Loss-of-function mutations in ISCA2 disrupt 4Fe-4S cluster machinery and cause a fatal leukodystrophy with hyperglycinemia and mtDNA depletion.

Iron-sulfur (Fe-S) clusters are essential cofactors for proteins that participate in fundamental cellular processes including metabolism, DNA replication and repair, transcriptional regulation, and the mitochondrial electron transport chain (ETC). ISCA2 plays a role in the biogenesis of Fe-S clusters and a recent report described subjects displaying infantile-onset leukodystrophy due to bi-allelic mutation of ISCA2. We present two additional unrelated cases, and provide a more complete clinical description that includes hyperglycinemia, leukodystrophy of the brainstem with longitudinally extensive spinal cord involvement, and mtDNA deficiency. Additionally, we characterize the role of ISCA2 in mitochondrial bioenergetics and Fe-S cluster assembly using subject cells and ISCA2 cellular knockdown models. Loss of ISCA2 diminished mitochondrial membrane potential, the mitochondrial network, basal and maximal respiration, ATP production, and activity of ETC complexes II and IV. We specifically tested the impact of loss of ISCA2 on 2Fe-2S proteins versus 4Fe-4S proteins and observed deficits in the functioning of 4Fe-4S but not 2Fe-2S proteins. Together these data indicate loss of ISCA2 impaired function of 4Fe-4S proteins resulting in a fatal encephalopathy accompanied by a relatively unusual combination of features including mtDNA depletion alongside complex II deficiency and hyperglycinemia that may facilitate diagnosis of ISCA2 deficiency patients.

De novo CCND2 mutations leading to stabilization of cyclin D2 cause megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome.

Activating mutations in genes encoding phosphatidylinositol 3-kinase (PI3K)-AKT pathway components cause megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome (MPPH, OMIM 603387). Here we report that individuals with MPPH lacking upstream PI3K-AKT pathway mutations carry de novo mutations in CCND2 (encoding cyclin D2) that are clustered around a residue that can be phosphorylated by glycogen synthase kinase 3ß (GSK-3ß). Mutant CCND2 was resistant to proteasomal degradation in vitro compared to wild-type CCND2. The PI3K-AKT pathway modulates GSK-3ß activity, and cells from individuals with PIK3CA, PIK3R2 or AKT3 mutations showed similar CCND2 accumulation. CCND2 was expressed at higher levels in brains of mouse embryos expressing activated AKT3. In utero electroporation of mutant CCND2 into embryonic mouse brains produced more proliferating transfected progenitors and a smaller fraction of progenitors exiting the cell cycle compared to cells electroporated with wild-type CCND2. These observations suggest that cyclin D2 stabilization, caused by CCND2 mutation or PI3K-AKT activation, is a unifying mechanism in PI3K-AKT-related megalencephaly syndromes.

Whole-exome-sequencing-based discovery of human FADD deficiency.

Germline mutations in FASL and FAS impair Fas-dependent apoptosis and cause recessively or dominantly inherited autoimmune lymphoproliferative syndrome (ALPS). Patients with ALPS typically present with no other clinical phenotype. We investigated a large, consanguineous, multiplex kindred in which biological features of ALPS were found in the context of severe bacterial and viral disease, recurrent hepatopathy and encephalopathy, and cardiac malformations. By a combination of genome-wide linkage and whole-exome sequencing, we identified a homozygous missense mutation in FADD, encoding the Fas-associated death domain protein (FADD), in the patients. This FADD mutation decreases steady-state protein levels and impairs Fas-dependent apoptosis in vitro, accounting for biological ALPS phenotypes in vivo. It also impairs Fas-independent signaling pathways. The observed bacterial infections result partly from functional hyposplenism, and viral infections result from impaired interferon immunity. We describe here a complex clinical disorder, its genetic basis, and some of the key mechanisms underlying its pathogenesis. Our findings highlight the key role of FADD in Fas-dependent and Fas-independent signaling pathways in humans.