CDKL5 disruption by t(X;18) in a girl with West syndrome

There is variability in the attainment of developmental milestones in the CDKL5 disorder

Background

Individuals with the CDKL5 disorder have been described as having severely impaired development. A few individuals have been reported having attained more milestones including walking and running. Our aim was to investigate variation in attainment of developmental milestones and associations with underlying genotype.

Methods

Data was sourced from the International CDKL5 Disorder Database, and individuals were included if they had a pathogenic or probably pathogenic CDKL5 mutation and information on early development. Kaplan-Meier time-to-event analyses investigated the occurrence of developmental milestones. Mutations were grouped by their structural/functional consequence, and Cox regression was used to investigate the relationship between genotype and milestone attainment.

Results

The study included 109 females and 18 males. By 5 years of age, only 75% of the females had attained independent sitting and 25% independent walking whilst a quarter of the males could sit independently by 1 year 3 months. Only one boy could walk independently. No clear relationship between mutation group and milestone attainment was present, although females with a late truncating mutation attained the most milestones.

Conclusion

Attainment of developmental milestones is severely impaired in the CDKL5 disorder, with the majority who did attain skills attaining them at a late age. It appears as though males are more severely impaired than the females. Larger studies are needed to further investigate the role of genotype on clinical variability.

A novel transcript of cyclin-dependent kinase-like 5 (CDKL5) has an alternative C-terminus and is the predominant transcript in brain

The X-linked cyclin-dependent kinase-like 5 (CDKL5) gene is an important molecular determinant of early-onset intractable seizures with infantile spasms and Rett syndrome-like phenotype. The gene encodes a kinase that may influence components of molecular pathways associated with MeCP2. In humans there are two previously reported splice variants that differ in the 5' untranslated exons and produce the same 115 kDa protein. Furthermore, very recently, a novel transcript including a novel exon (16b) has been described. By aligning both the human and mouse CDKL5 proteins to the orthologs of other species, we identified a theoretical 107 kDa isoform with an alternative C-terminus that terminates in intron 18. In human brain and all other tissues investigated except the testis, this novel isoform is the major CDKL5 transcript. The detailed characterisation of this novel isoform of CDKL5 reveals functional and subcellular localisation attributes that overlap greatly, but not completely, with that of the previously studied human CDKL5 protein. Considering its predominant expression in the human and mouse brain, we believe that this novel isoform is likely to be of primary pathogenic importance in human diseases associated with CDKL5 deficiency, and suggest that screening of the related intronic sequence should be included in the molecular genetic analyses of patients with a suggestive clinical phenotype.

Epilepsy caused by CDKL5 mutations

Mutations in the cyclin-dependent kinase-like 5 gene (CDKL5) have been identified in female patients with early onset epileptic encephalopathy and severe mental retardation with a Rett-like phenotype. Subsequently CDKL5 mutations were shown to be associated with more diverse phenotypes including mild epilepsy and autism without epilepsy. Furthermore, CDKL5 mutations were found in patients with Angelman-like phenotype. The severity of epilepsy associated with CDKL5 mutations was recently shown to correlate with the type of CDKL5 mutations and epilepsy was identified to involve three distinct sequential stages. Here, we describe the phenotype of a severe form of neurodevelopmental disease in a female patient with a de novo nonsense mutation of the CDKL5 gene c.175C > T (p.R59X) affecting the catalytic domain of CDKL5 protein. Mutations in the CDKL5 gene are less common in males and can be associated with a genomic deletion as found in our male patient with a deletion of 0.3 Mb at Xp22.13 including the CDKL5 gene. We review phenotypes associated with CDKL5 mutations and examine putative relationships between the clinical epilepsy phenotype and the type of the mutation in the CDKL5 gene.

Mutational spectrum of CDKL5 in early-onset encephalopathies: a study of a large collection of French patients and review of the literature

The CDKL5 gene has been implicated in the molecular etiology of early-onset intractable seizures with infantile spasms (IS), severe hypotonia and atypical Rett syndrome (RTT) features. So far, 48 deleterious alleles have been reported in the literature. We screened the CDKL5 gene in a cohort of 177 patients with early-onset seizures, including 30 men and 10 girls with Aicardi syndrome. The screening was negative for all men as well as for women with Aicardi syndrome, excluding the CDKL5 gene as a candidate for this neurodevelopmental disorder. We report 11 additional de novo mutations in CDKL5 in female patients. For the first time, the MLPA approach allowed the identification of a partial deletion encompassing the promoter and the first two exons of CDKL5. The 10-point mutations consist of five missenses (with recurrent amino acid changes at p.Ala40 and p.Arg178), four splicing variants and a 1-base pair duplication. We present a review of all mutated alleles published in the literature. In our study, the overall frequency of mutations in CDKL5 in women with early-onset seizures is around 8.6%, a result comparable with previous reports. Noteworthy, the CDKL5 mutation rate is high (28%) in women with early-onset seizures and IS.

Novel mutations in the CDKL5 gene, predicted effects and associated phenotypes

It has been found that CDKL5 gene mutations are responsible for early-onset epilepsy and drug resistance. We screened a population of 92 patients with classic/atypical Rett syndrome, 17 Angelman/Angelman-like patients and six idiopathic autistic patients for CDKL5 mutations and exon deletions and identified seven novel mutations: six in the Rett subset and one in an Angelman patient. This last, an insertion in exon 11, c.903_904 dupGA, p.Leu302Aspfx49X, is associated with a relatively mild clinical presentation as the patient is the only one capable of sitting and walking alone. Of the six mutations, two are de novo missense changes affecting highly conserved aminoacid residues, c.215 T > C p.Ile72Thr and c.380A > G p.His127Arg (present in a mosaic condition) found in two girls with the most severe clinical presentation, while the remaining are the splicing c.145 + 2 T > C and c.2376 + 5G > A, the c.1648C > T p.Arg550X and the MPLA-identified c.162_99del261 mutation. RNA characterisation of four mutations revealed the aberrant transcript of the missense allele (case 2) and not the stop mutation (case 3), but also allowed the splicing mutation (case 1) and the c.-162_99del261 (case 4) to be categorised as truncating. The obtained data reinforce the view that a more severe phenotype is due more to an altered protein than haploinsufficiency. Furthermore, the mutational repertoire of the CDKL5 gene is shown to be expanded by testing patients with phenotypical overlap to Rett syndrome and applying multiplex ligation-dependent probe amplification.