RUNX2-related metaphyseal dysplasia with maxillary hypoplasia: A rare skeletal disorder resembling SFRP4-related Pyle disease.

Metaphyseal dysplasia with maxillary hypoplasia with or without brachydactyly (MDMHB) is an ultra-rare skeletal dysplasia caused by heterozygous intragenic RUNX2 duplications, comprising either exons 3 to 5 or exons 3 to 6 of RUNX2. In this study, we describe a 14-year-old Belgian boy with metaphyseal dysplasia with maxillary hypoplasia but without brachydactyly. Clinical and radiographic examination revealed mild facial dysmorphism, dental anomalies, enlarged clavicles, genua valga and metaphyseal flaring and thin cortices with an osteoporotic skeletal appearance. Exome sequencing led to the identification of a de novo heterozygous tandem duplication within RUNX2, encompassing exons 3 to 7. This duplication is larger than the ones previously reported in MDMHB cases since it extends into the C-terminal activation domain of RUNX2. We review previously reported cases with MDMHB and highlight the resemblance of this disorder with Pyle disease, which may be explained by intersecting molecular pathways between RUNX2 and sFRP4. This study expands our knowledge on the genotypic and phenotypic characteristics of MDMHB and the role of RUNX2 in rare bone disorders.

Genotype-phenotype correlation in clubfoot (talipes equinovarus).

Clubfoot (talipes equinovarus) is a congenital malformation affecting muscles, bones, connective tissue and vascular or neurological structures in limbs. It has a complex aetiology, both genetic and environmental. To date, the most important findings in clubfoot genetics involve PITX1 variants, which were linked to clubfoot phenotype in mice and humans. Additionally, copy number variations encompassing TBX4 or single nucleotide variants in HOXC11, the molecular targets of the PITX1 transcription factor, were linked to the clubfoot phenotype. In general, genes of cytoskeleton and muscle contractile apparatus, as well as components of the extracellular matrix and connective tissue, are frequently linked with clubfoot aetiology. Last but not least, an equally important element, that brings us closer to a better understanding of the clubfoot genotype/phenotype correlation, are studies on the two known animal models of clubfoot-the pma or EphA4 mice. This review will summarise the current state of knowledge of the molecular basis of this congenital malformation.

Mutations in gene regulatory elements linked to human limb malformations.

Most of the human genome has a regulatory function in gene expression. The technological progress made in recent years permitted the revision of old and discovery of new mutations outside of the protein-coding regions that do affect human limb morphology. Steadily increasing discovery rate of such mutations suggests that until now the largely neglected part of the genome rises to its well-deserved prominence. In this review, we describe the recent technological advances permitting this unprecedented advance in identifying non-coding mutations. We especially focus on the mutations in cis-regulatory elements such as enhancers, and trans-regulatory elements such as miRNA and long non-coding RNA, linked to hereditary or inborn limb defects. We also discuss the role of chromatin organisation and enhancer-promoter interactions in the aetiology of limb malformations.

Functional analysis of novel RUNX2 mutations identified in patients with cleidocranial dysplasia.

RUNX2 (Runt-related transcription factor 2) is a master regulator of osteoblast differentiation, cartilage and bone development. Pathogenic variants in RUNX2 have been linked to the Cleidocranial dysplasia (CCD), which is characterized by hypoplasia or aplasia of clavicles, delayed fontanelle closure, and dental anomalies. Here, we report 11 unrelated Polish patients with CCD caused by pathogenic alterations located in the Runt domain of RUNX2. In total, we identified eight different intragenic variants, including seven missense and one splicing mutation. Three of them are novel: c.407T>A p.(Leu136Gln), c.480C>G p.(Asn160Lys), c.659C>G p.(Thr220Arg), additional three were not functionally tested: c.391C>T p.(Arg131Cys), c.580+1G>T p.(Lys195_Arg229del), c.652A>G p.(Lys218Glu), and the remaining two: c.568C>T p.(Arg190Trp), c.673C>T p.(Arg225Trp) were previously reported and characterized. The performed transactivation and localization studies provide evidence of decreased transcriptional activity of RUNX2 due to mutations targeting the Runt domain and prove that impairment of nuclear localization signal (NLS) affects the subcellular localization of the protein. Presented data show that pathogenic variants discovered in our patients have a detrimental effect on RUNX2, triggering the CCD phenotype.