Research Articles on CDKL5

CDKL5 Research Times (produced by the International Foundation for CDKL5 Research)

August 2011

 

Research Articles:

Recurrent mutations in the CDKL5 gene: Genotype-phenotype relationships. American Journal of Medical Genetics 2012. Link to abstract
This study from France looks at whether there is a relationship between mutation and severity of phenotype. A total of 26 CDKL5 mutations were identified in 358 unrelated females who had been referred with encephalopathy and early seizures. Most of these mutations were unique but 8 were recurrent mutations that had previously been described in other studies (including studies published by this group). By studying females with recurrent mutations in both this study and those from previous studies, the authors have drawn some conclusions about the relationship between site of mutation and severity of phenotype. They looked at mutations occurring in 3 particular areas of the CDKL5 protein, the ATP binding site and the ST kinase active site, both of which lie in the catalytic or kinase domain, and the large C-terminal domain of the protein. They found that mutations affecting the ST kinase site and the C-terminal domain produced more severe phenotypes (refractory epilepsy, limited hand skills, non-walkers) than mutations affecting the ATP binding site. The mutations identified were missense and truncating mutations in the ST kinase segment and frameshift mutations in the C-terminal domain. In the ATP binding site, the recurrent mutations were all missense mutations at amino acid position 40, and when compared to girls with other CDKL5 mutations (presumably in this same area), those with this particular mutation (which is termed p.Ala40Val) tended to present with better hand use and better ability to walk. They also looked at whether X-inactivation (see The story of CDKL5) might also have an effect on phenotype but were unable to draw any useful conclusion.
Note - Overall I found this quite compelling reading really and it certainly builds on previous studies that have suggested links between position of mutation and phenotype. I was a little unsure about the exact number of girls in this study who actually had CDKL5, but overall I think it is a good and useful study. The fact that certain mutations at the C-terminal of the protein (which is well away from the kinase domain) are also associated with more severe phenotypes presumably suggests something about the 3-D structure of the CDKL5 protein in terms of how the C-terminal interacts with the kinase domain. There is a suggestion that part of the C-terminal might stabilise the protein against degradation and mutations here might therefore accelerate degradation -a view perhaps supported by the study below regarding a novel transcript. Who knows!
Variant of Rett Syndrome and CDKL5 gene: Clinical and autonomic description of 10 cases. Neuropediatrics 2012. Link to abstract

This study from Italy describes features in 10 girls, 8 from Italy and 2 from Sweden, aged between 2 and 13 years. All had established CDKL5 mutations (4 missense, 3 frameshift, 1 nonsense and 2 deletions). Early-onset epilepsy was the predominant feature with 9 developing seizures by the 4th month of life and one at 6 months. Speech was absent in all 10 girls, while 8 had some sort of disturbance of their vision. All 10 had experienced disturbances in their sleep patterns during the first 4 months of life and 7 continued to do so at the time of the review. The autonomic aspect of the study focussed on breathing patterns. There are 4 recognised disorders of breathing in Rett Syndrome. Forceful breathing patterns were noted in 8 of the girls with CDKL5 while the remaining 2 girls exhibited the apneustic type.

Note - A study which again reinforces many of the features we are coming to recognise as part of the CDKL5 spectrum. The description of the breathing patterns is interesting as I have not encountered this in any other studies I have read (I may have missed something!). They also discuss other autonomic dysfunction such as cold hands and feet, all of which we recognise in Ellie.
A novel transcript of cyclin-dependent kinase-like 5 (CDKL5) has an alternative C-terminus and is the predominant transcript in brain. Human Genetics 2012. Link to abstrac

The molecular characterisation of the CDKL5 protein is still largely unknown. However, previous research has identified 3 different variants of the CDKL5 protein. This paper describes the theoretical existence of a 4th variant which the authors then go on to show not only exists, but is likely to be the predominant form expressed in the human brain. Although the CDKL5 gene contains 21 coding exons, this novel variant is shorter, only incorporating amino acids coded in exons up to 18. They conclude that this novel variant is likely to be of primary pathogenic importance in CDKL5-related conditions.
Note - This is quite a technical-based paper and took me several malts to get through!! It is estimated that the human body is made up of over 100,000 different proteins yet only contains some 25 - 30,000 genes. This means that on average, one gene may code for perhaps 4 - 5 proteins, and this seems to be the case with CDKL5. The authors of this paper are suggesting that they may have identified one particular form of the CDKL5 protein which is slightly shorter in length, amino acids coded in exons 19 - 21 are not incorporated, and may be the relevant form of the protein in CDKL5-related conditions. This therefore needs to be considered when screening children who present with features of CDKL5.

