Zimmermann-Laband syndrome (ZLS; MIM 135500) is a rare developmental disorder characterized by facial dysmorphism, gingival enlargement, nail aplasia/hypoplasia, hypertrichosis, and intellectual disability, with or without epilepsy. Recently, a whole exome sequencing based approach allowed the applicant's group to identify two disease genes (KCNH1 and ATP6V1B2). More recently, further patients with KCNH1 mutations with intellectual disability and a phenotype characterized by syndromic developmental delay, hypotonia and seizures have been reported.
The applicant proposes studies to characterize the molecular bases of ZLS. To this aim, different in vitro approaches will be used to dissect the functional mechanisms of pathogenesis. As recently reported, KCNH1 is a regulator of ciliogenesis. In order to gain insight in the deregulated mechanism caused by the ZLS causing KCNH1 missense mutations, we will examine cilium assembly and function as a potential molecular cause of dysfunction.
The functional role of KCNH1 in ciliogenesis will be assessed by evaluating the frequency of expression of cilia in wild type and mutants expressing cells. We will test also if pathogenic effect of the mutants relies on ciliary physiology. To this aim, we will characterize the subcellular localization of endogenous wild type and mutant KCNH1 on cultured skin fibroblasts obtained from subcutaneous biopsies of ZLS patients, as well as in transiently transfected cell lines.
Functional studies on disease-causing mutants could shed light on molecular network involved in the pathogenesis of this disorder characterized by multisystem involvement, providing clues on the differential effect that the causing mutations exert on ZLS related developmental processes.
Efforts aimed at disclosing molecular dysfunctions involved in pathogenesis are fundamental prerequisites that will provide molecular information with direct impact on diagnosis, prognosis, counseling, and patient management.
So far a total of 21 individuals with pathogenic KCNH1 variants have been published (Kortum, Caputo et al. 2015, Simons et al. 2015, Bramswig et al. 2015, Fukai et al. 2016, Mègarbanè et al. 2016). These patients carry 12 different KCNH1 mutations, suggesting the presence of mutation hot spots corresponding to the S4-S6 domains and highlighting the functional importance of those domains of KCNH1. Furthermore, these finding indicate that there is no correlation between the location of the affected residues and the clinical diagnosis.
Due to the limited number of individuals reported, it is difficult to draw accurate genotype-phenotype correlations. In this context, recruitment of additional patients, clinically well-characterized, and a detailed characterization of the molecular bases are needed to provide new tools for classification and diagnosis of this rare disorder.
Recently KCNH1 has been demonstrated as implicated in ciliogenesis (Sánchez et al. 2016), highlighting the relevance of non-canonical functions of the ion channel KCNH1, whose perturbation could underlie developmental processes. Functional assays to explore the mechanism involving KCNH1 in the pathogenesis of ZLS could shed light on the role of KCNH1 on developmental processes related to ZLS and ZLS related phenotypes providing clues on the differential effect that the pathogenic KCNH1 mutations exert on cellular mechanisms, e.g. cilium assembly/function.
Cilia transduce signals from extracellular stimuli to a cellular response that regulates proliferation, differentiation, transcription, migration, polarity and tissue morphology. Defects in this organelle impair the cellular responses to differentiation signals, leading to a group of diseases collectively termed ciliopathies, that show common clinical features, such as visceral cysts, defective digits, oral anomalies, cognitive impairment, and tumors. KCNH1 mutated patients show orofacial, digital abnormalities, and mental retardation, even with variable severity, and it has been suggested that their morphological features are highly reminiscent of ciliopathies (Pardo et al. 2016). Preliminary results reported that at least one KCNH1 mutation causing ZLS exerts a functional effect on ciliary resorption (Pardo et al. 2016).
The overlapping features of ZLS and other disorders, broad clinical spectrum, and absence of clinical features with pathognomonic value make diagnosis of ZLS challenging, especially in infancy, since some phenotypes (e.g. limb phenotype, gingival hypertrophy and epilepsy) can develop over time. Those limits exert direct impact on patient management due to the prognostic relevance of diagnosis (i.e. prevalence and extent of seizures, degree of cognitive impairment). Efforts aimed at disclosing molecular dysfunctions involved in pathogenesis are fundamental prerequisites that will provide molecular information with direct impact on diagnosis, prognosis, counseling, and patient management.