BMP4 mutations in tooth agenesis and low bone mass

Abstract

Objective: To identify an uncommon genetic cause of tooth agenesis (TA) by utilizing whole exome sequencing (WES) and targeted Sanger sequencing in a cohort of 120 patients with isolated TA.

Design: One deleterious mutation in the gene encoding bone morphogenetic protein 4 (BMP4) was identified in 6 unrelated patients with TA by WES. After that, the coding exons of BMP4 were examined in 114 TA patients using Sanger sequencing. Dual-energy X-ray absorptiometry (DEXA) was used to measure the bone mineral density of patients who carried a BMP4 mutation. Finally, preliminary functional studies of two BMP4 mutants were performed.

Results: We detected 3 novel missense mutations (c.58 G > A: p.Gly20Ser, c.326 G > T: p.Arg109Leu and c.614 T > C: p.Val205Ala) and 1 reported mutation in the BMP4 gene among 120 TA probands. The previously reported BMP4 mutation (c.751C > T: p.His251Tyr) was associated with urethra and eye anomalies. By extending the pedigrees, we determined that the tooth phenotypes had an autosomal dominant inheritance pattern, as individuals carrying a BMP4 mutation exhibit different types of dental anomalies. Interestingly, we observed that patients harboring a BMP4 mutation manifested early onset osteopenia or osteoporosis. Further in vitro functional assays demonstrated that two BMP4 mutants resulted in a decreased activation of Smad signaling. Therefore, a loss-of-function in BMP4 may contribute to the clinical phenotypes seen in this study.

Conclusions: We identified 4 mutations in the BMP4 gene in 120 TA patients. To our knowledge, this is the first study to describe human skeletal diseases associated with BMP4 mutations.

Keywords: BMP4; Dental anomalies; Osteopenia; Osteoporosis; Tooth agenesis.

Figure 1. Dental characteristics of patients with different BMP4 mutations.

(A) Panoramic radiograph and schematic of congenitally missing teeth of #217 proband. (B) Panoramic radiograph and schematic of congenitally missing teeth of #290 proband. (C) Panoramic radiograph and schematic of congenitally missing teeth of #69 proband. Asterisks and solid squares indicate the congenitally missing teeth. R, right; L, left; MAX, maxillary; MAND, mandibular.

Figure 2. Pedigrees of patients with tooth agenesis and mutational analysis of human BMP4 gene.

(A-C) Available DNA sequencing chromatograms showing three novel heterozygous mutations of c.614T > C, c.58G > A and c.326G > T in #217 proband, #290 proband and #551 proband, respectively. (D) Available DNA sequencing chromatograms showing a heterozygous mutation of c.751C > T in #69 proband and #423 proband. Black arrows indicate the proband on each family. Red arrows indicate point mutations. Black squares represent TA patients. Grey squares represent individuals with tooth anomalies. “NA” indicates that a DNA sample was not available.

Figure 3. Dual-energy X-ray absorptiometry (DEXA) examination of lumbar and hip.

(A) DEXA scan images of #217 proband carrying a BMP4 mutation, c.614T > C: p.Val205Ala, at the age of 15. (B) DEXA scan images of the father of #217 proband carrying the same BMP4 mutation with #217 proband, at the age of 48. (C) DEXA scan images of #423 proband carrying a BMP4 mutation, c.751C > T: p.His251Tyr, at the age of 20.

Figure 4. Distribution and conservation analysis of tooth agenesis associated BMP4 mutations.

(A) Schematic diagram of BMP4 gene and protein structure showing all the BMP4 mutations identified in patients with TA. Novel mutations are marked in red, and reported mutation are in black. (B) Analysis of sequence conservation of involved BMP4 amino acids among different species.