Feline precision medicine using whole-exome sequencing identifies a novel frameshift mutation for vitamin D-dependent rickets type 2

ObjectivesA 14-week-old female domestic longhair kitten presented with shifting lameness and disproportionately smaller size compared with a co-housed littermate.MethodsHematology and serum biochemical testing were conducted to investigate causes for delayed growth, and radiographs of the appendicular skeleton were obtained.ResultsThe afflicted kitten had marked hypocalcemia, mild hypophosphatemia and substantial elevations in alkaline phosphatase activity, as well as pathognomonic radiographic findings consistent with rickets. Skeletal changes and hypocalcemia prompted testing of concentrations of parathyroid hormone (PTH) and vitamin D metabolites. Endocrine testing demonstrated significant increases in serum concentrations of PTH and 1,25-dihydroxycholecalciferol (calcitriol), supporting a diagnosis of vitamin D-dependent rickets type 2. Provision of analgesia, supraphysiologic doses of calcitriol and calcium carbonate supplementation achieved normalization of the serum calcium concentration and restoration of normal growth, although some skeletal abnormalities persisted. Once skeletally mature, ongoing calcitriol supplementation was not required. Whole-exome sequencing (WES) was conducted to identify the underlying DNA variant. A cytosine deletion at cat chromosome position B4:76777621 in VDR (ENSFCAT00000029466:c.106delC) was identified and predicted to cause a stop codon in exon 2 (p.Arg36Glufs*18), disrupting >90% of the receptor. The variant was unique and homozygous in this patient and absent in the sibling and approximately 400 other cats for which whole-genome and whole-exome data were available.Conclusions and relevanceA unique, heritable form of rickets was diagnosed in a domestic longhair cat. WES identified a novel frameshift mutation affecting the gene coding for the vitamin D3 receptor, determining the likely causal genetic variant. Precision medicine techniques, including whole-exome and whole-genome sequencing, can be a standard of care in cats to identify disease etiologies, and to target therapeutics and personalize treatment.

【研究目标】一只14周龄的雌性家养长毛猫，表现为游走性跛行，且与同窝幼崽相比体型显著偏小。
【研究方法】通过血常规检测与血清生化检验探究其生长迟缓的病因，并对其附肢骨骼进行X线摄影检查。
【研究结果】患病幼猫存在显著低钙血症、轻度低磷血症，以及碱性磷酸酶活性大幅升高，同时出现符合佝偻病特征的确诊性X线影像学表现。骨骼病变与低钙血症提示需检测甲状旁腺激素（parathyroid hormone, PTH）与维生素D代谢物的浓度。内分泌检测显示血清甲状旁腺激素与1,25-二羟胆钙化醇（calcitriol，骨化三醇）浓度显著升高，据此确诊为2型维生素D依赖性佝偻病。通过镇痛治疗、超生理剂量的骨化三醇以及碳酸钙补充治疗，患病幼猫的血清钙浓度恢复正常，生长情况得以改善，但仍残留部分骨骼异常。待骨骼发育成熟后，无需继续补充骨化三醇。本研究通过全外显子测序（whole-exome sequencing, WES）排查潜在的致病DNA变异。在VDR（vitamin D receptor）基因的猫染色体B4位点76777621处检出一处胞嘧啶缺失（ENSFCAT00000029466:c.106delC），该变异被预测会在第2外显子中引入终止密码子（p.Arg36Glufs*18），导致受体90%以上的结构受损。该变异在该病例中为纯合型且为独特变异，在其同窝幼崽以及其余约400只已获得全基因组与全外显子测序数据的猫中均未检出。
【结论与临床意义】本研究确诊了一例独特的遗传性佝偻病病例，患畜为家养长毛猫。全外显子测序鉴定出了一处影响维生素D3受体（vitamin D3 receptor）编码基因的新型移码突变，确定了该疾病的潜在致病遗传变异。包括全外显子测序与全基因组测序在内的精准医学技术，可作为猫类疾病诊疗的常规标准手段，用于明确疾病病因、制定靶向治疗方案并实现个体化治疗。