High frequency of nonclassical steroid 21-hydroxylase deficiency.

Nonclassical steroid 21-hydroxylase deficiency is an autosomal recessive disorder that is defined by clinical and hormonal criteria that distinguishes it from the classical 21-hydroxylase deficiency. No estimates of the gene frequency of nonclassical 21-hydroxylase deficiency, also called attenuated, late-onset, acquired, and cryptic adrenal hyperplasia, have been published thus far. Here, we have used HLA-B genotype data in families containing multiple members affected with nonclassical 21-hydroxylase deficiency together with the results of quantitative hormonal tests to arrive at estimates of gene and disease frequencies for this disorder. We found nonclassical 21-hydroxylase deficiency to be a far more common disorder than classical 21-hydroxylase deficiency, which occurs in 1/8,000 births. The prevalence of the disease in Ashkenazi Jews was 3.7%; in Hispanics, 1.9%; in Yugoslavs, 1.6%; in Italians, 0.3%; and in the diverse Caucasian population, 0.1%. The gene for nonclassical 21-hydroxylase deficiency is in genetic linkage disequilibrium with HLA-B14 in Ashkenazi Jews, Hispanics, and Italians, but not in Yugoslavs or in a diverse, non-Jewish, Caucasian group. The penetrance of nonclassical 21-hydroxylase deficiency gene in the HLA-B14 containing haplotypes was incomplete. Thus, nonclassical 21-hydroxylase deficiency is probably the most frequent autosomal recessive genetic disorder in man and is especially frequent in Ashkenazi Jews, Hispanics, Italians, and Yugoslavs.     

Genetic mapping of the 21-hydroxylase locus: estimation of small recombination frequencies

The locus for 21-hydroxylase (CA21HB) has been mapped to the interval between the HLA-B and HLA-DR loci on chromosome 6. Several methods of estimating genetic distance were used to determine whether CA21HB is closer to HLA-B or HLA-DR based on data collected on 157 families ascertained through a proband with the classical form of 21-hydroxylase deficiency (CA12Hd). The results were inconclusive but serve to highlight the limitations of present methods of estimating genetic distance when recombination frequencies are of the order of .005.     

Genetic epidemiology of rheumatoid arthritis.

We conducted family studies and segregation analyses of rheumatoid arthritis (RA) that were based on consecutive patients with RA ascertained without regard to family history or known risk factors. First-degree relatives from 135 simplex and 30 multiplex families were included in the analyses. A highly penetrant recessive major gene, with a mutant allele frequency of .005, was identified as the most parsimonious genetic risk factor. Significant evidence for heterogeneity in risk for RA was observed for proband gender but not for proband age at onset. Kaplan-Meier risk analysis demonstrated significant evidence for differences in the distribution of risk among first-degree relatives. These analyses demonstrated that both proband gender and age at onset are important risk factors but that proband gender appears to be the more important determinant of risk, with relatives of male probands having the greatest cumulative risk for RA. In addition, log-linear modeling identified proband gender, familiality (multiplex or simplex), and an interaction term between these two variables as being adequate to define the distribution of risk in families. The pattern of risk for RA among susceptible individuals and its inheritance is thus heterogeneous. For future genetic analyses, families with an excess of affected males having a young age at onset may be the most informative in identifying the putative recessive gene and its modifiers.     

Segregation analysis of blood pressure and body mass index in a rural US community

To assess evidence for a gene with large effect on systolic blood pressure (SBP), diastolic blood pressure (DBP), and body mass index (BMI), we conducted segregation analyses on 261 nuclear families collected from a rural Caucasian community in Michigan. The families were ascertained through a hypertensive proband. Each phenotype was adjusted for significant covariate effects (e.g., gender and age). We used class D regressive models to conduct the segregation analyses. Our analysis results support the segregation of a major gene for BMI, but not for SBP or DBP. A recessive locus effect provided the best explanation for BMI where approximately 43% of the variance of BMI was due to this gene.     

Major recessive gene(s) with considerable residual polygenic effect regulating adult height: confirmation of genomewide scan results for chromosomes 6, 9, and 12

Segregation and linkage analyses were performed for adult height in a population of 200 Dutch families, each of which was ascertained through a proband with asthma. The best-fit model from the segregation analysis was a major recessive gene with a significant residual polygenic background. Models without a polygenic component were rejected. A genomewide scan was performed, and it confirmed previous linkage results for chromosomes 6q25 (LOD = 3.06, D6S2436), 9p1 (LOD = 2.09, D9S301), and 12q1 (LOD = 1.86, D12S375). Our results provide evidence that a combination of segregation and linkage approaches is valuable in understanding genetic determination of common complex traits.     

