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LRRC59 (leucine-rich repeat-containing protein 59) is a tail-anchored protein with a single transmembrane domain close to its C-terminal end that localizes to the endoplasmic reticulum (ER) and the nuclear envelope. Here, we investigate the mechanisms of membrane integration of LRRC59 and its targeting to the inner nuclear membrane (INM). Using purified microsomes, we show that LRRC59 can be post-translationally inserted into ER-derived membranes. The TRC-pathway, a major route for post-translational membrane insertion, is not required for LRRC59. Like emerin, another tail-anchored protein, LRRC59 reaches the INM, as demonstrated by rapamycin-dependent dimerization assays. Using different approaches to inhibit importin α/β-dependent nuclear import of soluble proteins, we show that the classic nuclear transport machinery does not play a major role in INM-targeting of LRRC59. Instead, the size of the cytoplasmic domain of LRRC59 is an important feature, suggesting that targeting is governed by passive diffusion.

The NAA10-NAA15 complex (NatA) is an N-terminal acetyltransferase that catalyzes N-terminal acetylation of ~40% of all human proteins. N-terminal acetylation has several different roles in the cell, including altering protein stability and degradation, protein localization and protein-protein interactions. In recent years several X-linked NAA10 variants have been associated with genetic disorders. We have identified a previously undescribed NAA10 c.215T>C p.(Ile72Thr) variant in three boys from two unrelated families with a milder phenotypic spectrum in comparison to most of the previously described patients with NAA10 variants. These boys have development delay, intellectual disability, and cardiac abnormalities as overlapping phenotypes. Functional studies reveal that NAA10 Ile72Thr is destabilized, while binding to NAA15 most likely is intact. Surprisingly, the NatA activity of NAA10 Ile72Thr appears normal while its monomeric activity is decreased. This study further broadens the phenotypic spectrum associated with NAA10 deficiency, and adds to the evidence that genotype-phenotype correlations for NAA10 variants are much more complex than initially anticipated.

N-terminal acetylation is a common protein modification in eukaryotes associated with numerous cellular processes. Inherited mutations in NAA10, encoding the catalytic subunit of the major N-terminal acetylation complex NatA have been associated with diverse, syndromic X-linked recessive disorders, whereas de novo missense mutations have been reported in one male and one female individual with severe intellectual disability but otherwise unspecific phenotypes. Thus, the full genetic and clinical spectrum of NAA10 deficiency is yet to be delineated. We identified three different novel and one known missense mutation in NAA10, de novo in 11 females, and due to maternal germ line mosaicism in another girl and her more severely affected and deceased brother. In vitro enzymatic assays for the novel, recurrent mutations p.(Arg83Cys) and p.(Phe128Leu) revealed reduced catalytic activity. X-inactivation was random in five females. The core phenotype of X-linked NAA10-related N-terminal-acetyltransferase deficiency in both males and females includes developmental delay, severe intellectual disability, postnatal growth failure with severe microcephaly, and skeletal or cardiac anomalies. Genotype-phenotype correlations within and between both genders are complex and may include various factors such as location and nature of mutations, enzymatic stability and activity, and X-inactivation in females., (© 2016 The Authors. **Human Mutation published by Wiley Periodicals, Inc.)

We report two brothers from a non-consanguineous Irish family presenting with a novel syndrome characterised by intellectual disability, facial dysmorphism, scoliosis and long QT. Their mother has a milder phenotype including long QT. X-linked inheritance was suspected. Whole exome sequencing identified a novel missense variant (c.128 A > C; p.Tyr43Ser) in NAA10 (X chromosome) as the cause of the family's disorder. Sanger sequencing confirmed that the mutation arose de novo in the carrier mother. NAA10 encodes the catalytic subunit of the major human N-terminal acetylation complex NatA. In vitro assays for the p.Tyr43Ser mutant enzyme showed a significant decrease in catalytic activity and reduced stability compared to wild-type Naa10 protein. NAA10 has previously been associated with Ogden syndrome, Lenz microphthalmia syndrome and non-syndromic developmental delay. Our findings expand the clinical spectrum of NAA10 and suggest that the proposed correlation between mutant Naa10 enzyme activity and phenotype severity is more complex than anticipated; the p.Tyr43Ser mutant enzyme has less catalytic activity than the p.Ser37Pro mutant associated with lethal Ogden syndrome but results in a milder phenotype. Importantly, we highlight the need for cardiac assessment in males and females with NAA10 variants as both patients and carriers can have long QT.