Pseudo vitamin deficiency rickets

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					Pseudo-vitamin D                            deficiency rickets
F H Glorieux
Genetics Unit, Shriners  Hospital and Departments of Surgery, Pediatrics and Human Genetics, McGill University, Montr\l=e'\al, Qu\l=e'\bec, Canada H3G   1A6
(Requests for offprints should be addressed to F H Glorieux)

In 1961, Prader et al. (1961) reported on a new form of                             protein which belongs to the steroid-thyroid-retinoic acid
vitamin D resistant rickets, which differed from the classic                        receptor superfamily of genes (Evans 1988). It is comprised
hypophosphatemic type (Albright et al. 1937) by its early                           of two functional domains, a steroid hormone binding
onset (within the first year of life), the development of                           domain and a DNA-binding domain. Mutations affecting
severe hypocalcemia with tetany, moderate hypophos-                                 both have been found in VDD2 families. In other in¬
phatemia and hyperaminoaciduria both reflecting second-                             stances there appears to be a VDR intranuclear retention
ary hyperparathyroidism, enamel hypoplasia and the                                  defect (Barsony et al. 1990).
complete correction of all clinical and biochemical evi-
dence of rickets (including stunted growth rate) with high                          Serum vitamin D metabolite levels
daily doses of vitamin D. In the initial report identifying
this hereditary form of pseudo-vitamin D deficiency                                 In most cases, serum calcitriol values are very low in
rickets (PDDR), transmission was reported to be auto-                               untreated PDDR patients (Delvin et al. 1981). Even when
somal dominant. Later, recessive inheritance was estab-                             treated with large doses of vitamin D causing major
lished (Fanconi & Prader 1969). In view of the fact that to                         increases in the circulating levels of calcidiol, calcitriol
maintain health, intake of vitamin D had to be consistently                         values do not reach the normal range (Scriver et al. 1978,
in vast excess of the recommended daily allowance                                   Delvin et al. 1981). Right after birth PDDR infants have
(400 IU), the term 'vitamin D dependency' (VDD)                                     very low calcitriol levels, months before any clinical
was proposed to describe the new syndrome (Scriver                                  evidence of rickets develops. Some investigators have
1970).                                                                              reservation at making low calcitriol levels an absolute
   In 1973,    through the availability of la,25(OH)2D3 as a                        criterion. Indeed, Balsan (1991) observed PDDR patients
therapeutic   agent, came the demonstration that PDDR                               with normal calcitriol levels. Those values, however,
was an  inborn error of vitamin D metabolism involving the                          should be considered as inappropriate in the face of rickets,
defective conversion of calcidiol (25(OH)D) to calcitriol                           hypocalcemia and secondary hyperparathyroidism. It may
(la25(OH)2D) (Fraser et al. 1973, and the biosynthetic                              well be that these differences reflect a degree of genetic
pathway shown in Fig. 1). Several groups subsequently                               heterogeneity among PDDR pedigrees, that will only be
showed that small (physiologic) daily amounts of calcitriol                         resolved at the molecular level.
or its analog, la(OH)D3, allowed excellent and prolonged                               Infant death by hypocalcemia or pulmonary infections
control of the abnormal phenotype (Balsan et al. 1975,                              was not infrequent in the past when the diagnosis was
Reade et al. 1975, Delvm et al. 1981). Thus the basic                               either missed (confused for a neurologic or respiratory
PDDR abnormality appears to be an altered activity of                               disease) or made too late. Treatment with large doses of
the renal la-hydroxylase (la-OHase) enzyme, although                                vitamin D remains the only solution in countries where
the exact nature of the mutation remains to be identified.                          calcitriol or its analogue are not available. However, the
As is often the case in autosomal recessive disorders,                              therapeutic doses are close to the toxic ones and renal
hétérozygotes for the trait cannot be characterized either                          damage remains a threat. Furthermore, adjustments made
clinically or biochemically, but only by analysis of their                          necessary by immobilization or pregnancy are much more
progeny.                                                                            difficult to monitor because of the large accumulated stores
   In 1978, another inborn error of vitamin D metabolism                            of vitamin D (and calcidiol) that may take months to be
was isolated in which a clinical picture of pseudo-vitamin                          reduced.
