FunMed PBL Write-up
Fragile x syndrome
1. Introduction
Fragile X syndrome (FXS) is an X-linked dominant genetic disorder. It is the most common genetically inherited condition related to mental retardation – the current estimate lies at 1 in 4000 males, and 1 in 5000-8000 females affected. (Lyons et al.) The cause of this disorder is a mutation in the FMR-1 gene, the Fragile X Mental Retardation Gene. To be more exact, an abnormal number (>200) of CGG repeats in this particular gene on the X chromosome.
The number of CGG repeats can vary in each person, and will affect the severity of expression of this disorder; 5-44 CGG repeats is classed as “normal”, 55-200 CGG repeats is defined as a “premutation” and numbers of 200+ CGG repeats is classified as the “full mutation”. (Bourgeois et al., 2009) The result of this are mild to severe cognitive impairments, usually more in males than females, and physical features such as an elongated face and prominent ears. (McLennan et al., 2011) Diagnosis of FXS is nowadays usually done by polymerase chain reaction (PCR) and Southern blot test. With these two routine testing methods, the number of CGG repeats and methylation of the FMR-1 gene can be established. (Saldarriaga et al., 2014) To date, no form of treatment specifically targeted at FXS exists, however conditions resulting from this syndrome, such as social anxiety, ADHD and learning disabilities can be treated and kept under control.
In the following write-up Fragile X Syndrome will be discussed in terms of:
1) X-Linked Genetic Disorders
2) The FMR-1 mutation specifically
3) Symptoms of Fragile X Syndrome
4) The link between FXS and autism
5) Fragile X-associated tremor/ataxia syndrome
6) Diagnosing FXS
7) Treatment options for FXS
8) Genetic counselling and dealing with the condition
2. Fragile X Syndrome
2.1 X-Linked Genetic disorders
X-linked genetic disorders, are conditions resulting from a mutation on the X chromosome. In females, X-linked dominant disorders would mean that the same gene is mutated or dysfunctional on both the X chromosomes. This is very rare, which is why X-linked dominant (and recessive) conditions usually tend to be more prevalent in men. While men usually tend to express the condition, females are often carriers, and can pass on the mutation to their children. When an affected female has children, 50% of her children will be affected, and 50% will be unaffected.
An affected male will have one faulty X chromosome and one normal Y chromosome. If an affected male has children with an unaffected female, he will pass the condition, the faulty X chromosome, to all his daughters, but can never pass the condition on to his sons. (NSW Government, 2016)
2.2 The FMR-1 mutation
The FMR-1 gene sits on the X chromosome at a position called Xq27.3. Contained within this gene is a repeating sequence of the trinucleotide CGG. Normal alleles contain up to 44 repeats, premutation alleles up to 200, and anything above 200 CGG repeats is classed as a full mutation. In affected individuals, methylation – adding methyl groups to DNA, rendering the genes inactive- of the cytosine in the CpG islets also occurs, in comparison to healthy individuals, where there is no methylation present. This methylation, in combination with gene silencing and thus the absence of the protein encoded by the FMR-1 gene (FMRP – Fragile X Mental Retardation Protein), leads to the full mutation.(Saldarriaga et al., 2014)
In some rare cases, additional frameshift and point mutations can also occur, leading to a dysfunction of FMRP.
FMRP is a protein involved in regulating mRNAs that code for proteins involved in the development of synapses, neural plasticity and brain development. FMRP has also been associated with multiple neurotransmitter systems, which can be impaired when there is a lack of FMRP or the available protein is dysfunctional. Due to its important role in brain development, a wide variety of intellectual deficits and behavioural problems can result from this mutation. (Saldarriaga et al., 2014, Darnell et al., 2011)
2.3 Symptoms
The severity of symptoms in FXS depends on the amount of FMRP produced, which again depends on the degree of methylation and the number of CGG trinucleotide repeats. The concentration of FMRP allows for a clinical spectrum to be established. In women, there is even greater variability in the expression of the phenotype, as the concentration of FMRP produced also depends on the ratio of activation of the affected X chromosome, and the compensation by the unaffected X chromosome. This means that an affected woman could show hardly any physical symptoms, but have learning disabilities depending on the ratio of cells with the normal allele present on the activated X chromosome. (Saldarriaga et al., 2014)
Physical Features
The classic FXS phenotype in adults includes an elongated face with a prominent forehead, large ears and enlarged testes (macroorchidism). Furthermore, hyperextensible finger joints and flat feet are also often observed. (McLennan et al., 2011)
Other medical conditions such as otitis media (middle-ear infections), gastrointestinal disorders, such as gastroesophageal reflux, and seizures can occur in these patients which is why they should be monitored and medically assessed frequently. It has been found that 20% of individuals affected with FXS have frequent seizures. (Hansen and Hagerman, 2005)
Behaviour
People affected with FXS, often show severe anxiety, attention deficit hyperactivity disorder (ADHD), and are sometimes also hypersensitive to sensory stimuli. In addition to this, it has been observed that FXS patients often show obsessive-compulsive disorder – like symptoms, in that they frequently tend to focus and persevere on certain topics. When food becomes their main point of obsession, overeating and obesity have often been found to occur. (McLennan et al., 2011)
