Incomplete penetrance for isolated congenital asplenia in humans with mutations in translated and untranslated RPSA exons.

Isolated congenital asplenia (ICA) is the only known human developmental defect exclusively affecting a lymphoid organ. In 2013, we showed that private deleterious mutations in the protein-coding region of RPSA, encoding ribosomal protein SA, caused ICA by haploinsufficiency with complete penetrance. We reported seven heterozygous protein-coding mutations in 8 of the 23 kindreds studied, including 6 of the 8 multiplex kindreds. We have since enrolled 33 new kindreds, 5 of which are multiplex. We describe here 11 new heterozygous ICA-causing RPSA protein-coding mutations, and the first two mutations in the 5'-UTR of this gene, which disrupt mRNA splicing. Overall, 40 of the 73 ICA patients (55%) and 23 of the 56 kindreds (41%) carry mutations located in translated or untranslated exons of RPSA. Eleven of the 43 kindreds affected by sporadic disease (26%) carry RPSA mutations, whereas 12 of the 13 multiplex kindreds (92%) carry RPSA mutations. We also report that 6 of 18 (33%) protein-coding mutations and the two (100%) 5'-UTR mutations display incomplete penetrance. Three mutations were identified in two independent kindreds, due to a hotspot or a founder effect. Finally, RPSA ICA-causing mutations were demonstrated to be de novo in 7 of the 23 probands. Mutations in RPSA exons can affect the translated or untranslated regions and can underlie ICA with complete or incomplete penetrance.

Whole-genome sequencing is more powerful than whole-exome sequencing for detecting exome variants.

We compared whole-exome sequencing (WES) and whole-genome sequencing (WGS) in six unrelated individuals. In the regions targeted by WES capture (81.5% of the consensus coding genome), the mean numbers of single-nucleotide variants (SNVs) and small insertions/deletions (indels) detected per sample were 84,192 and 13,325, respectively, for WES, and 84,968 and 12,702, respectively, for WGS. For both SNVs and indels, the distributions of coverage depth, genotype quality, and minor read ratio were more uniform for WGS than for WES. After filtering, a mean of 74,398 (95.3%) high-quality (HQ) SNVs and 9,033 (70.6%) HQ indels were called by both platforms. A mean of 105 coding HQ SNVs and 32 indels was identified exclusively by WES whereas 692 HQ SNVs and 105 indels were identified exclusively by WGS. We Sanger-sequenced a random selection of these exclusive variants. For SNVs, the proportion of false-positive variants was higher for WES (78%) than for WGS (17%). The estimated mean number of real coding SNVs (656 variants, ∼3% of all coding HQ SNVs) identified by WGS and missed by WES was greater than the number of SNVs identified by WES and missed by WGS (26 variants). For indels, the proportions of false-positive variants were similar for WES (44%) and WGS (46%). Finally, WES was not reliable for the detection of copy-number variations, almost all of which extended beyond the targeted regions. Although currently more expensive, WGS is more powerful than WES for detecting potential disease-causing mutations within WES regions, particularly those due to SNVs.

Neural migration. Structures of netrin-1 bound to two receptors provide insight into its axon guidance mechanism.

Netrins are secreted proteins that regulate axon guidance and neuronal migration. Deleted in colorectal cancer (DCC) is a well-established netrin-1 receptor mediating attractive responses. We provide evidence that its close relative neogenin is also a functional netrin-1 receptor that acts with DCC to mediate guidance in vivo. We determined the structures of a functional netrin-1 region, alone and in complexes with neogenin or DCC. Netrin-1 has a rigid elongated structure containing two receptor-binding sites at opposite ends through which it brings together receptor molecules. The ligand/receptor complexes reveal two distinct architectures: a 2:2 heterotetramer and a continuous ligand/receptor assembly. The differences result from different lengths of the linker connecting receptor domains fibronectin type III domain 4 (FN4) and FN5, which differs among DCC and neogenin splice variants, providing a basis for diverse signaling outcomes.

Following experimental stroke, the recovering brain is vulnerable to lipoxygenase-dependent semaphorin signaling.

Recovery from stroke is limited, in part, by an inhibitory environment in the postischemic brain, but factors preventing successful remodeling are not well known. Using cultured cortical neurons from mice, brain endothelial cells, and a mouse model of ischemic stroke, we show that signaling from the axon guidance molecule Sema3A via eicosanoid second messengers can contribute to this inhibitory environment. Either 90 nM recombinant Sema3A, or the 12/15-lipoxygenase (12/15-LOX) metabolites 12-HETE and 12-HPETE at 300 nM, block axon extension in neurons compared to solvent controls, and decrease tube formation in endothelial cells. The Sema3A effect is reversed by inhibiting 12/15-LOX, and neurons derived from 12/15-LOX-knockout mice are insensitive to Sema3A. Following middle cerebral artery occlusion to induce stroke in mice, immunohistochemistry shows both Sema3A and 12/15-LOX are increased in the cortex up to 2 wk. To determine whether a Sema3A-dependent damage pathway is activated following ischemia, we injected recombinant Sema3A into the striatum. Sema3A alone did not cause injury in normal brains. But when injected into postischemic brains, Sema3A increased cortical damage by 79%, and again, this effect was reversed by 12/15-LOX inhibition. Our findings suggest that blocking the semaphorin pathway should be investigated as a therapeutic strategy to improve stroke recovery.

Structure of the semaphorin-3A receptor binding module.

The semaphorins are a large group of extracellular proteins involved in a variety of processes during development, including neuronal migration and axon guidance. Their distinctive feature is a conserved 500 amino acid semaphorin domain, a ligand-receptor interaction module also present in plexins and scatter-factor receptors. We report the crystal structure of a secreted 65 kDa form of Semaphorin-3A (Sema3A), containing the full semaphorin domain. Unexpectedly, the semaphorin fold is a variation of the beta propeller topology. Analysis of the Sema3A structure and structure-based mutagenesis data identify the neuropilin binding site and suggest a potential plexin interaction site. Based on the structure, we present a model for the initiation of semaphorin signaling and discuss potential similarities with the signaling mechanisms of other beta propeller cell surface receptors, such as integrins and the LDL receptor.