D deficiency developed despite high circulating concen¬                                The treatment of choice is replacement therapy with
trations of endogenously produced calcitriol (Marx et al.                           calcitriol. If rickets is florid, the initial dose may be
1978). In some pedigrees, the phenotype was com¬                                    between 2 and 3 µg/day until bone is healed, thereafter
pounded with complete alopecia (Rosen et al. 1979). This                            the maintenance dose will vary between 0-25 and 1 µg/
second variety of pseudo-deficiency, also referred to as                            day and remain remarkably stable throughout growth and
'vitamin D dependency type        (VDD2) is caused by a                             adulthood (our oldest patient is a 50-year-old woman,
spectrum  of mutations affecting the vitamin D receptor                             maintained for the last 15 years on a daily dose of
(VDR) in target tissues. The human VDR is a 50 kDa                                  0 25µg/day).
             Figure 1 Vitamin D biosynthetic pathway. The precursot of vitamin D3, 7-dehydrocholesterol, is present in
             the skin. Upon exposure to sunlight it is converted to pre-vitamin D3 which will thermally ¡sometize to
             cholecalciferol (vitamin D3). The compound may be stored in fat and muscle tissues from which it is
             released to reach the liver where it is converted by a hepatic microsomal hydroxylase to                         Q\>
             25-hydtoxycholecalciferol (25(OH)D3). The latter is biologically inert and requires a further hydroxylation in
             the kidney to 1,25-dihydroxyvitamin D3 (1a,25(OH)2D3), the hormonal form of the vitamin. Its synthesis is
             triggered by a decrease in serum calcium which induces the release of parathyroid hormone. Parathyroid
             hormone travels to the kidney and increases tubulat reabsorption of calcium and the excretion of
             phosphate. In addition, possibly through a lowering of intracellular phosphate concentration, this peptide
             hormone increases the conversion of 25-hydroxy- into 1,25-dihydroxyvitamin D. The alternate kidney
             conversion (25-hydroxy- to 24,25-dihydroxyvitamin D) is currently thought to be a detoxification route, no
             specific physiologic role having yet been assigned to 24,25-dihydroxyvitamin D (24,25(OH)2D3).

Placenta studies                                                       their collaboration and the one of their relatives set out to
                                                                       map the PDDR locus by using DNA markers and linkage
While in non-pregnant subjects lct-OHase activity is                   analysis to approach the primary defect in PDDR. We
mainly located in kidney, we have demonstrated that this               found that the gene responsible for the disease was linked
enzyme was also present in cells isolated from human                   to polymorphic restriction fragment length polymorphism
decidua (Delvin et al. 1985). Interestingly, when studying             (RFLP) markers in region   14 of the long arm of chromo¬
decidual tissues from two PDDR patients, we found                      some 12 (12ql4). Multipoint linkage analysis and studies of
that it did not have the capacity to produce calcitriol,               haplotypes and recombinants strongly suggest the localiza¬
indicating that decidua and kidney are both targets for                tion of the PDDR locus between the collagen type II alpha
the PDDR mutation (Glorieux et al. 1995). Although the                 1    (COL2A1) locus and        a   cluster of three anonymous
physiologic importance of this defect is unclear, the                  probes (D12S14, D12S17 and D12S6) which segregate as
finding provides us with a unique, albeit infrequent,                  a three-marker haplotype. Linkage disequilibrium be¬
source of mutant cells.                                                tween PDDR and this three-marker haplotype supports
                                                                       the notion of a founder effect in the studied population.
