Diagnostic implications of pitfalls in causal variant identification based on 4577 molecularly characterized families.

Despite large sequencing and data sharing efforts, previously characterized pathogenic variants only account for a fraction of Mendelian disease patients, which highlights the need for accurate identification and interpretation of novel variants. In a large Mendelian cohort of 4577 molecularly characterized families, numerous scenarios in which variant identification and interpretation can be challenging are encountered. We describe categories of challenges that cover the phenotype (e.g. novel allelic disorders), pedigree structure (e.g. imprinting disorders masquerading as autosomal recessive phenotypes), positional mapping (e.g. double recombination events abrogating candidate autozygous intervals), gene (e.g. novel gene-disease assertion) and variant (e.g. complex compound inheritance). Overall, we estimate a probability of 34.3% for encountering at least one of these challenges. Importantly, our data show that by only addressing non-sequencing-based challenges, around 71% increase in the diagnostic yield can be expected. Indeed, by applying these lessons to a cohort of 314 cases with negative clinical exome or genome reports, we could identify the likely causal variant in 54.5%. Our work highlights the need to have a thorough approach to undiagnosed diseases by considering a wide range of challenges rather than a narrow focus on sequencing technologies. It is hoped that by sharing this experience, the yield of undiagnosed disease programs globally can be improved.

A null founder variant in NPNT, encoding nephronectin, causes autosomal recessive renal agenesis.

Congenital anomalies of the kidney and urinary tract (CAKUT) are a spectrum of abnormalities affecting morphogenesis of the kidneys and other structures of the urinary tract. Bilateral renal agenesis (BRA) is the most severe presentation of CAKUT. Loss of either nephronectin (NPNT) or its receptor ITGA8 leads to failure of metanephric kidney development with resulting renal agenesis in murine models. Very recently, a single family with renal agenesis and a homozygous truncating variant in NPNT was reported. We report two families in whom genome-wide linkage analysis showed an autozygous locus linked to BRA (at least one member has unilateral renal agenesis) at 4q24, with an LOD score of ~3. Exome sequencing detected a nonsense variant in NPNT in both families within the linkage interval. The pathogenicity of this variant was supported by reverse transcription polymerase chain reaction data showing complete nonsense-mediated decay of the NPNT transcript. Our report confirms the candidacy of NPNT in renal agenesis in humans and shows that even complete loss of function can be compatible with the formation of a single kidney.

Missense NAA20 variants impairing the NatB protein N-terminal acetyltransferase cause autosomal recessive developmental delay, intellectual disability, and microcephaly.

PURPOSE: N-terminal acetyltransferases modify proteins by adding an acetyl moiety to the first amino acid and are vital for protein and cell function. The NatB complex acetylates 20% of the human proteome and is composed of the catalytic subunit NAA20 and the auxiliary subunit NAA25. In five individuals with overlapping phenotypes, we identified recessive homozygous missense variants in NAA20. METHODS: Two different NAA20 variants were identified in affected individuals in two consanguineous families by exome and genome sequencing. Biochemical studies were employed to assess the impact of the NAA20 variants on NatB complex formation and catalytic activity. RESULTS: Two homozygous variants, NAA20 p.Met54Val and p.Ala80Val (GenBank: NM_016100.4, c.160A>G and c.239C>T), segregated with affected individuals in two unrelated families presenting with developmental delay, intellectual disability, and microcephaly. Both NAA20-M54V and NAA20-A80V were impaired in their capacity to form a NatB complex with NAA25, and in vitro acetylation assays revealed reduced catalytic activities toward different NatB substrates. Thus, both NAA20 variants are impaired in their ability to perform cellular NatB-mediated N-terminal acetylation. CONCLUSION: We present here a report of pathogenic NAA20 variants causing human disease and data supporting an essential role for NatB-mediated N-terminal acetylation in human development and physiology.