Considering the Evaluation of Developmental Toxicity Based on Studies of the Mechanism of Occurrence of Supernumerary Ribs

Although supernumerary ribs have been examined in regulatory developmental toxicity studies, their toxicological significance remains controversial because of the unclear etiology and strain-dependent background incidence of this condition. In the current study, we established a rat model of drug-induced supernumerary ribs by administering flucytosine to pregnant Sprague–Dawley rats, a strain with low natural occurrence of this skeletal anomaly. The results revealed that flucytosine exposure induced supernumerary ribs in a dose- and timing-dependent manner, accompanied by dysregulated expression of specific Hox genes. Altered expression was correlated with axial patterning defects and, in some cases, limb abnormalities. Comparative genetic analysis between Sprague–Dawley rats and BrlHan:WIST@GALAS rats, which exhibit high spontaneous incidence (~50%) of supernumerary ribs, revealed sequence variation in the upstream regulatory region of Hoxa9, particularly affecting miR-196b. These findings demonstrate that temporal dysregulation of Hox gene networks and micro RNA (miRNA)-mediated modulation are key factors in the pathogenesis of supernumerary ribs. Moreover, our results suggest that genetic background should be considered when interpreting skeletal variations in developmental toxicity studies. Mechanistic understanding of these findings may improve the relevance of endpoints in risk assessment and regulatory decision-making.