Vitamin D-Dependent Rickets Type 3: A Case Report and Systematic Review

When and How to Evaluate Vitamin D Status? A Viewpoint from the Belgian Bone Club

Low serum vitamin D levels have been associated with a variety of health conditions which has led the medical community but also the general population to evaluate vitamin D status quite liberally. Nevertheless, there remain questions about the efficacy and cost-effectiveness of such a broad and untargeted approach. This review therefore aims to summarize the current evidence and recommendations on when and how to evaluate vitamin D status in human health and disease. For the general population, most guidelines do not recommend universal screening but suggest a targeted approach in populations at risk. Also, some guidelines do not even recommend evaluating vitamin D status when vitamin D substitution is indicated anyway, such as in children or patients receiving anti-osteoporosis drugs. In those guidelines that recommend the screening of vitamin D status, serum 25(OH)D levels are universally proposed as the preferred screening tool. However, little attention is given to analytical considerations and almost no guidelines discuss the timing and frequency of screening. Finally, there is the known variability in diagnostic thresholds for defining vitamin D insufficiency and deficiency. Overall, the existing guidelines on the evaluation of vitamin D status differ broadly in screening strategy and screening implementation, and none of these guidelines discusses alternative screening modes, for instance, the vitamin metabolic ratio. Efforts to harmonize these different guidelines are needed to enhance their efficacy and cost-effectiveness.

The Role of Intestinal Cytochrome P450s in Vitamin D Metabolism

Vitamin D hydroxylation in the liver/kidney results in conversion to its physiologically active form of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. 1,25(OH)2D3 controls gene expression through the nuclear vitamin D receptor (VDR) mainly expressed in intestinal epithelial cells. Cytochrome P450 (CYP) 24A1 is a catabolic enzyme expressed in the kidneys. Interestingly, a recently identified mutation in another CYP enzyme, CYP3A4 (gain-of-function), caused type III vitamin D-dependent rickets. CYP3A are also expressed in the intestine, but their hydroxylation activities towards vitamin D substrates are unknown. We evaluated CYP3A or CYP24A1 activities on vitamin D action in cultured cells. In addition, we examined the expression level and regulation of CYP enzymes in intestines from mice. The expression of CYP3A or CYP24A1 significantly reduced 1,25(OH)2D3-VDRE activity. Moreover, in mice, Cyp24a1 mRNA was significantly induced by 1,25(OH)2D3 in the intestine, but a mature form (approximately 55 kDa protein) was also expressed in mitochondria and induced by 1,25(OH)2D3, and this mitochondrial enzyme appears to hydroxylate 25OHD3 to 24,25(OH)2D3. Thus, CYP3A or CYP24A1 could locally attenuate 25OHD3 or 1,25(OH)2D3 action, and we suggest the small intestine is both a vitamin D target tissue, as well as a newly recognized vitamin D-metabolizing tissue.

Genetic causes of hypophosphatemia

Phosphate is a key component of mineralized tissues and is also part of many organic compounds. Phosphorus homeostasis depends especially upon intestinal absorption, and renal excretion, which are regulated by various hormones, such as PTH, 1,25-dihydroxyvitamin D, and fibroblast growth factor 23. In this review we provide an update of several genetic disorders that affect phosphate transporters through cell membranes or the phosphate-regulating hormones, and, consequently, result in hypophosphatemia.

Vitamin D deficiency or resistance and hypophosphatemia

Vitamin D is mainly produced in the skin (cholecalciferol) by sun exposure while a fraction of it is obtained from dietary sources (ergocalciferol). Vitamin D is further processed to 25-hydroxyvitamin D and 1,25-dihydroxy vitamin D (calcitriol) in the liver and kidneys, respectively. Calcitriol is the active form which mediates the actions of vitamin D via vitamin D receptor (VDR) which is present ubiquitously. Defect at any level in this pathway leads to vitamin D deficient or resistant rickets. Nutritional vitamin D deficiency is the leading cause of rickets and osteomalacia worldwide and responds well to vitamin D supplementation. Inherited disorders of vitamin D metabolism (vitamin D-dependent rickets, VDDR) account for a small proportion of calcipenic rickets/osteomalacia. Defective 1α hydroxylation of vitamin D, 25 hydroxylation of vitamin D, and vitamin D receptor result in VDDR1A, VDDR1B and VDDR2A, respectively whereas defective binding of vitamin D to vitamin D response element due to overexpression of heterogeneous nuclear ribonucleoprotein and accelerated vitamin D metabolism cause VDDR2B and VDDR3, respectively. Impaired dietary calcium absorption and consequent calcium deficiency increases parathyroid hormone in these disorders resulting in phosphaturia and hypophosphatemia. Hypophosphatemia is a common feature of all these disorders, though not a sine-qua-non and leads to hypomineralisation of the bone and myopathy. Improvement in hypophosphatemia is one of the earliest markers of response to vitamin D supplementation in nutritional rickets/osteomalacia and the lack of such a response should prompt evaluation for inherited forms of rickets/osteomalacia.

CYP3A4 Mutation Causes Vitamin D-Dependent Rickets Type 3: A Case Report in Saudi Arabia

Rickets is a childhood disorder of vitamin D deficiency that is characterized by growth retardation and impairment in skeletal mineralization. Vitamin D deficiency is usually due to decreased dietary vitamin D intake, decreased sunlight exposure, or genetic defects. A recurrent gain-of-function missense mutation (p.I301T) in the gene encoding CYP3A4 has been identified as a cause of excessive inactivation of vitamin D metabolites that causes vitamin D-dependent rickets type 3 (VDDR3). We hereby report a case of a six-year-old girl with poor growth and bone deformities, such as genu valgum. In addition, the patient has a strong family history of short stature and bone deformities. She continues to receive multidisciplinary care, and the finding of a heterozygous missense variant in CYP3A4: c.902 T > C; p.Ile301Thr in the CYP3A4 gene confirms the diagnosis of VDDR3. To our knowledge, this is the first case to be reported in Saudi Arabia and the fourth case in the literature. Our findings highlight the importance of vitamin D in those with high activity in CYP3A4 to maintain vitamin D hemostasis, and we need to reach optimal doses to help them maintain their biochemical and radiological finding within the normal range.

Approach to Rickets: Is It Calciopenic or Phosphopenic?

Rickets is a childhood disorder of decreased mineralization of bone tissue. It is either calciopenic or phosphopenic, according to the deficient mineral. Calcium, phosphate, and vitamin D metabolism should be known to understand the pathophysiology of rickets. A deficiency of calcium or vitamin D can be caused by several conditions. These conditions lead to defective osteoid mineralization, impaired chondrocyte differentiation, and apoptosis in the growth plate, resulting in clinical and radiological findings of rickets. Rickets developing as a result of vitamin D deficiency is the most frequently encountered form. Vitamin D-dependent rickets classification is made according to genetic abnormalities of enzymes that are involved in vitamin D metabolism. Phosphopenic rickets is divided mainly into 2 categories that are FGF23 related or not. A systemic approach that includes a detailed history, physical examination, and laboratory evaluation is required when performing a diagnostic evaluation. Vitamin D and calcium supplementation should be used to treat nutritional rickets. To prevent rickets and its morbidities, vitamin D prophylaxis in the newborn period is suggested. High dose of vitamin D3, 1.25(OH)2D, and calcium are treatment choices in vitamin D-dependent rickets according to its subgroup. If conventional treatment consisting of phosphate and calcitriol is ineffective in the treatment of phosphopenic rickets, Burosumab is the new treatment option.