How exome sequencing is shedding light on the complexity of Mendelian disorders: some examples from Sardinia

The total number of Mendelian disorders is estimated to be around 7,000 and while each is individually rare, together, these genetic conditions contribute significantly to morbidity, mortality, and healthcare costs.In the last decade there has been a paradigm shift in their investigation due to the development of powerful new DNA sequencing technologies, such as whole exome sequencing. Although our knowledge of the diversity of Mendelian phenotypes is progressively increasing, substantial gaps remain. Up to 50% of patients affected by a rare genetic disorder never receive a diagnosis.We focused our attention on such Mendelian disorders and in a collaborative effort we studied by WES a cohort of heterogeneous samples affected by Crisponi/Cold-induced sweating syndrome-like, syndromic Intellectual Disabilities and Epileptic Encephalopathies.The results of our work along with others reported in the literature, are contributing to reveal the extensive clinical variability and genetic complexity underlying Mendelian phenotypes and inheritance, to provide insight into study design and approach and analytical strategies and to identify novel mechanisms.Our increasing knowledge on the genetic basis of rare disorders is shedding light on the “complex” nature of the “simple” Mendelian disorders and that “true monogenic” disorders are very rare, underscoring the current challenges of clinical diagnostics and discovery.