Distal renal tubular acidosis, autoimmune thyroiditis, enamel hypomaturation, and tooth agenesis caused by homozygosity of a novel double-nucleotide substitution in SLC4A4

LRP4 mutations, dental anomalies, and oral exostoses

BACKGROUND

In order to generate a normal set of teeth, fine-tuning of Wnt/β-catenin signaling is required, in which WNT ligands bind to their inhibitors or WNT inhibitors bind to their co-receptors. Lrp4 regulates the number of teeth and their morphology by modulating Wnt/β-catenin signaling as a Wnt/β-catenin activator or inhibitor, depending on its interactions with the partner proteins, such as Sostdc1 and Dkk1.

AIM

To investigate genetic etiologies of dental anomalies involving LRP4 in a Thai cohort of 250 children and adults with dental anomalies.

DESIGN

Oral and radiographic examinations and whole exome sequencing were performed for every patient.

RESULTS

Two novel (p.Leu1356Arg and p.Ala1702Gly) and three recurrent (p.Arg263His, p.Gly1314Ser, and p.Asn1385Ser) rare variants in low-density lipoprotein receptor-related protein 4 (LRP4: MIM 604270) were identified in 11 patients. Oral exostoses were observed in five patients.

CONCLUSION

Antagonism of Bmp signaling by Sostdc1 requires the presence of Lrp4. Mice lacking Lrp4 have been demonstrated to have alteration of Wnt-Bmp-Shh signaling and an abnormal number of incisors. Therefore, the LRP4 mutations found in our patients may disrupt Wnt-Bmp-Shh signaling, thereby resulting in dental anomalies and oral exostoses. Root maldevelopment in the patients suggests an important role of LRP4 in root morphogenesis.

Review of childhood genetic nephrolithiasis and nephrocalcinosis

Nephrolithiasis (NL) is a common condition worldwide. The incidence of NL and nephrocalcinosis (NC) has been increasing, along with their associated morbidity and economic burden. The etiology of NL and NC is multifactorial and includes both environmental components and genetic components, with multiple studies showing high heritability. Causative gene variants have been detected in up to 32% of children with NL and NC. Children with NL and NC are genotypically heterogenous, but often phenotypically relatively homogenous, and there are subsequently little data on the predictors of genetic childhood NL and NC. Most genetic diseases associated with NL and NC are secondary to hypercalciuria, including those secondary to hypercalcemia, renal phosphate wasting, renal magnesium wasting, distal renal tubular acidosis (RTA), proximal tubulopathies, mixed or variable tubulopathies, Bartter syndrome, hyperaldosteronism and pseudohyperaldosteronism, and hyperparathyroidism and hypoparathyroidism. The remaining minority of genetic diseases associated with NL and NC are secondary to hyperoxaluria, cystinuria, hyperuricosuria, xanthinuria, other metabolic disorders, and multifactorial etiologies. Genome-wide association studies (GWAS) in adults have identified multiple polygenic traits associated with NL and NC, often involving genes that are involved in calcium, phosphorus, magnesium, and vitamin D homeostasis. Compared to adults, there is a relative paucity of studies in children with NL and NC. This review aims to focus on the genetic component of NL and NC in children.

Mutations in LRP5 and BMP4 are associated with mesiodens, tooth agenesis, root malformation, and oral exostoses

WNT/β-catenin and BMP signaling pathways play important roles in the process of tooth development. Dysregulation of WNT/β-catenin and BMP signaling is implicated in a number of human malformations, including dental anomalies. Whole exome and Sanger sequencing identified seven patients with LRP5 mutations (p.Asn1121Asp, p.Asp856Asn, p.Val1433Met, and p.Val1245Met) and six patients with BMP4 mutations (p.Asn150Lys, p.Gly168Arg, p.Arg269Gln, and p.Ala42Glu). All patients were affected with isolated dental anomalies (dental anomalies with no other structural defects), including mesiodens, tooth agenesis, unseparated roots, narrow roots, shortened and tapered roots, and taurodontism. Five and a patients with LRP5 and BMP4 mutations had oral exostoses, respectively. Protein models of LRP5 mutations indicate the possible functional effects of the mutations. Here we report for the first time that mutations in LRP5 are associated with dental anomalies. LRP5 appears to be the first gene related to pathogenesis of mesiodens. We also show for the first time that in addition to tooth agenesis, mutations in BMP4 are also implicated in root maldevelopment and torus mandibularis. Sharing the phenotypes of the patients with LRP5 and BMP4 mutations, which include root maldevelopment, tooth agenesis, and torus mandibularis, implicates cross-talks between WNT/β-catenin and BMP signaling pathways, especially during root development.