Learning from massive testing of mitochondrial disorders: UPD explaining unorthodox transmission.

The heterogeneous group of mitochondrial disorders (MDs) is by far the largest class of inborn errors of metabolism, having a collective incidence of 1.6 in 5000.1 Over the past decade, routine clinical use of next-generation sequencing (NGS) has allowed a considerable number of cases to receive a precise molecular diagnosis. At present, however, NGS approaches, including targeted multigene panels and exome sequencing, still fail to achieve molecular definition in about half of patients with MD with a confirmed biochemical and clinical diagnosis.2 These unresolved cases could be due to new (‘hyper-rare’) genes/loci; alternatively, they could be linked to limits of current NGS strategies: indeed, mosaicisms requiring high sequencing depth for detection, genomic rearrangements including small and large deletions/insertions ( 50 bp, respectively), and structural changes such as chromosomal rearrangements (inversions, translocations, deletions, duplications) all usually escape traditional NGS analysis pipelines. The chronic limited use of whole-genome sequencing in diagnostic settings may contribute to the substantial number of unresolved cases. The presence of repeat expansions and variants in deep intronic and regulatory regions2 may also provide an explanation for ‘missing heritability’. Although relatively rare, UPD might explain the unsolved transmission of homozygous mutations or aberrant patterns of imprinting,3 even in MDs. Here, we describe the cases of two children, each with a clinical diagnosis of MD, in whom chromosomal microarray analysis (CMA) allowed detection of UPD and helped to substantiate a homozygous variant identified by NGS. This study was conducted in accordance with Italian National Health System guidelines and the Declaration of Helsinki. Patient 1 is a child who, when first seen by us, presented with generalised tonic-clonic seizures, hypotonia and …