A major gene for primary hypoalphalipoproteinemia.

Sixteen kindreds were ascertained through probands clinically determined to have primary hypoalphalipoproteinemia, characterized by bottom decile high-density lipoprotein cholesterol (HDL-c), but otherwise normolipidemic. Age- and sex-adjusted, standardized HDL-c levels on 64 individuals in 14 nuclear families in which the proband was a parent were analyzed using the unified mixed model of segregation analysis as implemented in the computer program POINTER. The analysis proceeded by using the likelihood of offspring conditional on the parental phenotypes (conditional likelihood), which appears to overcome the limitation of possible heterogeneity in the selection criteria and provides an appropriate correction for the ascertainment. In these families, the multifactorial contribution to the phenotype appears to be small and significant only in the offspring generation. Although it was not possible to resolve the dominance pattern at the major locus since none of a recessive, additive, or dominant hypothesis could be firmly rejected, these families provided clear evidence for a major gene. Genetic heterogeneity is still a possibility, even within "primary" hypoalphalipoproteinemia.     

Genetic analysis of multiply-affected families of insulin-dependent diabetes mellitus (IDDM) probands

The present study combines segregation and linkage information on 30 families ascertained through a proband and a first degree relative affected with insulin-dependent diabetes mellitus (IDDM). An autosomal dominant model with incomplete penetrance was much more likely to fit the family data than a recessive model, whether or not linkage to HLA was assumed. The lod scores for linkage to HLA were 2.46 at theta M = theta F = 0.00 for dominant and 1.45 at theta M = theta F = 0.22 for a recessive model. The results are discussed in light of heterogeneity in likelihood and lod scores when the families are grouped by familial types, which indicate that the increase in likelihood of a dominant hypothesis can be attributed to the parent-child families and not the sib-sib families.     

Segregation and linkage analyses in families of patients with bipolar,unipolar, and schizoaffective mood disorders.

Hypotheses of single major locus transmission (autosomal and X chromosome) of major affective disorder (i.e., bipolar, unipolar, and schizoaffective) are tested using the Elston-Stewart likelihood method of pedigree segregation analysis. The sample consists of families of varying size ascertained through patients treated at the National Institute of Mental Health in Bethesda, Maryland. We test hypotheses on subsamples of families according to: (1) diagnosis of proband (75 bipolar I, 22 bipolar II, 18 unipolar, and six schizoaffective); (2) extreme value of a biological trait in the proband ("low" monoamine oxidase, "low" cerebrospinal fluid serotonin metabolite 5-HIAA); and (3) positive response to lithium in the proband. We cannot find evidence for single major locus transmission of major affective disorder from segregation analysis in any subsample of family even when the diagnostic classification of ill phenotypes is widened to include possible affective "spectrum" diagnoses. In addition, linkage studies of 21 autosomal markers do not provide evidence for single major locus transmission of illness. The maximum lod score, found for 30 families at the MNS locus, was 1.39 at 20% recombination.     

Major genes regulating total serum immunoglobulin E levels in families with asthma

Immunoglobulin E (IgE) has a major role in the pathogenesis of allergic disorders and asthma. Previous data from 92 families, each identified through a proband with asthma, showed evidence for two major genes regulating total serum IgE levels. One of these genes mapped to 5q31-33. In the current study, the segregation analysis was extended by the addition of 108 probands and their families, ascertained in the same manner. A mixed recessive model (i.e., major recessive gene and residual genetic effect) was the best-fitting and most-parsimonious one-locus model of the segregation analysis. A mixed two-major-gene model (i.e., two major genes and residual genetic effect) fit the data significantly better than did the mixed recessive one-major-gene model. The second gene modified the effect of the first recessive gene. Individuals with the genotype aaBB (homozygous high-risk allele at the first gene and homozygous low-risk allele at the second locus) had normal IgE levels (mean 23 IU/ml), and only individuals with genotypes aaBb and aabb had high IgE levels (mean 282 IU/ml). A genomewide screening was performed using variance-component analysis. Significant evidence for linkage was found for a novel locus at 7q, with a multipoint LOD score of 3. 36 (P=.00004). A LOD score of 3.65 (P=.00002) was obtained after genotyping additional markers in this region. Evidence for linkage was also found for two previously reported regions, 5q and 12q, with LOD scores of 2.73 (P=.0002) and 2.46 (P=.0004), respectively. These results suggest that several major genes, plus residual genetic effects, regulate total serum IgE levels.     

