Finally the identification of predictive biomarkers of
Finally, the identification of predictive biomarkers of response will also allow the more recently discovered CDK inhibitors to be explored in particular genetically-defined contexts, for example by building on recent observations that KRAS mutant tumors are highly sensitive to CDK1 inhibition, and that CDK2 or CDK9 inhibition is synthetically lethal in MYC-addicted tumor Caffeine (Costa-Cabral et al., 2016, Poon et al., 2016).
Conflict of interest
Acknowledgements This work was supported by Cancer Research UK (grant numbers C309/A11566, C368/A6743 and A368/A7990). We acknowledge Cancer Research UK funding to the Cancer Research UK Cancer Therapeutics Unit at The Institute of Cancer Research. Paul Workman is a Cancer Research UK Life Fellow. We thank Nicky Evans for editorial assistance and our many colleagues and collaborators for helpful discussions.
Introduction Pathologic variants in cyclin-dependent kinase-like 5 (CDKL5)1, 2, 3, 4, 5 cause CDKL5 deficiency disorder (CDD; OMIM 300203, 300672), a developmental encephalopathy. Developmental encephalopathies share common constellations of features that extend beyond traditional criteria of autism spectrum disorder or intellectual disability such as treatment-resistant epilepsy, movement disorders, and autonomic dysfunction. Pathologic variants in CDKL5 cause early life epilepsy in one in 40,000 to 60,000 live births,7, 8, 9 half to a third as prevalent as Dravet (1:20,000 to 50,000)10, 11 or Rett (1:10,000 females) syndrome. Common features include infantile-onset refractory epilepsy, hypotonia, developmental delay, intellectual disability, and visual impairment.13, 14, 15 CDD is an X-linked disorder that affects females more than males (∼4:1) (Olson et al., unpublished data, 2018) as males with germline variants have no normal CDKL5 gene and may not survive fetal life. CDD was initially identified as the early seizure variant of Rett syndrome, but only 23.7% of females and no males with CDD met criteria for typical or atypical Rett syndrome and diagnosis of atypical Rett syndrome is even rarer in recent clinical experience (Olson et al., unpublished data, 2018).13, 16 The literature on CDD includes case series and data from the International CDKL5 Disorder Database, based on caregiver questionnaires.13, 17, 18, 19, 20, 21 Prospective data collection is occurring through the Natural History Study for Rett and Rett-related disorders (U54 HD061222; ClinicalTrials.gov: NCT00299312/NCT02738281) and through a clinic-based study by the International Foundation for CDKL5 Research Centers of Excellence (COEs). Initial sites were Boston Children's Hospital, Children's Hospital Colorado, and Cleveland Clinic. The COEs provide comprehensive care and collaborate on research for CDD. The COEs have collected data on >93 individuals with CDD between 0 and 34 years to inform the typical features and spectrum of CDD (Olson et al., unpublished data, 2018).
CDKL5 protein and molecular biology CDKL5 is a serine threonine kinase. The N-terminal catalytic domain starts in exon 2 and the long C-terminus may have a regulatory role. CDKL5 is highly expressed in the brain, predominantly in neuronal nuclei and dendrites, with peak expression in early postnatal life, when symptoms typically begin.23, 24, 25, 26 The CDKL5 protein has roles in cell proliferation, neuronal migration, axonal outgrowth, dendritic morphogenesis and synapse development, and function in the adult brain. CDKL5 has multiple transcripts because of alternative splicing in mice and humans. The primary brain isoform is hCDKL5_1. Pathogenic missense variants occur exclusively within the catalytic domain except for the recurrent missense variant p.Val718Met, which affects splicing. A male individual mosaic for this variant followed in our COEs has a “typical” CDD phenotype but has walked independently since age two years. Somatic mosaicism in probands, perhaps more often in males, and presumed parental mosaicism is described; unaffected parents with a full germline CDKL5 variant have not been described.15, 19, 29, 30, 31, 32, 33 Thus parental testing is critical to assess variants of uncertain significance in CDKL5. There are neither biomarkers nor is there a functional assay for variants of uncertain significance, both would be beneficial to the field.