Bi-allelic MCM10 variants associated with immune dysfunction and cardiomyopathy cause telomere shortening.

Minichromosome maintenance protein 10 (MCM10) is essential for eukaryotic DNA replication. Here, we describe compound heterozygous MCM10 variants in patients with distinctive, but overlapping, clinical phenotypes: natural killer (NK) cell deficiency (NKD) and restrictive cardiomyopathy (RCM) with hypoplasia of the spleen and thymus. To understand the mechanism of MCM10-associated disease, we modeled these variants in human cell lines. MCM10 deficiency causes chronic replication stress that reduces cell viability due to increased genomic instability and telomere erosion. Our data suggest that loss of MCM10 function constrains telomerase activity by accumulating abnormal replication fork structures enriched with single-stranded DNA. Terminally-arrested replication forks in MCM10-deficient cells require endonucleolytic processing by MUS81, as MCM10:MUS81 double mutants display decreased viability and accelerated telomere shortening. We propose that these bi-allelic variants in MCM10 predispose specific cardiac and immune cell lineages to prematurely arrest during differentiation, causing the clinical phenotypes observed in both NKD and RCM patients.

Implementation of a genomic medicine multi-disciplinary team approach for rare disease in the clinical setting: a prospective exome sequencing case series.

BACKGROUND: A multi-disciplinary approach to promote engagement, inform decision-making and support clinicians and patients is increasingly advocated to realise the potential of genome-scale sequencing in the clinic for patient benefit. Here we describe the results of establishing a genomic medicine multi-disciplinary team (GM-MDT) for case selection, processing, interpretation and return of results. METHODS: We report a consecutive case series of 132 patients (involving 10 medical specialties with 43.2% cases having a neurological disorder) undergoing exome sequencing over a 10-month period following the establishment of the GM-MDT in a UK NHS tertiary referral hospital. The costs of running the MDT are also reported. RESULTS: In total 76 cases underwent exome sequencing following triage by the GM-MDT with a clinically reportable molecular diagnosis in 24 (31.6%). GM-MDT composition, operation and rationale for whether to proceed to sequencing are described, together with the health economics (cost per case for the GM-MDT was £399.61), the utility and informativeness of exome sequencing for molecular diagnosis in a range of traits, the impact of choice of sequencing strategy on molecular diagnostic rates and challenge of defining pathogenic variants. In 5 cases (6.6%), an alternative clinical diagnosis was indicated by sequencing results. Examples were also found where findings from initial genetic testing were reconsidered in the light of exome sequencing including TP63 and PRKAG2 (detection of a partial exon deletion and a mosaic missense pathogenic variant respectively); together with tissue-specific mosaicism involving a cytogenetic abnormality following a normal prenatal array comparative genomic hybridization. CONCLUSIONS: This consecutive case series describes the results and experience of a multidisciplinary team format that was found to promote engagement across specialties and facilitate return of results to the responsible clinicians.

Insights from early experience of a Rare Disease Genomic Medicine Multidisciplinary Team: a qualitative study.

Whole-exome/whole-genome sequencing (WES/WGS) has the potential to enhance genetic diagnosis of rare disease, and is increasingly becoming part of routine clinical care in mainstream medicine. Effective translation will require ongoing efforts in a number of areas including: selection of appropriate patients, provision of effective consent, pre- and post-test genetic counselling, improving variant interpretation algorithms and practices, and management of secondary findings including those found incidentally and those actively sought. Allied to this is the need for an effective education programme for all members of clinical teams involved in care of patients with rare disease, as well as to maintain public confidence in the use of these technologies. We established a Genomic Medicine Multidisciplinary Team (GM-MDT) in 2014 to build on the experiences of earlier successful research-based WES/WGS studies, to address these needs and to review results including pertinent and secondary findings. Here we report on a qualitative study of decision-making in the GM-MDT combined with analysis of semi-structured interviews with GM-MDT members. Study findings show that members appreciate the clinical and scientific diversity of the GM-MDT and value it for education and oversight. To date, discussions have focussed on case selection including the extent and interpretation of clinical and family history information required to establish likely monogenic aetiology and inheritance model. Achieving a balance between effective use of WES/WGS - prioritising cases in a diverse and highly complex patient population where WES/WGS will be tractable - and meeting the recruitment targets of a large project is considered challenging.

A Recurrent Mosaic Mutation in SMO, Encoding the Hedgehog Signal Transducer Smoothened, Is the Major Cause of Curry-Jones Syndrome.

Curry-Jones syndrome (CJS) is a multisystem disorder characterized by patchy skin lesions, polysyndactyly, diverse cerebral malformations, unicoronal craniosynostosis, iris colobomas, microphthalmia, and intestinal malrotation with myofibromas or hamartomas. Cerebellar medulloblastoma has been described in a single affected individual; in another, biopsy of skin lesions showed features of trichoblastoma. The combination of asymmetric clinical features, patchy skin manifestations, and neoplastic association previously led to the suggestion that this could be a mosaic condition, possibly involving hedgehog (Hh) signaling. Here, we show that CJS is caused by recurrent somatic mosaicism for a nonsynonymous variant in SMO (c.1234C>T [p.Leu412Phe]), encoding smoothened (SMO), a G-protein-coupled receptor that transduces Hh signaling. We identified eight mutation-positive individuals (two of whom had not been reported previously) with highly similar phenotypes and demonstrated varying amounts of the mutant allele in different tissues. We present detailed findings from brain MRI in three mutation-positive individuals. Somatic SMO mutations that result in constitutive activation have been described in several tumors, including medulloblastoma, ameloblastoma, and basal cell carcinoma. Strikingly, the most common of these mutations is the identical nonsynonymous variant encoding p.Leu412Phe. Furthermore, this substitution has been shown to activate SMO in the absence of Hh signaling, providing an explanation for tumor development in CJS. This raises therapeutic possibilities for using recently generated Hh-pathway inhibitors. In summary, our work uncovers the major genetic cause of CJS and illustrates strategies for gene discovery in the context of low-level tissue-specific somatic mosaicism.

Factors influencing success of clinical genome sequencing across a broad spectrum of disorders.

To assess factors influencing the success of whole-genome sequencing for mainstream clinical diagnosis, we sequenced 217 individuals from 156 independent cases or families across a broad spectrum of disorders in whom previous screening had identified no pathogenic variants. We quantified the number of candidate variants identified using different strategies for variant calling, filtering, annotation and prioritization. We found that jointly calling variants across samples, filtering against both local and external databases, deploying multiple annotation tools and using familial transmission above biological plausibility contributed to accuracy. Overall, we identified disease-causing variants in 21% of cases, with the proportion increasing to 34% (23/68) for mendelian disorders and 57% (8/14) in family trios. We also discovered 32 potentially clinically actionable variants in 18 genes unrelated to the referral disorder, although only 4 were ultimately considered reportable. Our results demonstrate the value of genome sequencing for routine clinical diagnosis but also highlight many outstanding challenges.