Furthermore, FXS patients, especially children, tend to be non-verbal, especially when they are placed in situations they deem uncomfortable due to social anxiety. This also makes it very hard for them to articulate what they are feeling and if they are experiencing any pain, and so many symptoms may go unnoticed. (Kidd et al., 2014)
Regarding the cognitive phenotype, the majority of males and females with FXS show declines in IQ scores from childhood through to adolescence, but there is a lack of research evidence as to the IQ development into late adulthood. Borghraef et al. investigated intellectual decline though adulthood, and observed a significant overall IQ decline, especially evident in the verbal area, with a decrease in the use of language over time. (Hansen and Hagerman, 2005)
Premutation carriers have also been found to have behavioural traits similar to people with the full mutation. Some of these symptoms include shyness, anxiety, agoraphobia, depression, avoidance of eye contact and interpersonal sensitivity. It should be noted however, that the stress of raising a child with FXS is highly likely to add to the development and severity of these psychological traits.(Bourgeois et al., 2009)
2.4 Fragile X Syndrome and Autism
Autism and Autism Spectrum Disorder (ASD) are conditions commonly associated with FXS. 30% of males with FXS are diagnosed with autism, and an additional 30% of males who do not meet the criteria for autism are classed as having an ASD. (McLennan et al., 2011) However, why children with FXS develop autism, is still unknown. Furthermore, it is unclear, if FXS and autism are two distinct disorders that are very likely to co-occur, or if they are part of each other’s spectrum. (Bailey et al., 2001)
2.5 Fragile x-associated tremor/ataxia syndrome
Carriers of the FXS premutation (55-200 CGG repeats) can also be affected by Fragile X-associated tremor/ataxia syndrome (FXTAS). This neurodegenerative condition usually affects men older than 50 years of age. (Sallansonnet-Froment et al., 2010)
This syndrome of kinetic tremor, cerebellar gait ataxia and parkinsonism has also been found to occur in people in “grey zone CGG expansions” (45-54 CGG repeats). The explanation for the manifestation of FXTAS in people with grey zone expansions is likely to be the fact that molecular changes in FMR-1 are similar to those seen in the premutation, such as decreased levels of FMRP. (Hall et al., 2012)
It has been found that the age of onset of FXTAS strongly correlates with the CGG repeat length, which makes the incidence of FXTAS in premutation carriers much higher than in grey zone expansion carriers.
People affected with FXTAS show symptoms such as decreased reflexes, progressive loss of bowel and bladder control, slow and lurching gait, rest tremor, and in some cases also a decline in cognitive functions. Usually the onset of cognitive function decline is not very obvious at the time and is often missed, until it starts to affect behaviour. One of the most striking cognitive deficits observed in FXTAS is the initiation of goal-directed activity and the inhibition of inappropriate behaviour. The neurological symptoms of FXTAS are usually milder in females, due to compensation of the expression of FMR-1 and production of FMRP from the healthy X chromosome. (Berry-Kravis et al., 2007)
2.6 Diagnosis
Early diagnosis of FXS is usually made in children around the age of 3, that show a delay or a lack of speech development. Children are referred for molecular testing when they show delayed motor development, hand flapping, eye contact avoidance, hyperactivity and signs of ADHD, anxiety and reoccurring mid-ear infections. (Bagni et al., 2012)
In the past, karyotyping was the main form of diagnosis used for FXS, however nowadays, this has mostly been replaced by Polymerase Chain Reaction (PCR) and Southern blot test. PCR allows for the region on the X chromosome containing the FMR-1 gene (Xq27.3) to be amplified, and the number of CGG repeats to be determined. Southern blotting is then utilised to determine the methylation status. (Bourgeois et al., 2009, Saldarriaga et al., 2014)
These two tests are usually ordered for patients that show the physical characteristics of FXS as well as displaying intellectual disabilities. Once the patient has tested positive for FXS, all family members suspected of being FXS carriers undergo molecular testing. PCR and southern blotting can also be done on material collected though chorionic villus sampling, thus allowing for a prenatal diagnosis. (Saldarriaga et al., 2014)
2.7 Treatment
Currently there is no form of medication for FXS. Instead, the standard approach is to ameliorate the behavioural traits arising from the condition, such as anxiety, aggression, learning disabilities and ADHD. Occupational therapy and special education teachers can be employed to aid children with FXS at school, although they should be integrated into the normal classroom setting if possible. (Hansen and Hagerman, 2005)
As children and adults affected with FXS tend to be hypersensitive to touch, smell, sound and images, stimulant medication has been shown to be an effective therapy too. They have a calming effect on the individuals, who are easily overwhelmed in situations where there is a potential sensory overload, such as a supermarket, which could cause severe anxiety and even aggression. (Hansen and Hagerman, 2005)
In the last decade, however, substantial amount of research has gone into developing cures targeted specifically at FXS, with a potential for eventual cure. The two approaches currently being considered are:
a) Reactivation of the affected gene, FMR-1 and
b) Compensating for the lack of FMRP.
