Type and Level of RMRP Functional Impairment Predicts Phenotype in the Cartilage Hair Hypoplasia–Anauxetic Dysplasia Spectrum

Abstract

Mutations in the RMRP gene lead to a wide spectrum of autosomal recessive skeletal dysplasias, ranging from the milder phenotypes metaphyseal dysplasia without hypotrichosis and cartilage hair hypoplasia (CHH) to the severe anauxetic dysplasia (AD). This clinical spectrum includes different degrees of short stature, hair hypoplasia, defective erythrogenesis, and immunodeficiency. The RMRP gene encodes the untranslated RNA component of the mitochondrial RNA-processing ribonuclease, RNase MRP. We recently demonstrated that mutations may affect both messenger RNA (mRNA) and ribosomal RNA (rRNA) cleavage and thus cell-cycle regulation and protein synthesis. To investigate the genotype-phenotype correlation, we analyzed the position and the functional effect of 13 mutations in patients with variable features of the CHH-AD spectrum. Those at the end of the spectrum include a novel patient with anauxetic dysplasia who was compound heterozygous for the null mutation g.254_263delCTCAGCGCGG and the mutation g.195C-->T, which was previously described in patients with milder phenotypes. Mapping of nucleotide conservation to the two-dimensional structure of the RMRP gene revealed that disease-causing mutations either affect evolutionarily conserved nucleotides or are likely to alter secondary structure through mispairing in stem regions. In vitro testing of RNase MRP multiprotein-specific mRNA and rRNA cleavage of different mutations revealed a strong correlation between the decrease in rRNA cleavage in ribosomal assembly and the degree of bone dysplasia, whereas reduced mRNA cleavage, and thus cell-cycle impairment, predicts the presence of hair hypoplasia, immunodeficiency, and hematological abnormalities and thus increased cancer risk.

Functional characterization of theDrosophila MRP(mitochondrial RNA processing) RNA gene

“…6). This result is consistent with a defect in a subset of previously described RNase MRP mutants that has been described in other species (Schmitt and Clayton 1993;Lygerou et al 1996;Hermanns et al 2005;Thiel et al 2005Thiel et al , 2007.…”

“…Essentially uniform expression of dMRP RNA throughout the Drosophila life cycle ( Fig. 2A) is consistent with its role in fundamental cellular processes such as ribosome biogenesis, mitochondrial DNA replication, and cell cycle regulation (Chang and Clayton 1987;Schmitt and Clayton 1993;Lygerou et al 1996;Gill et al 2004;Thiel et al 2007). Further, dMRP RNA localizes to the nucleolus, the same compartment to which the bulk of S. cerevisiae RNase MRP (Gill et al 2006) and mammalian MRP RNA localize ( Fig.…”

“…S. cerevisiae and human MRP RNA genes have been known for some time to cleave the rRNA precursor leading to the formation of the 59 end of the 5.8S S rRNA. A reduction in the level of 5.8S S , accompanied by an increase in the level of the unprocessed 5.8S L , in MRP RNA mutants is a strong indication of this function (Schmitt and Clayton 1993;Lygerou et al 1996;Hermanns et al 2005;Thiel et al 2005Thiel et al , 2007. Homozygous dMRP EY08633 mutants display a similar change in relative abundances of the two forms of 5.8S rRNAs, indicating a similar function for this gene in Drosophila (Fig.…”

“…In human cells, a defect in rRNA processing and regulation of cyclin B mRNA were also observed; however, direct cleavage of these substrates by RNase MRP has not yet been demonstrated (Hermanns et al 2005;Thiel et al 2005Thiel et al , 2007. Recently, human MRP RNA (RMRP) was identified as a component of a novel complex with telomerase reverse transcriptase (TERT) that functions as an RNA-dependent RNA polymerase.…”

“…Mutations that result in increased cyclin B mRNA levels are correlated with symptoms characteristic of CHH. These patients show less severe skeletal defects but have higher incidences of immunodeficiency, hematological abnormalities, and cancer (Thiel et al 2005(Thiel et al , 2007.…”