rehabilitation with coordination and balance exercises can help patients with postural problems.
Diagnosis
When an individual with mental retardation or autism presents some of the characteristic features of those mentioned above, it is suspected that they may be affected by the syndrome. But it is not enough to detect somatic symptoms and mental deficiency to give a positive diagnosis of the disorder, but it is necessary to resort to genetic diagnosis for it to be definitive.
Classically, the definitive diagnosis of the disease was established cytogenetically, by the expression of the fragile site in certain culture conditions.
A fragile site is a chromosomal region or band that appears as an unstained interruption and that can be broken while working with the sample, resulting in chromosome fragments of defined size.
But as already stated, the fragile site is not expressed in vivo. For it to become apparent, it is necessary to culture the patient's cells (lymphocytes or fibroblasts) in a medium low in folic acid and deoxythymidine triphosphate (dTTP) for at least one cell cycle. That is, it is necessary for the DNA synthesis stage (S) to occur at least once for this constriction to manifest itself. Whether or not it appears in a cell of an affected person is a probabilistic fact and its patency usually occurs in 5-20% of cells, so it is necessary to observe many cells before being able to give a cytogenetically negative diagnosis. Instead, it is enough to find a cell or few cells with the constriction to give a cytogenetically positive diagnosis.
Light microscopy and chromosome banding show an elongated and condensed region near the end of the long arm of the X chromosome, between band q27 and q28, although we know that it is exactly at q27.3. Under the electron microscope it has the appearance of a secondary constriction behind which remains a large satellite.
The first fragile site on the X chromosome to be detected was the FRAXA site, which is the one that has been described so far, and is also the most abundant. This fragile site affects the FMR-1 gene.
Subsequently, other less frequent fragile sites were also detected on the X chromosome, the most important being the FRAXE site, since it is associated with mild intellectual disability and affects the FMR- gene. 2, whose function is not very clear. It is located at Xq28.
As for the other sites, FRAXD and FRAXF are poorly studied and the associated phenotype is unknown or little known.
It is known that FRAXD is at Xq27.2, very close to the FRAXA site, and that its constriction is inducible by high doses of aphidicolin, as Sutherland already demonstrated in 1989. It is detected in only 1-2% of patients. patients, so it is not very significant, and it can also be detected by the usual procedure.
FRAXF appears in people without the condition as a chromosome lesion at Xq26. Not much else is known about this fragile site.
Since the 1980s, at least twelve other heritable secondary constrictions have been discovered on other chromosomes, but none of them is associated with a particular phenotype.
In 1981 and 1982, Tommerup et al. and Jacobs et al. demonstrated that pharmacological inhibition of thymidylate synthetase (TYMS) is effective in inducing X marking fragile in cultured cells.
In 1991, Griffiths and Strachan described a technique that allows visualizing the fragile site and performing prometaphase banding in the same specimen.
Currently, molecular techniques are preferred for definitive diagnosis, since knowing the number of repeats in the sequence can be very useful to study the inheritance of the disease within a family, since it allows studying unaffected individuals without carriers, unaffected carriers, and affected carriers. In the latter, it also makes it possible to study the degree of methylation, which is decisive in the manifestation of the syndrome.
Another diagnostic technique consists in the use of restriction enzymes and subsequent electrophoresis of the fragments in order to find bands of abnormal length.
By combining enzymes that are sensitive to methylation with others that are not but that have the same recognition sequence and cutting pattern, abnormal methylation in the fragile site can be detected in both affected males and female carriers. Some affected males appear to be mosaics, with the coexistence of a long methylated fragment and a short normal unmethylated fragment.
The use of the Southern blot with EcoRI and EagI digestions is a simple test to distinguish the normal genotype, the premutation and the complete mutation.
You can also use the pfx3 test or PCR followed by sequencing to find out the exact number of repeats, especially if they exceed 130.
A technique that is interesting because it is minimally invasive is the use of mouse monoclonal antibodies against the FMR-1 protein in a patient's blood smear. It is very minimally invasive, since it only requires one or two drops of blood. An adaptation of this same test has been used to make the diagnosis with hair roots instead of blood samples. This can be useful if we think that some patients have personality disorders that frequently manifest as aggressiveness. A simple way to solve the possible inconvenience of obtaining a blood sample is to collect detached capillaries on the sheets, on the pillow or by using a comb or brush to make the diagnosis.
In contrast, an invasive technique, but one that can be used if you do not want to resort to molecular techniques, consists in the analysis of olfactory neuroblasts, because they are accessible neurons that can be regenerated and that are closely linked to the brain.
