Cartilage-hair hypoplasia (CHH)
- Rna component of mitochondrial rna processing endoribonuclease (RMRP)
Cartilage-hair hypoplasia (CHH) is an inherited growth disorder characterized by short-limb dwarfism, hypoplastic hair, defective immunity, and hypoplastic anemia. Mutations of RMRP gene at chromosome 9p21-12, are responsible for CHH. RMRP encodes the RNA component of mitochondrial RNA processing endoribonuclease (RNase MRP) which is thought to be involved in many important processes in the cell. Mutations in RMRP gene are shown to lower its expression level and lead to transcripts with grater instability. Loss of mutant RNA transcripts seems to be a critical feature in CHH pathogenesis.In addition to CHH, mutations in this gene may also cause Anauxetic dysplasia (OMIM#607095) and Metaphyseal dysplasia without hypotrichosis (OMIM#250460). CHH begins in utero; affected newborns have disproportionatly short limbs. Median adult height for males is 131 cm and 122 cm for females. Other clinical manifestations include hypoplastic hair and defective immunity, which occasionally causes severe complications of chickenpox and also accouns for an increased risk of malignancy, mainly lymphomas. Hypoplastic anemia during childhood occurs in almost all cases. Hirschprung disease is part of the CHH phenotype, although it is present in only a small fraction of cases. Diagnosis of CHH is based on characteristic X-ray changes of bones and other clinical findings. It can be confirmed by DNA testing of the RMRP gene.