Clinical genetic characteristics of metaphyseal chondrodysplasia, McKusick type (cartilage-hair hypoplasia) in children caused by mutations in the RMRP gene: authors’ observations and literature review

Introduction. Metaphyseal chondrodysplasia, McKusick type (MCD) (OMIM: #250250) (cartilage-hair hypoplasia) is a rare, autosomal recessive disorder with main clinical manifestations including disproportionate short stature, hair thinning and hypotrichosis. Some patients with MCD develop cellular and humoral immunodeficiency, bronchiectases and Hirschsprung disease. Such patients have an increased risk of developing malignant tumors and hypoplastic anemia. MCD is one of the rare monogenic disorders caused by mutations in the RMRP gene encoding a non-coding RNA instead of a protein. So far, 123 pathogenic RMRP variants have been described. The evidence of clinical genetic correlations in patients with different types and localization of gene mutations will facilitate further understanding of pathogenetic mechanisms of the disorder and enable to predict the spectrum and severity of clinical symptoms in individual patients.

Materials and methods. We present the first description of clinical genetic characteristics of two Russian patients with MCD caused by mutations in the RMRP gene, along with the comparison of our results with literature data. In both cases the diagnosis was confirmed by analyzing the RMRP gene sequence using the direct Sanger sequencing technique.

Results. Analysis of specific clinical signs observed during clinical examination of our patients in comparison with those reported in literature has shown the presence of typical skeletal and extraskeletal manifestations suggestive of MCD. In Russian patients we found the major mutation previously described in the Amish and Finnish populations, n.71A>G, present in compound heterozygous state, along with two other mutations: in one patient with an earlier described n.80G>A mutation, and in the other — a newly detected n.76C>T mutation. All detected mutations were mapped to a highly conserved region of the first domain that plays a major role in the functioning of the endoribonuclease complex.

Conclusions. Considering the small size of the RMRP gene and presence of specific signs of MCD, the most accurate and inexpensive method of molecular genetic analysis is the detection of mutations in the RMRP gene using direct automated Sanger sequencing. Timely diagnosis of MCD enables to choose the correct follow-up strategy for patients with this disorder.

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