CDKL5 in different atypical Rett syndrome variants: Description of the first eight cases from Spain. Journal of Pediatric Epilepsy 2012. Link to text

This study is from the Saint Joan de Deu Children's Hospital in Barcelona. Over a 26 year period, 408 patients were diagnosed with classical or variant Retts. There were 258 children identified who had mutations of the MECP2 gene. Of the remaining 150 without a genetic diagnosis, testing for CDKL5 mutations was offered to 53 girls and 9 boys who had either early epilepsy, polymorphic seizures or drug-resistant seizures. They found 8 (15%) of the girls, but none of the boys, had novel pathogenic mutations of the CDKL5 gene. There were variations in the phenotypes of the 8 children, compared to those described in other studies and which have been taken to be characteristic of CDKL5. For instance, the 3 stages of epilepsy as described by Bahi-Buisson were not observed, with only 4 children developing seizures within the first few months of life. There were 3 who developed their seizures at 11 months, 2 and 3 years of age and 1 who did not appear to have developed seizures at all. There were 2 children who regained some speech having previously lost it. The authors report some association between the severity of the phenotype and the location of the mutation with the more severely affected having a mutation at the proximal region of the gene. The authors conclude that testing for CDKL5 must be mandatory in patients with Rett syndrome and early epilepsy, in those with epileptic encephalopathy with onset in the first year of life as well as in those with classical or variant Retts without an identifiable MECP2 gene mutation.

Note - This article clearly throws a slight spanner in the works in that some variation is introduced into what, up to now, have been considered fairy constant characteristics of children with CDKL5, particularly in relation to the presence of early seizures. It therefore at least raises the possibility that the spectrum of CDKL5 phenotypes may be broader than first thought. This issue of the Journal also has an editorial by Jeffrey Neul from the Texas Children's Hospital, which includes a short review of this article. He concludes that perhaps CDKL5 should now be recognised as a distinct clinical disorder with specific unique characteristics rather than as a variant of Rett syndrome, and that not to do so might be a disservice to both clinical medicine and to those with the condition. Bravo!

Further Note - A subsequent Letter to the Editor regarding this article has pointed out that one of the described mutations, which affects almost the very end of the C-terminal domain, may only have "minimal" clinical effect. This is because a mutation at this site (which is in exon 20) affects that part of the CDKL5 gene which is not used to make the particular form of the protein that may be relevant in CDKL5 disorders (see last paper above). This may explain some of the variation in presentation described in the paper, in that the individual with this mutation may not in fact have a CDKL5 related disorder. The plot thickens!!!

Historic, clinical and prognostic features of epileptic encephalopathies caused by CDKL5 mutations. Paediatric Neurology 2012. Link to abstract.

This is a retrospective study from the Mayo Clinic, looking at the clinical features and treatment of seizures in 6 children (4 girls, 2 boys) with CDKL5. The onset of seizures ranged from 1 to 3 months of age. Particular features are discussed including dysphagia (difficulty swallowing) and cortical visual impairment. It is also noted that the 2 males in the group appeared less affected by certain features than the females. No particular treatment eliminated seizures, but topiramate, vigabatrin and the ketogenic diet were the most helpful at reducing their frequency.

Note - This is a study of a small number of children with a relatively short follow-up period, less than 3 years in 2 children and no more than a year in the other 4. The children studied are all young (eldest less than 3 years of age at review). The prognostic features referred to in the title seem to be the presence of the CDKL5 mutation itself and the resultant early-onset of seizures. Vagal Nerve Stimulation (VNS) does not appear to have been used as a treatment option, possibly because of the age of the children involved, although a recent study has suggested that VNS is a safe and effective treatment for resistant epilepsy in young children. The authors of the above study rightly conclude that more research is needed

Adult Phenotypes in Angelman- and Rett- Like Syndromes. Molecular Syndromology. Published online: January 13 2012. Link to abstract.