Tests using the drug 5-aza-deoxycytidine (5-azadC) have shown restoration of transcription and translation of the FMR-1 gene when administered to human cells. This reactivation lead to demethylation of the allele. However, this technology is still very new, and has, due to safety issues, such as the induction of apoptosis, not yet been tested in vivo. (Bagni et al., 2012)
3. Genetic counselling
Genetic counselling plays a vital role in providing pre-and post-test information to individuals and their family, as well as giving support when dealing with potentially positive test results. It is very important for families to fully understand the condition they are dealing with, so they can make informed decisions about their parenting future and can effectively communicate with other family members who might also have to undergo the genetic screening. Parents might be distressed and anxious about the prospect of raising an affected child, so it is important for them to be educated about the different treatment and support possibilities. Furthermore, parents who have been diagnosed with the premutation also have to understand the possible implications this might have for themselves, in terms of developing conditions such as FXTAS (Fragile X-associated tremor/ataxia syndrome).
Genetic counsellors should therefore be able to provide pre-and post-screening psychological support, be able to explain the condition to the family and aid them in planning their future and installing all necessary support schemes for the affected individual. (Finucane et al., 2012)
4. Conclusion
Although FXS is the most frequent genetic cause for intellectual disabilities, further investigation is still required in many areas, such as finding a pharmaceutical treatment for this condition. To date, FXS is treated mainly though occupational and behavioural therapies, extra learning support for affected children, and stimulant medication to alleviate sensory overload.
In recent years however, more progress has been made in finding medication that potentially could reactivate the FMR-1 gene or compensate for a lack or dysfunction of FMRP. Tests using 5-azadC have been shown to reverse methylation on the affected allele in vitro, but have not been investigated in vivo due to safety issues. (Bagni et al., 2012)
In the case of the PBL scenario, Andrew showed the classical signs of FXS – shyness, avoidance of eye contact, hyperactivity, irritability, delayed motor and speech development, long face, large ears and a prominent jaw. After genetic testing, it was found that Andrew had the full mutation and his mother, Monica, was tested positive for the premutation and had some physical symptoms of FXS herself. The family was then referred to a genetic counsellor, a highly important step to educate the affected family on FXS, make sure that all family members suspected of being carriers undergo genetic screening, and offer advice on dealing with the condition.
5. References
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BAILEY, D. B., HATTON, D. D., SKINNER, M. & MESIBOV, G. 2001. Autistic behavior, FMR1 protein, and developmental trajectories in young males with fragile X syndrome. J Autism Dev Disord, 31, 165-74.
BERRY-KRAVIS, E., ABRAMS, L., COFFEY, S. M., HALL, D. A., GRECO, C., GANE, L. W., GRIGSBY, J., BOURGEOIS, J. A., FINUCANE, B., JACQUEMONT, S., BRUNBERG, J. A., ZHANG, L., LIN, J., TASSONE, F., HAGERMAN, P. J., HAGERMAN, R. J. & LEEHEY, M. A. 2007. Fragile X-associated tremor/ataxia syndrome: clinical features, genetics, and testing guidelines. Mov Disord, 22, 2018-30, quiz 2140.
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FINUCANE, B., ABRAMS, L., CRONISTER, A., ARCHIBALD, A. D., BENNETT, R. L. & MCCONKIE-ROSELL, A. 2012. Genetic counseling and testing for FMR1 gene mutations: practice guidelines of the national society of genetic counselors. J Genet Couns, 21, 752-60.
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HALL, D., TASSONE, F., KLEPITSKAYA, O. & LEEHEY, M. 2012. Fragile X-associated tremor ataxia syndrome in FMR1 gray zone allele carriers. Mov Disord, 27, 296-300.
HANSEN, R., J & HAGERMAN, R., J 2005. Fragile X Syndrome, New York, The Guildford Press.
KIDD, S. A., LACHIEWICZ, A., BARBOUTH, D., BLITZ, R. K., DELAHUNTY, C., MCBRIEN, D., VISOOTSAK, J. & BERRY-KRAVIS, E. 2014. Fragile X syndrome: a review of associated medical problems. Pediatrics, 134, 995-1005.
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SALDARRIAGA, W., TASSONE, F., GONZÁLEZ-TESHIMA, L. Y., FORERO-FORERO, J. V., AYALA-ZAPATA, S. & HAGERMAN, R. 2014. Fragile X syndrome. Colomb Med (Cali), 45, 190-8.
SALLANSONNET-FROMENT, M., DE GRESLAN, T., ROUX, X., BOUNOLLEAU, P., OUOLOGUEM, M., TAILLIA, H., RICARD, D. & RENARD, J. L. 2010. [Tremor/ataxia syndrome related to Fragile X premutation]. Presse Med, 39, 187-95.