Single-nucleotide polymorphisms in intron 2 of CYP21P: evidence for a higher rate of mutation at CpG dinucleotides in the functional steroid 21-hydroxylase gene and application to segregation analysis in congenital adrenal hyperplasia.

BACKGROUND: Intron 2 of CYP21, the functional steroid 21-hydroxylase gene contains several single-nucleotide polymorphisms (SNPs). We tested the hypothesis that intron 2 of the pseudogene, CYP21P, might also be polymorphic and provide markers for segregation analysis of this region of the genome, including observable markers for segregation analysis of CYP21 gene deletions. A comparison of SNPs in both genes might provide insights into the rates of mutation in these duplicated genes. METHODS: After amplification with PCR, we examined restriction site polymorphisms in intron 2 of CYP21P in 24 members of the parental generation of the Centre d'Etude du Polymorphisme Humain families and selected offspring. RESULTS: Intron 2 of CYP21P contains frequent SNPs around nucleotide 398 and nucleotide 509, which can be typed by PCR/restriction enzyme digestion with HaeIII. Of the 48 CYP21P alleles examined, 44 could be characterized unambiguously. Of these 44 alleles, 4 were deleted, and the frequencies of restriction at the polymorphic HaeIII sites were 20 of 40 at nucleotide 398 and 30 of 40 at nucleotide 509. Both polymorphisms result from C-->T transitions that occur at CpG dinucleotides. The frequencies of C at these nucleotides in CYP21P are significantly higher than at the corresponding nucleotides in CYP21 of the same individuals (P <0.01). CONCLUSION: These data suggest that these CpG dinucleotides are more frequently mutated in CYP21 than in CYP21P, and that several mutations at CpG dinucleotides in the coding regions of CYP21 might result from CpG instability rather than the more usually proposed mechanism of gene conversion. These frequent SNPs provide useful markers for studying both allelic segregation of CYP21, particularly for chromosomes with known CYP21 deletions, and for investigating the origin of these polymorphisms.

Characterization of frequent polymorphisms in intron 2 of CYP21: application to analysis of segregation of CYP21 alleles.

The gene encoding adrenal steroid 21-hydroxylase, CYP21, is located in the MHC class III region. Most cases of congenital adrenal hyperplasia (CAH) are caused by mutations in this gene, and most mutations appear to arise from gene conversion-like events involving the transfer of deleterious sequences from the pseudogene, CYP21P, which is located within 30 kb of CYP21. Approximately 20-30% of mutations are caused by deletions of CYP21. The second intron of CYP21 is polymorphic, and several base substitutions that include nt395, nt453, and nt601 have been reported; however, the frequencies of these polymorphisms are unknown. Using a combination of cleavase fragment length polymorphism analysis and direct sequencing, we examined the sequence of intron 2 in seven wild-type CYP21 genes and determined the frequency of polymorphisms at nt395, nt453, and nt601 in 48 chromosomes from the parental generation of Centre d'Etude du Polymorphisme Humain families. The observed frequencies of bases at these positions were as follows: 395C, 0.17; 395T, 0.83; 453C, 0.71; 453T, 0.29; 601A, 0.1; and 601C, 0.9. Using a PCR/restriction digestion approach to examine these intragenic markers, we could follow the segregation of alleles in informative families with 21-hydroxylase deficiency and identify deletions of CYP21. We emphasize that this method should be used in conjunction with other molecular genetic techniques for diagnosis of CAH. In addition to their potential use in families with CAH, these markers may be of use in genetic studies of the MHC in humans.

Minnesota rickets. Need for a policy change to support vitamin D supplementation.

Vitamin D deficiency rickets, once considered the most common disease of early childhood, was reported to have disappeared by the 1960s. However, during a recent 18-month period, seven cases of nutritional rickets were diagnosed in the Twin Cities metropolitan area. All of the patients were born at term and were breastfed without supplementation vitamins. Three of the patients were Caucasian, three were African American, and one was biracial. This case series demonstrates the risk of nutritional rickets in breastfed infants in our northern climate, regardless of race. In hopes of eradicating this completely preventable disease, we advocate a uniform policy of vitamin D supplementation to breastfed infants.

Effects of growth hormone on kidney function in pediatric transplant recipients.

Recent evidence suggests that treatment with recombinant human growth hormone (rhGH) after a successful kidney transplant improves the growth rate of children with short stature. We prospectively investigated eight children (6 boys, 2 girls), focusing on acute rejection episodes and changes in serum creatinine levels during rhGH treatment. The children (mean age 11.6 +/- 3.4 years) received rhGH daily (0.04-0.05 mg/kg subcutaneously). Seven patients completed at least 12 months (20 +/- 8 months) of rhGH treatment. Their mean serum creatinine level was 1.3 +/- 0.7 mg/dl 12 months before, and increased to 3.4 +/- 4.2 mg/dl after 12 months of rhGH treatment, but did not achieve statistical significance (P = 0.06). Their mean calculated glomerular filtration rate was 58 +/- 20 ml/min per 1.73 m2 12 months before, and decreased to 38 +/- 21 ml/min per 1.73 m2 after 12 months of rhGH treatment, but did not achieve statistical significance (P = 0.08). Of the seven patients, two developed acute rejection after 5 and 6 rejection-free years; three lost their grafts and returned to dialysis. These preliminary observations describe untoward renal events in children receiving rhGH treatment after a kidney transplant.

The clitoral index: a determination of clitoral size in normal girls and in girls with abnormal sexual development.

Clitoral measurements were obtained in normal children; small increases in clitoral dimensions were associated with growth and puberty. Patients with androgen excess had major increases in clitoral size, clearly different from normal values.

Molecular and endocrine characterization of a mutation involving a recombination between the steroid 21-hydroxylase functional gene and pseudogene.

The gene encoding steroid 21-hydroxylase activity, P450c21B, is located in the major histocompatibility complex (MHC) class III region, in close proximity to a highly homologous pseudogene, P450c21A. Recombinations between P450c21B and P450c21A have been shown to result in deficiency of 21-hydroxylase activity, the usual cause of congenital adrenal hyperplasia (CAH). A mutant P450c21 gene from a patient with simple virilizing CAH was identified and shown to be consistent with a recombination between P450c21A and P450c21B. Sequence analysis of the mutant gene showed the recombination site to be located between the first exon and the second intron. The mutant gene encodes a leucine instead of the normal proline at codon 31. This mutation resides on a chromosome bearing the HLA-B44 serotype. A comparison of mutations associated with HLA-B44 and that normally found with the HLA-Bw47 serotype suggests that the HLA-B44 mutations are of more ancient origin. The patient's homologous chromosome has a deletion of P450c21B. Endocrinological testing therefore allows for testing of the mutant gene in genetic isolation. Such testing demonstrated that the patient was capable of producing aldosterone and retaining sodium in response to a low-sodium diet, indicating that the mutant gene encodes an enzyme with partial 21-hydroxylase activity.