This study establishes a new measurement method for quantification of mouse gutter shaped root (GSR). To evaluate GSR, we made clear the completion period of root formation by continuous observation from 9 days after birth to 82 days using a micro-CT. We observed that the mouse root formation is complete at 35 days after birth. We established a schema to evaluate the Dental Root Fusion Rate (DRFR) on a CT image and we could evaluate DRFR as a quantitative trait using this image. Consequently we could establish a new method for measurement of mouse gutter shaped roots as a quantitative trait. Materials and Methods MiceWe used C57L/J mice and ICR mice to determine the mouse dental root formation completion period of the M 2 . Mice of inbred strain C57L/J were obtained from Jackson Laboratory (Bar Harbor, ME) and ICR mice and AKR/J mice were obtained from Japan Clea Co. We used C57L/J mice (n)61ס as a model mouse of GSR. ICR mice (n)5ס and AKR/J (n)7ס mice have normal root in M 2 . ICR mice are control mice for longitudinal study. AKR/J mice are control mice for measurement
Keywords: AKXL RI mice; Gutter-shaped root; QTL analysis; dental root fusion rate. IntroductionAbnormal root morphogenesis in human mandibular second or third molars occurs frequently. One of these abnormalities is known as gutter-shaped root (GSR), in which the root appears to be fused. It is well known that root canal treatment of teeth with fused roots is challenging and has a worse prognosis than that for a normal root (1). Teeth with GSR are also likely to have a poor prognosis when affected by periodontal disease. The morphological features of GSR have been reported by many investigators (2,3).It has been reported that C57L/J mice have fused roots similar to GSR on their lower second molars, and they are therefore one of the most useful animal models for studying the cause of GSR formation (4). Previous research has suggested that an autosomal-dominant inheritance pattern affects the development of mouse GSR, and that several genes may be involved (5,6). However, the major gene responsible has not yet been specified. Arita et al. recently reported a new method for measuring mouse GSR as a quantitative trait using micro-CT imaging (7). Therefore, quantitative trait locus (QTL) analysis has become feasible for determining the candidate genes responsible for GSR formation. QTL analysis has been used successfully for identifying chromosomal regions that exert quantitative effects due to poly genes, determining traits such as body weight and susceptibility to alcoholism (8,9).Recombinant inbred (RI) strains of mice are useful for the study of complex traits such as body weight (10,11). RI mouse strains are derived from systematic inbreeding of randomly selected pairs of the F2 generation of a cross between two different inbred strains. The AKXL RI strain is an existing strain derived from the mouse AKR/J and C57L/J progenitor strains, both of which have been well characterized, and show differences in a variety of phenotypes, such as cholesterol gallstone formation (12,13). AKR/J mice do not have abnormal tooth roots. Therefore, it is thought that different strains of AKXL RI mice may have a variety of root shapes in the lower second molar. In this study, we focused on identifying the chromosomal regions involved in mouse GSR formation. Here we report genetic analysis of GSR formation in AKXL RI strains using QTL analysis. Materials and Methods Experiment 1 MiceA total of 44 mice obtained from paternal strains (2 males and 2 females for each of C57L/J, AKR/J, BALB/ cAnNCrlCrlj, C57BL/6J, C3H/HeJ, and DBA/2J) and F1
C57L/J mice have about 90-100% incidence of gutter-shaped root (GSR) on the lower second molars (M2). Many investigators have reported the morphological features of this anomaly, but the development mechanisms are not well-understood. A classic genetic study on the cause of GSR in C57L/J mice suggested that a genetic factor with autosomal-dominant inheritance affected the development of mouse GSR. The purpose of this study was to identify genome regions of mouse GSR with novel method. We used N2 backcross mice (N=153) from C57L/J mice, AKR/J mice and 103 informative polymorphic Mit (Massachusetts Institute of Technology) markers in this study. To evaluate phenotype, we calculate Dental root fusion rate (D.R.F.R.) using micro-CT. Here, quantitative trait locus (QTL) analysis was used to detect the chromosomal regions responsible for the mouse GSR in C57L/J mice. Highly significant value was obtained in the approximately 5.5 cM area from D6Mit284 (30.6 cM) to D6Mit230 (36.1 cM) on chromosome 6, strongly suggesting the existence of a major gene associated with the components of gutter-shaped root. Many investigators have reported the morphological features of this anomaly 3) , but its cause is not known. It has been reported that C57L/J mice are one of the most useful models for studying its cause; the incidence rate of GSR on the lower second molar tooth is known to be about 90-100%. A classic genetic study on the cause of GSR in C57L/J mice was reported to suggest that a genetic factor with autosomal-dominant inheritance affected the development of mouse GSR, and that a few genes were involved 4,5). Although a responsible gene has been searched by linkage analysis using microsatellite markers in several genetic studies, gene locus (the root fusioninduced gene in the lower second molar of mouse, allele symbol is TRFIN) is not identified yet. The
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