rnaset2 mutant zebrafish model familial cystic leukoencephalopathy and reveal a role for RNase T2 in degrading ribosomal RNA.

T2-family acidic endoribonucleases are represented in all genomes. A physiological role for RNase T2 has yet to be defined for metazoa. RNASET2 mutation in humans is linked with a leukoencephalopathy that arises in infancy characterized by cortical cysts and multifocal white matter lesions. We now show localization of RNASET2 within lysosomes. Further, we demonstrate that loss of rnaset2 in mutant zebrafish results in accumulation of undigested rRNA within lysosomes within neurons of the brain. Further, by using high field intensity magnetic resonance microimaging, we reveal white matter lesions in these animals comparable to those observed in RNASET2-deficient infants. This correlates with accumulation of Amyloid precursor protein and astrocytes at sites of neurodegeneration. Thus we conclude that familial cystic leukoencephalopathy is a lysosomal storage disorder in which rRNA is the best candidate for the noxious storage material.

Pelizaeus-Merzbacher-like disease is caused not only by a loss of connexin47 function but also by a hemichannel dysfunction.

Autosomal recessive mutations in the GJA12/GJC2 gene encoding the gap junction protein connexin47 (C x 47) cause a form of Pelizaeus-Merzbacher-like disease (PMLD) with hypomyelination, nystagmus, impaired psychomotor development and progressive spasticity. We investigated the functional consequences of four C x 47 missense mutations (G149S, G236R, T265A, and T398I) and one C x 47 complex mutation (A98G_V99insT) by immunoblot analysis and immunocytochemistry in transfected communication-incompetent HeLa cells and in OLI-neu cells. All studied C x 47 mutants, except G236R, generated stable proteins in transfected HeLa cells and OLI-neu cells. The mutants T265A and A98G_V99insT were retained in the ER, T398I formed gap junctional plaques at the plasma membrane, and G149S showed both, structures at the plasma membrane and ER localization. Two-microelectrode voltage clamp analyses in Xenopus laevis oocytes injected with wild-type and mutant C x 47 cRNA revealed reduced hemichannel currents for G236R, T265A, and A98G_V99insT. In contrast, T398I revealed hemichannel currents comparable to wild-type. For C x 47 mutant T398I, our results indicate a defect in hemichannel function, whereas C x 47 mutants G149S, G236R, T265A, and A98G_V99insT are predicted to result in a loss of C x 47 hemichannel function. Thus, PMLD is likely to be caused by two different disease mechanisms: a loss of function and a dysfunction.

RNASET2-deficient cystic leukoencephalopathy resembles congenital cytomegalovirus brain infection.

Congenital cytomegalovirus brain infection without symptoms at birth can cause a static encephalopathy with characteristic patterns of brain abnormalities. Here we show that loss-of-function mutations in the gene encoding the RNASET2 glycoprotein lead to cystic leukoencephalopathy, an autosomal recessive disorder with an indistinguishable clinical and neuroradiological phenotype. Congenital cytomegalovirus infection and RNASET2 deficiency may both interfere with brain development and myelination through angiogenesis or RNA metabolism.