This paper looks at the evolution of the clinical characteristics (phenotype) in adults with various syndromes, and includes a female who was diagnosed with a splice site mutation in intron 7 of CDKL5 at the age of 47. She had developed seizures after 6 months and a subsequent delay in psychomotor development. Her motor development had only been mildly delayed, but she hadn't learned to speak and she had profound intellectual disability. Her seizures had persisted despite multiple drug treatments. She was small, being at the lower end of centile charts and displayed a number of facial characteristics including prominent cheekbones, mild prognathism, deep-set eyes and blepharochalasis. A number of other characteristics are also described. The authors state that this is the oldest reported patient with a CDKL5 mutation. They also then review adult phenotypes reported in other published studies, which, with the adult in this study comprises a group of 10 adults with an age range from 18 to 47 years. From this review they surmise that apart from difficulty with seizure control, adults with CDKL5 may also have medical problems such as secondary scoliosis, contractures, feeding difficulties and airway infections.

Note - It is probably with some caution that we should apply the findings in this relatively small group of adults to every child now growing up with CDKL5, because, as many of us are aware, although our children share many characteristics, they also display variable levels of function and development. Furthermore, the fact that CDKL5 is now recognised as a separate condition may improve their longer-term outcome through earlier diagnosis and subsequent improvement in treatment and care. Although factors that may influence their longer term development are still to be determined, we are sure to learn more as longitudinal studies continue.

Genes of Early-Onset Epileptic Encephalopathies: From Genotype to Phenotype. Paediatric Neurology 2012. Link to text.

This is a review of genetic disorders associated with early-onset epileptic encephalopathies and the associated phenotypes, including CDKL5. The authors report that to-date 53 pathologic mutations of CDKL5 have been reported consisting of 12 missense, 4 nonsense, 8 splice-site, 22 deletions and 7 frameshift mutations. They state that no correlation has been established between the site of mutation and clinical severity and then go on to discuss a more severe epileptic encephalopathy as described by Bahi-Buisson et al. They review the 3 stages of epilepsy in CDKL5 as described by Bahi-Buisson et al and the EEG findings as described in studies by Pintaudi et al and Melani et al.

Note - A good review article although a lot of information to digest. There also seems to be some ambiguity here about what is known so far about the site of mutation and the severity of the phenotype. This paper states that there is no correlation between severity and site of mutation (giving references), but then refers to the study by Bahi-Buisson which suggests that there may be. This is the ambiguity I can't quite sort out - the CDKL5 gene has been designated Online Mendelian Inheritance in Man OMIM® no. *300203 (the * indicates that this designation applies to the gene and its mutations) and it is to this designation that there is said to be no relation between site and severity. However, there is also another designation, OMIM® #300672 which is a descriptive designation for a phenotype (indicated by the #) and refers to a more severe form of epileptic encephalopathy also associated with CDKL5 -and which according to the study by Bahi-Busson has a phenotype that may well be related to the site of mutation. If the genetic basis for both designations lies within the CDKL5 gene, why have 2 designations or are they being considered as distinct entities? There must be a straight-forward answer here but for now this remains slightly confusing (to me at least) and although I contacted OMIM® to clarify this, I am not sure that I am any the wiser for the response (I would be grateful for clarification from anybody!!) Clearly, the bottom line is that more children with CDKL5 mutations need to be studied to understand the relationship between site of mutation and severity of phenotype, and ultimately, severity will probably depend on several factors, and not just the site of mutation.

A survey of parents of children with cortical or cerebral visual impairment. Journal of Visual Impairment and Blindness 2010. Link to text.

This study summarises the results of a survey of 80 parents who have children with Cortical or Cerebral Visual Impairment (CVI). The various aetiologies (causes) are reviewed along with rehabilitation and intervention strategies. The most effective rehabilitation is said to be that which involves parents and other family members. Motion, particularly of a coloured object is the most important factor in improving the visual response. The survey also looks at 3 issues - how do parent's receive the diagnosis of CVI, what educational support do children with CVI receive, and what do parents of children with CVI feel about the support they and their children receive.