Complex segregation analysis provides compelling evidence for a major gene underlying obsessive-compulsive disorder and for heterogeneity by sex

Evidence from twin and family studies supports a genetic etiology for obsessive-compulsive disorder (OCD). The purpose of this study was to test whether a major gene is implicated in a proportion of families with OCD. Complex segregation analyses of 153 families (80 case and 73 control), ascertained in the Johns Hopkins OCD Family Study, provided strong evidence for a major gene. A Mendelian-dominant model, with significant sex effects and with residual familial effects, best explained the observed data. Stratification of the sample by the sex of probands provided further evidence of heterogeneity with respect to familial aggregation. Segregation analyses of 86 families with a female proband and of the 67 families with a male proband suggested that a Mendelian-dominant model with familial residual effects was the most parsimonious model explaining the inheritance of OCD in both subgroups.     

HLA linkage and B14, DR1, BfS haplotype association with the genes for late onset and cryptic 21-hydroxylase deficiency.

Classical congenital adrenal hyperplasia due to 21-hydroxylase deficiency (21-OH-def) has been established to be an HLA-linked, recessive monogenetic disease. However, two nonclassical forms of 21-OH-def have also been described: "cryptic" 21-OH-def, which has been shown to be HLA-linked, and "late onset" 21-OH-def, for which the status of linkage to HLA has been less certain. We now describe studies of eight additional unrelated probands with symptomatic, "late onset" 21-OH-def, and conclude that this form is also HLA-linked. Both "late onset" and "cryptic" 21-OH-def are highly associated with the same HLA antigens and markers (HLA-B14, HLA-DR1, and Bf type S) in individuals from different ethnic and geographical backgrounds. Since both "late onset" and "cryptic" 21-OH-def appear to occur in individuals with one classical 21-OH-def (21-OHCAH) allele who in addition have another 21-OH-def allele, as well as in individuals who appear to be homozygous for variant 21-PH-def alleles, and since both late onset and cryptic 21-OH-def appear to occur in the same families, our data suggest that these syndromes may represent different clinical expressions of similar or identical nonclassical 21-OH-def alleles.     

Genetic epidemiology of vitiligo: multilocus recessivity cross-validated.

Vitiligo is a dermatological disorder characterized by hypopigmentary patches that tend to become progressive over time. There are reports of extensive familial aggregation. A genetic model for this disorder was earlier proposed by us. This model postulates that recessive alleles at multiple unlinked autosomal loci interact epistatically in the pathogenesis of vitiligo. The present family study was primarily undertaken to cross-validate the proposed genetic model. Data on 194 families from the United States were collected. Each family was ascertained through an affected proband. Analyses of these data reveal that approximately 20% of probands have at least one first-degree relative afflicted with vitiligo. All types of first-degree relatives of probands show a significant risk of developing vitiligo. Results of segregation and robustness analyses reveal that the genetic model postulated by us previously is the most parsimonious model for the present family data set.     

Mapping the locus for hereditary hemochromatosis: localization between HLA-B and HLA-A.

We studied a family with HLA-linked hereditary hemochromatosis in which an informative recombination occurred within the HLA region. The father, an obligate heterozygote for hereditary hemochromatosis, had HLA haplotypes A2,B13 and A11,B27. The mother, also an obligate heterozygote, had HLA haplotypes A29,B44 and A2,B7. Three haplotypes were found among three homozygous affected offspring. Two affected siblings were HLA-identical with haplotypes A2,B13 and A29,B44. The proband had HLA haplotypes A2,B13 and A2,B44, the latter a recombinant haplotype inherited from her mother. Since the maternal hemochromatosis allele was linked to the A29,B44 haplotype, and since the proband has hemochromatosis, the maternal hemochromatosis allele was transmitted to the proband with the B44 antigen. This is the first known example of recombination in an individual with HLA-linked hemochromatosis in whom the hemochromatosis allele appeared to segregate with the HLA-B antigen instead of the -A antigen. The possibility of either a double reciprocal recombination event or a gene conversion event cannot be excluded. Combined with earlier observations of segregation of the hemochromatosis allele with the A locus in HLA recombinants, the findings in this pedigree map the hemochromatosis locus between the HLA-B and HLA-A loci rather than outside the HLA region.     

A genetical model for vitiligo.

A genetical model is found to provide a good fit to family data on vitiligo. The model postulates that recessive alleles at a set of four unlinked diallelic loci are involved in the causation of the disorder. Under this multiple recessive homozygosis model, for normal X affected families ascertained through the affected parent, the expected segregation probability is .063; the estimated value is 0.53, which is not significantly different from the expected value. For normal X normal families ascertained through an affected offspring, the expected segregation probability is .037; the estimated value is .04.     

