# Comprehensive Genetic Exploration of Fused Teeth by Whole Exome Sequencing
## 1 Introduction  
Structurally, the tooth consists of a crown above and in the upper part of the gums, and root(s) in the alveolar bone; it is composed of enamel, dentin, and cementum [1]. Dentin has a dental pulp within it, which contains nerves and blood vessels. Anomalous tooth shapes can occur [1, 2], such as fused teeth, defined as the union of two or more teeth involving their enamel, dentin, and cementum [3]. Fused teeth usually appear as large crowns which contain a groove between the two originating teeth and cause malocclusion [1, 2, 3]. Caries, periodontal issues, and aesthetic problems can, therefore, result and may require dental treatment [4]. The most frequent fused area involves the mandibular incis, followed by the maxillary incis, and, rarely, the molar area [1, 5, 6]. The incidence of fused teeth has been reported to be 0.5-2.5% in primary dentition and 0.1% in permanent dentition [7, 8, 9]. Similarly, a meta-analysis has shown that the frequency of bilateral fused teeth is 0.02% for both primary and permanent dentition [10]. No sex difference has been observed [11]. A higher incidence has been observed in Asians and Native Americans [2]. The aetiology of fused teeth remains unclear. Currently, it is considered that direct contact between the teeth caused by physical force or pressure during tooth development causes fusion [1, 2, 3]. Genetically, the development of fused teeth has been suggested to be autosomal dominant with reduced penetrance [2]. A report of multiple fused teeth cases in a five-member family reported two members with fused teeth in the primary dentition [12]. In addition, hypodontia of the permanent teeth and fused teeth in the primary dentition ofa mother as well as fused teeth in the primary dentition of her 5-year-old daughter have been reported [13]. In relation to the possible genetic origin of tooth developmental disorders, dental agenesis (absence of teeth) has been reported to be related to several gene variants. [PERSON] et al. [14] performed a meta-analysis of 101 articles covering 522 patients to determine the genotype-phenotype correlations between dental agenesis and the genes msh homeobox \(1\) (_MSX1_), paired box 9 (_PAX9_), axin 2 (_AX1N2_), paired like homeodomain 2 (_PITX2_), wnt family member 10A (_WNT10A_), nemo (_NMO_), ectodysplasin A (_EDA_), ectodysplasin A receptor (_EDAR_), EDAR associated death domain (_EDARADD_), and gremlin 2, DAN family BMP antagonist (_GREM2_). [PERSON] et al. [15] analysed whole exome sequence (WES) data from four Japanese families and found that supernumerary teeth were associated with variants such as SCO-spondin (_SSPO_). [PERSON] et al. [16] discovered an association between shovel-shaped incisors and _EDAR_ in 100 Japanese subjects from Tokyo and 100 subjects from the Sakashima Islands using the polymerase chain reaction (PCR) direct sequencing method. [PERSON] et al. [17] investigated tautorodntism, microdontia, and dens invaginatus using WES for first-family, second-family, and linkage analyses and reported that these were related to kinesin family member 4A (_KIF4A_). However, no research has been conducted on gene variants causing fused teeth. Investigating the genetic causes of tooth developmental disorders and dental agenesis could be potentially useful in understanding the mechanism of fused teeth formation. Given that similar conditions are gene-related, we hypothesised that fused teeth develop at least partially due to genetic factors. Further, we hypothesised that fused teeth follow mendelian inheritance based on reports regarding the familial expression of fused teeth. Many genetic diseases are found in the exome region. Hence, searching for single nucleotide variants (SNVs) and small insertion/deletion (indel) in the exome region is an effective method for investigating the genetic basis of a disease [18]. SNVs cause amino acid substitution, and amino acid substitution can cause pathogenic mutations.