Towards Personalised Medicine in Bardet-Biedl Syndrome

Personalised medicine is becoming routine in the treatment of common diseases such as cancer, but has lagged behind in the field of rare diseases. It is currently in the early stages for the treatment of BardetBiedl syndrome. Advances in the understanding of ciliary biology and diagnostic techniques have opened up the prospect of treating BBS in a patient-specific manner. Owing to their structure and function, cilia provide an attractive therapeutic target and genetic therapies are being explored in ciliopathy treatment. Promising avenues include gene therapy, gene editing techniques and splicecorrecting and read-through therapies. Targeted drug design has been successful in the treatment of genetic disease and research is underway in the discovery of known and novel drugs to treat BBS. Introduction Bardet-Biedl syndrome (BBS) is one of a family of disorders known as ciliopathies, all of which exhibit a defect in the structure or function of cilia. Cilia are evolutionarily conserved, complex organelles that project from the apical cell membranes of most cells in vertebrates. Their structure differs depending on whether they are motile or immotile. Immotile cilia, also known as primary cilia are implicated in BBS aetiology. These have roles in cell signaling pathways, including sonic hedgehog and WNT signaling[1, 2]. They have been implicated in the development of cancer and obesity as well as ciliopathies[1, 3]. The basal body, which is a specialised centriole, anchors the cilium to the apical cell membrane, and along with the BBSome, which consists of BBS1, BBS2, BBS4, BBS5, BBS7, BBS8 and BBS9 proteins, is involved in the formation and maintenance of cilia[4]. Other BBS proteins, BBS6, BBS10 and BBS12, form a chaperonin complex purported to facilitate protein folding [5]. Mutations in any of these and some other genes localizing primarily to the basal body can cause BBS syndrome Ciliopathies share a number of features including obesity, structural brain defects, retinal degeneration, situs inversus, skeletal abnormalities, cognitive impairment and structural renal and hepatobiliary defects[6]. While most of these features can be seen in BBS, the primary features are rod-cone dystrophy resulting in blindness, postaxial polydactyly, obesity, cognitive impairment, hypogonadism or genital abnormalities and renal anomalies[7]. Minor features, such as facial dysmorphism are also considered in the diagnostic criteria. It is a multisystem disease which begins during embryonic development and involves many organs and body systems, making treatment very challenging. It is thought to affect approximately 1 in 100,000 in North America and up to 1 in 160,000 people in parts of Northern Europe, but a higher prevalence is seen in some isolated or consanguineous populations, for example 1 in 13,500 in Kuwaiti Bedouins and 1 in 18,000 in Newfoundland [7-10]. BBS is inherited in an autosomal recessive manner, though rare cases with more complex modes of inheritance have been reported[11-13]. Currently, 21 genes are known to be implicated in BBS, but mutations cannot be found in about 20% of patients, suggesting that additional genes are yet to be identified [14-16]. Mutations in BBS1 and BBS10 account for approximately 50% and 20% of cases in the United Kingdom[17]. There is significant phenotypic variability between patients with BBS, even between patients with identical mutations and patients in the same family. The basis for this variability is not understood, though genetic modifiers have been identified in other ciliopathies[18, 19]. It is also possible that environmental factors and epigenetic modifications play a role. Some genotype-phenotype correlations have been made but these are not always consistent. For example, patients with mutations in BBS1 have a lower risk of developing renal disease and appear to develop eye disease later[17, 20-22]. Genes known to cause BBS have also been implicated in other ciliopathies, for example CEP290 mutations causes Joubert syndrome, Meckel syndrome, Leber congenital amaurosis and Senior-Loken syndrome as well as BBS[22-26]. Personalised medicine is defined by the Horizon 2020 advisory group as "a medical model using characterization of individuals’ phenotypes and genotypes (e.g. molecular profiling, medical imaging, lifestyle data) for tailoring the right therapeutic strategy for the right person at the right time, and/or to determine the predisposition to disease and/or to deliver timely and targeted prevention"[27]. The identification of biomarkers is allowing early detection of disease and relapse, opening up previously unavailable avenues for treatment. However, this has proved challenging in complex, multisystem disorders, particularly those, like BBS, that manifest in utero and has yet to catch up with other fields, such as oncology. However, the potential for benefit to these patients is enormous, given the burden of morbidity and mortality. Deep phenotyping, diagnostics and patient stratification Deep phenotyping in Bardet-Biedl syndrome Peter Robinson has defined deep phenotyping as “the precise and comprehensive analysis of phenotypic abnormalities in which the individual components of the phenotype are observed and described” and is a vital tool in personalised medicine for rare disease [28]. Accurate and detailed phenotypic information about patients enables stratification for research, clinical trials and treatment. One of the ways to ensure that such data are routinely collected is for patients to attend a specialised multidisciplinary clinic. The National Health Service England has commissioned a specialist BBS clinic, where patients are seen by clinicians including a clinical geneticist, endocrinologist, ophthalmologist, renal physician, psychologist, dietician, speech and language therapist and a family advocate. Detailed clinical data is recorded and a standardized set of investigations including comprehensive genetic testing is performed at each visit. These data allow patients involved in research to be accurately stratified, essential in the search for biomarkers and treatment targets. The use of Human Phenotype Ontology (HPO) terms aids in the standardisation and accurate recording of data[29]. The recording of data in a standardised format enables the integration of clinical data with other datasets. Diagnostics Until recently, diagnostics for diseases such as BBS was done on a gene by gene basis, with the most likely gene, in this case BBS1, being sequenced first. If no mutation was found, the next most likely gene would be sequenced and so on. However, in the last few years, diagnostics for conditions with significant locus heterogeneity has moved to panel-based next generation sequencing (NGS)[30-32]. Currently the approach to BBS testing in the UK is to perform exome sequencing and analyse a panel of 21 BBS genes. This has replaced a panel of 11 BBS-related genes. The diagnostic yield is approximately 80%[33]. This approach is faster and more cost effective than previous strategies. The remaining 20% are sent for whole genome sequencing. As the cost of next-generation sequencing decreases, the utility of whole genome sequencing (WGS) is beginning to be evaluated in the diagnosis of ciliopathies including BBS. There are both technical and diagnostic advantages to using WGS over WES[34]. One advantage is that intronic regions can be examined for areas of interest. Mutations in regulatory regions have occasionally been reported in disease[35]. In the case of BBS, mutations outside coding regions have been reported[26, 36]. WGS is superior to WES in that copy number variants can be detected across the genome thus providing a better chance of an individual molecular diagnosis. An accurate molecular diagnosis is important even in the absence of confirmed genotype-phenotype correlations. Firstly, it allows advice to be given about recurrence risk. Secondly, it allows the stratification of patients for research. Thirdly, in the age of personalised medicine, this knowledge will be required for tailoring treatments to the patient. The diagnostic yield of WGS has yet to be evaluated in this patient population. Understanding variability in Bardet-Biedl syndrome One of the major goals in BBS and ciliopathy research is to understand the variability between patients. Both genetic and environmental factors are likely to influence this[4, 37]. The search for genetic and epigenetic modifiers is ongoing. This is particularly challenging in rare diseases, especially those exhibiting significant phenotypic variability because of the difficulty of assembling large, homogenous cohorts. In smaller or phenotypically varied cohorts only genetic modifiers with a very strong effect will