Note - This article has been written by parents of children with Cortical Visual Impairment (CVI) . This is not something that we read so much about in CDKL5 articles, as the focus tends to be on seizures, developmental delay and mobility. However, children with CDKL5 certainly do suffer from CVI and although this article covers a broad spectrum of issues, many are relevant to us which I think is very helpful.

Gastrointestinal Disorders in Children with Neurodevelopmental Disabilities.

Developmental Disabilities Research Reviews 2008. Link to abstract.

As the title suggests, this article reviews the various gastrointestinal problems that children with neurodevelopmental disabilities can suffer. It systematically describes problems from the mouth and swallowing, through to the lower bowel and constipation. Underlying causes are discussed and the various treatment options reviewed. The author points out how difficulties with eating may be confused with behavioural food avoidance or aversion, and that abdominal discomfort can present as chronic irritability, crying or dystonic (involuntary twisting or repetitive) movements of the face and neck. More than 80% of children with gastro-oesophagel reflux will have recurrent vomiting. Constipation is discussed and defined as opening of bowels less frequently than 3 times per week or the regular need for laxatives. Constipation is a common problem in children with disabilities and can go unrecognised for years. Treatment requires a consistent approach by willing parents and carers.

Note - Although this review tends to refer frequently to children with cerebral palsy, many of the problems discussed will apply to children with CDKL5. I have highlighted 2 particular issues that are discussed, gastro-oesophageal reflux and constipation, as these will be familiar to most of us. There is one particular telling line in the discussion about gastro-oesophagel reflux, which says "It is ironic that the learning deficit that usually accompanies neurological impairment also impairs the ability of the individual to communicate the main symptom... which is pain." A good and very useful review article - I have a full copy if anyone would like to read it

Melatonin therapy for circadian rhythm sleep disorders in children with multiple disabilities: what have we learned in the last decade? Developmental Medicine & Child Neurology 2004. Link to text.

Sleep is thought to be a learned neurological process that develops in response to cues from the environment. This is a process that we all go through, but which is impaired in children with neurodevelopmental problems. As a result they can retain their fragmented "baby" pattern of sleeping, potentially throughout their life. Circadian rhythm sleep disorders (CRSD) occur when there is a dissociation between sleep-wake behaviour and the environment. Parents should promote the sleep learning process through "highly regulated strict sleep hygiene". Melatonin is produced by the pineal gland in the brain, and although previous research has established that it can induce and maintain sleep, it's exact mechanism of action remains unclear. It does not promote sleep in the same way as a hypnotic (sleeping tablet) does, but may act by inhibiting neuronal excitability in the central nervous system. Exposure to light tends to decrease melatonin secretion while darkness promotes it. CRSD's tend to be associated with disturbance in melatonin secretion. There are also non-circadian rhythm sleep disorders due to pain from such things as reflux or orthopaedic problems, or from nocturnal seizures or related drug treatment. In the treatment of CRSD it is important to choose the correct melatonin formulation depending of the particular sleep disorder. Discontinuation for a few days is also recommended as children can acquire normal sleep patterns over time. Some studies suggest that children with neurodevelopmental disabilities, who have CRSD can show a 70-90% quick response. However, not all children respond equally and there may be some with certain conditions who may not benefit. No major side-effects of melatonin have been reported in paediatric studies.

Note - This review contains a lot of information about sleep disorders and the role of melatonin. One of the authors published the original study on the role of melatonin in sleep disorders, and so the enthusiasm for melatonin is understandable. They do state that much of the skepticism regarding melatonin was based on lack of quality control, unsubstantiated benefit claims, misconceptions about the action of melatonin and oversimplification about the relationship between melatonin and CRSD's. Perhaps the most relevant points for parents and carers are made in the first few paragraphs where they state that the majority of children with neurodevelopmental disabilities are said to have long-standing sleep disorders, and that the management is often neglected due to the inadequate training of doctors in these problems.