FRAGILE X SYNDROME: A disease of twists and turns

Genetics plays a crucial role in controlling our health. Mutations and epigenetic factors are significant factors controlling the genotype and phenotype of organisms. Keeping this in mind, we will proceed towards mental retardation disorders that are caused due to alterations in genomic sequences and, in terms, influence a myriad of changes in the processes relating to the gene sequence, in specific we will talk about “Fragile X Syndrome”.

It was in 1943 that Martin and Bell observed a secondary constriction in the X chromosomes of some patients showing mental retardation. This constriction seemed to be a fragile point on the chromosome, thus conferring its name. Upon checking for the family history and pedigree, it was concluded that this gene in the X chromosome where the constriction was observed was the second most common reason for mental retardation. They named it the “Martin Bell syndrome”. As research progressed, it was pretty evident that this gene is responsible for numerous phenotypes other than mental retardation, like macrocephaly, macroorchidism, high-arched palates, repeated behaviours, seizures, decreased muscle toning and many more. Often, older women show early menopause. Generally, the individuals having FXS are initially doubted to be suffering from Autism due to similarity in symptoms.  However, clinical diagnosis depicts the obvious. There are cases where individuals suffer from both.

As the disease shows a higher frequency of occurrence in males, it was believed to follow the X-linked recessive inheritance pattern. However, a numerous pedigree analysis of families of affected ones showed something new. Phenotypes of the disease were observed in mothers thought to be carriers. More interestingly, these mothers’ sons, though carrying the mutated gene, did not seem to get affected. Sherman named these individuals the “Non-Transmitting Males(NTM)”. This observation is what describes the Sherman paradox. Sherman’s intensive pedigree analysis showed the inconsistency of the disease with the recessive X-linked inheritance pattern.

To understand this better, let us focus on the gene responsible for the disease. 

Hypermethylation of the FMR1 gene ( Fragile X mental retardation 1) located on the q arm of the X chromosome is responsible for this. This gene is a 17-exon-long sequence. There is a CGG repetitive sequence downstream of the promoter region of exon 1. Also, there is a CpG island 250 bp upstream of the repetitive sequence. These repetitive sequences are rich in C, which attracts the DNA methylase enzyme and is thus responsible for the disease. The repetitive sequences and the CpG island form an unstable mutation that increases its length over generations. Abnormally lengthened repeated sequence, caused by slipped strand mispairing, confuses the DNA methylase enzyme while adding a methyl group to the template strand, causing extensive methylation. The presence of such bulkier groups in such high numbers prevents the binding of transcription factors, thus barring transcription. Thus, not synthesising the FMRP (Fragile X mental retardation protein), thus resulting in the diseased phenotype.

WHY IS THE ABSENCE OF FMRP RESPONSIBLE FOR THE DISEASED PHENOTYPE?

FMRP is a mRNA binding protein that binds to the mRNA present in the brain neurons. These mRNA molecules produce proteins that act as neurotransmitters. Thus, maintenance of the synaptic plasticity of the neuron is another function of FMRP. The neurotransmitters

help conduct electrical signals from one neuron to another. FMRP acts as a repressor molecule. The binding of FMRP prevents the production of these synaptic proteins, thus closing the signal relay. This provides time for the synaptonemal complex to relax before transducing another signal. Which is necessary for the effective functioning of the neuron.

In the absence of FMRP, these neurotransmitters are continuously synthesised. This leads to fatigue and consequent collapse of neuron ends in the brain, a significant reason for mental retardation.

Several Factors affect the synthesis and concentration of FMRP negatively and positively. For example, the presence of Metabotropic Glutamate receptors (mGluR)s seems to retard the synthesis of FMRP. Activation of these receptors prevents the translational block given by FMRP. Also, different signal pathways act differently, controlling the expression of FMRP. Knowledge of the pathways acting on FMRP action is necessary for therapeutic and diagnostic basis.

INHERITANCE PATTERN:

Based on the number of CGG repeats the gene has four alleles;

1. 1. The wild type– In the unaffected Non-carrier individuals, 45-54 repeats of the trinucleotide sequences are found. There is little to no chance of these repeats to extend over the generations.
   2. Pre-mutation– If the repeat exceeds more than 54 to 200 repeats,  there is a high frequency that individuals in the upcoming generation may have total mutations.  This does not happen for the immediate generation following. However, exceptions do exist. These individuals generally experience frequent tremors and seizures collectively known as the “Fragile X associated tremor ataxia syndrome (FXTAS)”.

• Partially unmethylated– Occurs if the repeat sequence exceeds 200 but remains partially or entirely unmethylated.

• Full mutation– occurs when the repeat sequence is more than 200, and extensive hypermethylation occurs in the upstream region of the gene.

However, what causes the difference between the third and fourth alleles needs to be better understood.

Symptoms are usually seen in the individuals having second or the fourth allele, with extremes in the latter case.

THERAPEUTIC APPROACHES THROUGH EXPERIMENTS ON ANIMAL MODELS

Intensive research has studied the effects of the FMR1 gene in animals, such as Mice, Drosophila, Zebrafish, etc. In these animals, the gene homologous to the FMR1 is mutated so that the corresponding protein is not produced, and the functioning of the target organ is studied. Knockout mouse models are the most commonly used model organisms. 

Extensive research has focused on the drugs that can produce the phenotypes that failed to be produced in FXS Patients. Focusing on pathways that can reverse the diseased phenotype requires in-depth knowledge of the developmental mechanisms of humans. Of course, research in these fields was quite successful on animal models, but clinical trials on humans could have been more fruitful. Also, there are not enough neurogenic studies on the effects of FMRP in different brain regions, further complicating diagnosis. Futuristic approaches to deciphering the mystery behind the different symptoms could help cure the disease to a certain extent.