Basic and clinical aspects of congenital adrenal hyperplasia

Defective steroid 21-hydroxylation is the most common of the biochemical defects causing hyperplasia of the adrenal cortex. The genetic mode of transmission of all enzyme abnormalities seen in cortisol biosynthesis is autosomal recessive. Steroid 21-hydroxylase deficiency has three currently accepted forms: the simple virilizing and salt-wasting variants of the classical deficiency, and the nonclassical (attenuated) form, which shows a wide clinical range of effects and whose characterization emerged from co-ordinated hormonal testing and family studies. More recent molecular genetic studies have started to identify specific mutations altering 21-hydroxylase activity. Defects in the other enzymes occur more rarely and are less well known, although initial work with abnormal 11 beta-hydroxylase and 3 beta-hydroxylase indicates that allelic gene defects may be correlated with different clinical phenotypes seen for these disorders also. The gene for the enzyme steroid 21-hydroxylase, a cytochrome P-450, is situated within the major histocompatibility complex on the p arm of human chromosome 6, proximal to the HLA-B antigen locus. Linkage disequilibria between certain B and DR alleles and classical and nonclassical 21-hydroxylase deficiency permit the use of HLA genotyping in conjunction with hormonal evaluation for diagnosis of this disorder and for identification of carrier haplotypes in population studies. Test programs have shown the feasibility of neonatal screening for 21-hydroxylase deficiency by blood-spot hormonal assay for elevated 17-hydroxyprogesterone. Prenatal detection of disease currently depends on HLA serotyping of cultured aminocytes jointly with measurement of amniotic 17-hydroxyprogesterone (13-18 week gestation); molecular genetic techniques with more specific nuclear probes will improve the specificity of this test and will in addition permit even earlier definitive fetal genotyping by chorionic villus biopsy (6-10 week gestation).     

Complex segregation analysis of febrile convulsions.

Complex segregation analysis was performed on 467 nuclear families ascertained through febrile-convulsion probands. The probands were identified as having their first febrile convulsion while residents of Rochester, MN, during the years 1935-64. Parents and first- and second-degree relatives of probands were identified through the Olmsted County, MN, record-linkage system. Diagnoses of convulsive activity were made from review of medical records. The genetic models investigated included both single-major-locus and polygenic models, with likelihoods computed jointly on children and parents as well as being conditioned on parental phenotype. Possible heterogeneity was investigated by means of analyses of frequency of febrile convulsions in the proband. Analyses of the entire data set indicated that the single-major-locus models could be rejected. The most parsimonious model for these data was the pure polygenic (or common familial environment) model with a large heritable component (68% +/- 7%). However, when families were partitioned on the basis of frequency of febrile convulsions in the proband, significant heterogeneity was present. Our results indicated that the polygenic model was strongly corroborated in families of probands with a single febrile convulsion. In families of probands with multiple febrile convulsions, evidence was consistent with a single-major-locus model with nearly dominant seizure susceptibility.     

Genetic analysis of juvenile periodontitis in families ascertained through an affected proband.

Data from 28 families ascertained through a proband with juvenile periodontitis were used to test a series of Mendelian models of inheritance that included both autosomal and X-linked transmission. There was strong evidence of familial aggregation of this progressive dental disease, and the best-fitting model was an autosomal recessive model. Because of the rather limited age range for expression of the disease in this situation, simulations were done, in a model-choice analysis using samples of this size, to assess the chance of mistaking an autosomal dominant disease (with masking of the affected phenotype outside a specified age range) for an autosomal recessive disease. While the rate of Type II error was fairly high (40%) when competing models in these simulations were compared, these data suggest that it is reasonable to infer that juvenile periodontitis is an autosomal recessive disorder.     

Parathyroid hormone gene analysis in autosomal hypoparathyroidism using an intragenic tetranucleotide (AAAT) n polymorphism

We have identified a polymorphic tetranucleotide consisting of (AAAT) _n within the first intron of the parathyroid hormone (PTH) gene, and have used this to investigate the segregation of the PTH gene and idiopathic hypoparathyroidism in 7 affected and 21 unaffected members from three families. An association between the PTH locus and autosomal dominant idiopathic hypoparathyroidism in one family was excluded by observing recombination between the two loci. In the remaining two families with autosomal recessive idiopathic hypoparathyroidism, the PTH locus was not similarly excluded. We had previously demonstrated a donor splice site mutation of the PTH gene in one of these families, and PTH gene abnormalities were therefore sought in the second of these families. DNA sequence analysis of the three exons, together with 4 exon-intron boundaries and the promoter region of the PTH gene revealed no abnormalities, thereby indicating molecular pathology at another locus. Thus, our analysis of idiopathic hypoparathyroidism reveals genetic heterogeneity for this disorder. In addition, our indentification of a microsatellite polymorphism of the PTH gene should help further segregation studies of this locus in families with parathyroid disorders.     