MacKenzie et al., in 2006, studied the possibility of using ectoderm derivatives for the diagnosis of the syndrome.
Prenatal diagnosis
In the case of families with a history of the syndrome, prenatal diagnosis can contribute to improving the quality of life of the offspring, especially women who carry the premutation.
Applied to a gestating embryo in early stages of development, it can be used to make the decision to abort or not in the event that it is detected that it has the complete mutation and that it is suspected that it will suffer mental deficiency with a high probability serious.
It can also be used to find out if embryos in later stages of development have a high probability of suffering from the syndrome and thus adapt the environment in which the child will develop and start treatment at an early age, in order to improve cognitive abilities.
To carry out this diagnosis, amniocyte cultures or sequencing from chorionic villi can be used. The pfxa3 technique can also be used to try to detect an abnormal 2.3kb band.
In more developed embryos, a blood sample may be taken for a blood smear along with mouse monoclonal antibodies against the FMR-1 protein.
Prenatal diagnosis can also be used in women with the premutation who have used in vitro fertilization. Prior to embryo implantation, various molecular diagnostic methods can be used with the intent of selecting healthy embryos.
Prior to prenatal diagnosis, there is genetic counseling, based on genetic and phenotypic studies of the parents and their relatives.
Treatment
The treatment of patients with fragile X syndrome is quite complex and its effectiveness is quite limited. It involves multiple professionals: specialists in special education, occupational therapists, psychologists, speech therapists, speech therapists, educators and doctors. Genetic counseling focused on the families involved is essential, where genetic counseling plays a fundamental role. The spectrum of commitment to treatment is a matter discussed in detail between the physician and the family.
Children affected by the syndrome usually require speech therapy and occupational therapy, which can be mediated through the patient's educational center. Boys in particular have significant sensory integration problems. Behavioral techniques along with fine and gross motor coordination therapies can calm the patient's mood. Severe behavioral disorders require the intervention of educators and psychologists who teach the family behavioral techniques.
The use of psychotropic medication is a useful tool in many cases. Improving concentration and decreasing aggressiveness, if present, are the main objectives in early childhood. Among those affected by this syndrome, and particularly in preschool-age children, stimulant medications, such as methylphenidate, are often associated with increased irritability. Clonidine, which has a calming action, helps control symptoms of hyperactivity and aggression in most children with fragile X.
Careful monitoring with periodic electrocardiograms should be performed if any type of psychotropic medication is used.
In school-age children, stimulants (methylphenidate, dextroamphetamine, and Adderall) are effective in about 60% of cases. With regard to anticonvulsant agents, such as carbamazepine or valproic acid, they are the main choice in cases of significant emotional instability. When the patient suffers from anxiety, restlessness or aggressiveness, Iinhibitors are also used. b>Selective Rerotonin S uptake drugs (SSRIs), such as fluoxetine, sertraline, fluvoxamine, or citalopram.
Different molecules with neurotonic action are being tested, including AMPA receptor agonists and selective metabotropic glutamate receptor antagonists, which could have great application in the pharmacological treatment of the syndrome. Likewise, a recent article suggests that pharmacological intervention on the endocannabinoid system or the intracellular cascade of the mTOR protein could also be beneficial for the treatment of the symptoms of this syndrome.
There is an interdisciplinary research group led by Dr. Yolanda de Diego Otero at the Malaga Biomedical Research Institute (IBIMA), which is advancing in the development of a new treatment for Fragile X syndrome. The Spanish Medicines Agency has approved the clinical trial, financed mainly by the Ministry of Health and Social Policy, which is currently being developed to verify the effectiveness of antioxidant compounds in improving behavioral and learning disorders in those affected by the syndrome., a discovery that has been protected by an invention patent.
The latest results of his research have allowed us to describe a new therapeutic target, to design specific treatments for Fragile X Syndrome and to investigate its effects on the disease. Two of the most prestigious scientific journals in the field of neuroscience, "Neuropsychopharmacology" and "Journal of Pineal Research", have published the most recent results of the research, in two complementary articles where it is described for the first time that there are compounds that control part of the symptoms, acting on the elimination and control of the production of free radicals, the altered biochemical mechanism in the brain of the mouse affected with the syndrome, as previously described by this same research group. The oxidative stress regulating compounds counteract the production of free radicals and improve the behavior and learning of mice affected by Fragile X Syndrome. Recently, the research group of the Mental Health Clinical Management Unit of the University Hospital of Malaga has shown improvements at the cognitive and behavioral levels in patients treated with an antioxidant compound.
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