                                                                       Furthermore, this localization of PDDR between flanking
Linkage analysis                                                       markers allows for the prediction of the carrier status in
                                                                       individuals at risk in the analyzed families as well as early
Although quite rare, PDDR is present with unusual                      diagnosis in newborns. The accuracy of carrier detection
frequency in a subset of the French Canadian population.               could exceed 99% in families informative for the markers
Over the years we have followed 36 patients and with                   used (Labuda et al. 1990).
Mapping     PDDR and VDR                                            different from that of PDDR, previously assigned to 12ql4
                                                                    (Labuda et al. 1990), thus excluding it as a target of the
Recently     the VDR gene has also been assigned to                 PDDR mutation.
chromosome 12 by Southern blot analysis of a panel of                  To understand the basis of the PDDR genotype it is
human—Chinese hamster cell hybrid DNAs. VDR cDNA                    necessary to characterize an appropriate cDNA probe
was also shown to reveal Apal polymorphism, allowing its            corresponding to la-OHase activity. Preliminary results
use as an RFLP marker in linkage analysis. By in situ               towards that goal have been recently obtained in this Unit,
hybridization VDR was found to map in the 12ql2-14                  by St-Arnaud et al. (1996). Using a probe centered on the
region. Simultaneously, by using samples of 21 from our             heme binding domain of the rat 24R-OHase cDNA we
PDDR families, we established that the VDR and PDDR                 have screened a cDNA library from kidneys of vitamin
loci were mapping in close proximity on 12ql2—14. It is             D-deficient rats under reduced stringency conditions to
likely that the genetic distance between VDR and PDDR               identify and clone the la-OHase cDNA. A 2-4 kb full-
is in the range of a few centiMorgans which in physical             length clone was identified which codes for a protein of
terms can correspond to 1-10 megabases. We find, at                 predicted molecular mass of 55 kDa. Amino acid sequence
present, no specific reason for this proximity, but its             similarity with the 24R-OHase enzyme was 78% within
functional significance might be established in the future          the heme binding domain, but clear divergence was seen
(Labuda et al. 1992).                                               outside this region for an overall sequence similarity of
                                                                    26%. Transfection of embryonal carcinoma cells with the
                                                                    sense la-OHase expression vector produced a vitamin
Molecular defect in PDDR                                            D metabolite that co-migrates on HPLC with the
                                                                     la25(OH)2D3 standard. The compound was also authen¬
Although   it is clear that PDDR is due to a deficiency of          ticated using two different radioreceptor assays. This rat
the renal mitochondrial la-OHase, leading to insufficient            la-OHase cDNA probe will allow cloning of its human
synthesis of calcitriol (Fraser et al. 1973), the exact target of   homolog, which will then be mapped to its genomic
the mutation is not known. It could affect either modula¬           location and the hypothesis that it will co-localize with the
tors of the enzyme activity or the structure of the enzyme          PDDR locus tested. If it did not, then the PDDR
itself. The la-OHase is made of three distinct moieties:            mutation is probably not affecting the integrity of
cytochrome P450, ferredoxin and ferredoxin reductase                 la-OHase but rather a controlling factor of its activity.
(reviewed in Chazarían 1990). A priori, each of these could             Finally, heterogeneity (in the sense of different loci
be affected by the mutation. However, it has recently been          being involved) cannot be excluded in PDDR. We are
shown that the two components of the electron-transfer              currently gathering specimens from PDDR pedigrees
system which are shared with other cytochrome P450s are             from other ethnic groups to ascertain whether the PDDR
encoded by single copy genes expressed in all steroido¬             locus maps there at the same genomic location as in the
genic tissues and located on chromosome 11 for the                  French Canadian families so far studied. These studies
ferrodoxin (Morel et al. 1988), and on chromosome 17 for            should shed an interesting light on the control of vitamin
the ferredoxin reductase (Solish et al. 1988). Since PDDR           D bioactivation.
patients express no evident deficiencies in the activity of
other cytochrome P450 systems, the P450 specific com¬
ponent of la-OHase remains as the most likely target for            References
the PDDR mutation.
   The fact that 24R-OHase activity is normal in PDDR
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