The effect of proband designation on segregation analysis

In many family studies, it is often difficult to know exactly how the families were ascertained. Even if known, the circumstances under which the families came to the attention of the study may violate the assumptions of classical ascertainment bias correction. The purpose of this work was to investigate the effect on segregation analysis of violations of the assumptions of the classical ascertainment model. We simulated family data generated under a simple recessive model of inheritance. We then ascertained families under different "scenarios." These scenarios were designed to simulate actual conditions under which families come to the attention of-and then interact with-a clinic or genetic study. We show that how one designates probands, which one must do under the classical ascertainment model, can influence parameter estimation and hypothesis testing. We demonstrate that, in some cases, there may be no "correct" way to designate probands. Further, we show that interactions within the family, the conditions under which the genetic study must function, and even social influences can have a profound effect on segregation analysis. We also propose a method for dealing with the ascertainment problem that is applicable to almost any study situation.     

Congenital adrenal hyperplasia: update on prenatal diagnosis and treatment

The diagnostic term congenital adrenal hyperplasia (CAH) applies to a family of inherited disorders of steroidogenesis caused by an abnormality in one of the five enzymatic steps necessary in the conversion of cholesterol to cortisol. The enzyme defects are translated as autosomal recessive traits, with the enzyme deficient in more than 90% of CAH cases being 21-hydroxylase. In the classical forms of CAH (simple virilizing and salt wasting), owing to 21-hydroxylase deficiency (21-OHD), androgen excess causes external genital ambiguity in newborn females and progressive postnatal virilization in males and females. Non-classical 21-OHD (NC21OHD) refers to the condition in which partial deficiencies of 21-hydroxylation produce less extreme hyperandrogenemia and milder symptoms. Females do not demonstrate genital ambiguity at birth.

The gene for adrenal 21-hydroxylase, CYP21, is located on chromosome 6p in the area of HLA genes. Specific mutations may be correlated with a given degree of enzymatic compromise and the clinical form of 21-OHD. NC21OHD patients are predicted to have mild mutations on both alleles or one severe and one mild mutation of the 21-OH locus (compound heterozygote). In most cases the mutation groups represent one diagnosis (e.g., Del/Del with SW CAH), however we have found several non-correlations of genotype to phenotype. Non-classical and classical patients were found within the same mutation group. Phenotypic variability within each mutation group has important implications for prenatal diagnosis and treatment.

Prenatal treatment of 21-OHD with dexamethasone has been utilized for a decade. An algorithm has been developed for prenatal diagnosis and treatment, which, when followed closely, has been safe for both the mother and the fetus, and has been effective in preventing ambiguous genitalia in the affected female newborn. This is an instance of an inborn metabolic error successfully treated prenatally.

Since 1986, prenatal diagnosis and treatment of congenital adrenal hyperplasia due to 21-hydroxylase deficiency (21-OHD) has been carried out in 403 pregnancies in The New York Hospital–Cornell Medical Center. In 280, diagnoses were made by amniocentesis, while 123 were diagnosed using chorionic villus sampling. Of the 403 pregnancies evaluated, 84 babies were affected with classical 21-OHD. Of these, 52 were females, 36 of whom were treated prenatally with dexamethasone. Dexamethasone administered at or before 10 weeks of gestation (23 affected female fetuses) was effective in reducing virilization. Thirteen cases had affected female sibs (Prader stages 1–4); 6 of these fetuses were born with entirely normal female genitalia, while 6 were significantly less virilized (Prader stages 1–2) than their sibs, and one was Prader stage 3. Eight newborns had male sibs; 4 were born with normal genitalia, 3 were Prader stages 1–2, and 3 were born Prader stages 3–4. No significant or enduring side effects were noted in either the mothers or the fetuses, indicating that dexamethasone treatment is safe. Prenatally treated newborns did not differ in weight, length, or head circumference from untreated, unaffected newborns.

Based on our experience, proper prenatal diagnosis and treatment of 21-OHD is effective in significantly reducing or eliminating virilization in the newborn female. This spares the affected female the consequences of genital ambiguity of genital surgery, sex misassignment, and gender confusion.