Role of Gln 85 of human CYP27A1 in 25-hydroxyvitamin D(3)-binding and protein folding

The serum vitamin D metabolome: What we know and what is still to discover

Vitamin D, referring to the two forms, D2 from the diet and D3 primarily derived from phototransformation in the skin, is a prohormone important in human health. The most hormonally active form, 1α,25-dihydroxyvitamin D (1α,25(OH)₂D), formed from vitamin D via 25-hydroxyvitamin D (25(OH)D), is not only important for regulating calcium metabolism, but has many pleiotropic effects including regulation of the immune system and has anti-cancer properties. The major circulating form of vitamin D is 25(OH)D and both D2 and D3 forms are routinely measured by LC/MS/MS to assess vitamin D status, due to their relatively long half-lives and much higher concentrations compared to 1α,25(OH)₂D. Inactivation of both 25(OH)D and 1α,25(OH)₂D is catalyzed by CYP24A1 and 25-hydroxyvitamin D3 3-epimerase. Initial products from these enzymes acting on 25(OH)D3 are 24R,25(OH)₂D3 and 3-epi-25(OH)D3, respectively, and both of these can also be measured routinely in some clinical laboratories to further document vitamin D status. With advances in LC/MS/MS and its increased availability, and with the help of studies with recombinant vitamin D-metabolizing enzymes, many other vitamin D metabolites have now been detected and in some cases quantitated, in human serum. CYP11A1 which catalyzes the first step in steroidogenesis, has been found to also act on vitamins D3 and D2 hydroxylating both at C20, but with some secondary metabolites produced by subsequent hydroxylations at other positions on the side chain. The major vitamin D3 metabolite, 20S-hydroxyvitamin D3 (20S(OH)D3), shows biological activity, often similar to 1α,25(OH)₂D3 but without calcemic effects. Using standards produced enzymatically by purified CYP11A1 and characterized by NMR, many of these new metabolites have been detected in human serum, with semi-quantitative measurement of 20S(OH)D3 indicating it is present at comparable concentrations to 24R,25(OH)₂D3 and 3-epi-25(OH)D3. Recently, vitamin D-related hydroxylumisterols derived from lumisterol3, a previtamin D3 photoproduct, have also been measured in human serum and displayed biological activity in initial in vitro studies. With the current extensive knowledge on the reactions and pathways of metabolism of vitamin D, especially those catalyzed by CYP24A1, CYP27A1, CYP27B1, CYP3A4 and CYP11A1, it is likely that many other of the resulting hydroxyvitamin D metabolites will be measured in human serum in the future, some contributing to a more detailed understanding of vitamin D status in health and disease.

Late-onset spinal form xanthomatosis without brain lesion: a case report

BACKGROUND

Cerebrotendinous xanthomatosis (CTX) is a rare autosomal recessive sterol storage disease caused by a mutated sterol 27-hydroxylase (CYP27A1) gene. Patients with typical CTX show neurological dysfunction including bilateral cataracts, paresis, cerebral ataxia, dementia, and psychiatric disorders, and magnetic resonance imaging (MRI) has revealed symmetrical lesions in the cerebellar white matter.

CASE PRESENTATION

We report the case of a patient with late-onset spinal form CTX without brain lesion. He showed pyramidal tract signs, and impaired joint position and vibration sensation in the lower limbs. Cervical sagittal MRI demonstrated a longitudinally extensive white matter abnormality in the dorsal column of the C2-C7 spinal cord; however, a brain MRI revealed an absence of lesions, including in the cerebellar white matter. Genetic analysis of CYP27A1 revealed that the patient was compound heterozygous for p.Gln85Arg in exon 1, a novel mutation, and p.Arg405Gln in exon 7, a previously reported mutation.

CONCLUSION

This is the first report of late-onset spinal form CTX without typical neurological symptoms, and the first report of p.Gln85Arg in CYP27A1. We speculate that spinal form CTX without brain lesion is a clinically and radiologically rare variation of CTX. Therefore, spinal xanthomatosis should be included in the differential diagnosis of chronic myelopathy even with late-onset and/or no other typical neurological findings.

Late-onset spinal form xanthomatosis without brain lesion: a case report

Background

Cerebrotendinous xanthomatosis (CTX) is a rare autosomal recessive sterol storage disease caused by a mutated sterol 27-hydroxylase (CYP27A1) gene. Patients with typical CTX show neurological dysfunction including bilateral cataracts, paresis, cerebral ataxia, dementia, and psychiatric disorders, and magnetic resonance imaging (MRI) has revealed symmetrical lesions in the cerebellar white matter.

Case presentation

We report the case of a patient with late-onset spinal form CTX without brain lesion. He showed pyramidal tract signs, and impaired joint position and vibration sensation in the lower limbs. Cervical sagittal MRI demonstrated a longitudinally extensive white matter abnormality in the dorsal column of the C2-C7 spinal cord; however, a brain MRI revealed an absence of lesions, including in the cerebellar white matter. Genetic analysis of CYP27A1 revealed that the patient was compound heterozygous for p.Gln85Arg in exon 1, a novel mutation, and p.Arg405Gln in exon 7, a previously reported mutation.

Conclusion

This is the first report of late-onset spinal form CTX without typical neurological symptoms, and the first report of p.Gln85Arg in CYP27A1. We speculate that spinal form CTX without brain lesion is a clinically and radiologically rare variation of CTX. Therefore, spinal xanthomatosis should be included in the differential diagnosis of chronic myelopathy even with late-onset and/or no other typical neurological findings.

Cytochromes P450 are essential players in the vitamin D signaling system

From earliest development on, the vitamin D receptor (VDR) is expressed in most cells of the mammalian body. The VDR is a nuclear, ligand-induced transcription factor that regulates in complex with hormonally active vitamin D the expression of more than 900 genes involved in a wide array of physiological functions (e.g. calcium homeostasis, growth control, differentiation, cognition, immune response, etc.). Accordingly, severe health problems are associated to vitamin deficiencies. Synthesis of the major active form 1α,25(OH)₂D₃ from vitamin D and subsequent metabolism are exclusively controlled by specific P450-forms. Synthesis, a two-step process, starts with a 25-hydroxylation primarily by CYP2R1 (CYP27A1, CYP2J2, and CYP3A4 may also contribute) and a subsequent 1α-hydroxylation via CYP27B1. Circulating in the bloodstream, 1α,25(OH)₂D₃ acts at sites of VDR expression (target sites) in an endocrine way. However, it is also capable of autocrine/paracrine functions since various target tissues are fully competent in 1α,25(OH)₂D₃ synthesis, as illustrated by three examples. 1α,25(OH)₂D₃ levels are short-lived: the hormone upregulates its rapid metabolism by CYP24A1 that attacks repeatedly the vitamin D C₂₀₋₂₇ side chain, thereby producing a complex cascade of transient metabolites with increasing polarity. Most of these metabolites still retain 1α,25(OH)₂D₃-like activities on the VDR, contributing to the overall effect that is commonly attributed to 1α,25(OH)₂D₃. As selective inhibitors of CYP24A1 increase the lifetime and thereby the function of vitamin D metabolites, they will help exploring whether and which intrinsic activities distinct metabolites possess. It appears likely that this strategy may unmask important regulators of new functions.