Calculating risk changes after negative mutation test outcomes for autosomal dominant hereditary late-onset disorders

We demonstrate, in a specific scenario, the effect of negative test results from relatives in families at risk for an autosomal dominant hereditary late-onset disorder. A hypothetical pedigree, of a family at risk of Huntington's disease, was used to demonstrate the consequences for the risk status of various family members in the case where relatives have been tested, and found to be mutation negative. We argue that accurate assessment of conditional probabilities in clinical genetics is important for individuals at risk for hereditary disorders with Mendelian transmission patterns; our formulae offer the opportunity -- when simplifying assumptions are met -- to determine the changed risk status of individuals in such cases.     

Approaches to familial aggregation: hypothesis testing and estimation when families are selected through parent probands under a variant of single ascertainment

Epidemiologic approaches to testing and estimating familial aggregation of a disease consist of comparing rates of disease in relatives of individuals with the disease (known as case probands) with rates of disease in relatives of individuals without the disease (known as control probands). Gold et al. (J Am Stat Ass 1967;62: 409-420) derived an explicit mathematical model and sampling methods, under which this approach is equivalent to testing the null hypotheses that the disease risk in families is homogenous. A basic assumption of this model is that every family member has the same risk of disease and that disease status is independent among family members, although the disease risk may vary between families. When the disease is suspected of having a genetic component, rather than being purely environmental, this model has been shown to be appropriate for detecting disease aggregation in siblings, when relatives are siblings of probands. This model however is unrealistic for use in nuclear families when the affected status of offspring is not independent of the affected status of parents, and these families are selected through an affected or an unaffected parent, so that a parent is the proband and relatives are offspring of probands. We extend the Gold et al. model to allow for the disease risk in offspring to vary with the affected status of the parent. We assume that families are selected through affected and unaffected parents, under a variation of single ascertainment. Under this study design, we show that the usual test of association between affected status of probands and relatives, performed by comparing sample proportions of affected relatives of affected and unaffected probands, respectively, is no longer equivalent to a test of homogeneity of disease risk in offspring. Instead, it is equivalent to testing that the disease risk in offspring is independent of the number of affected parents. This test reduces to a test of homogeneity if and only if one assumes that the variation in disease risk in offspring, between families, is solely due to the variation in the number of affected parents. As a result, we show that under this study design, the standard chi2 test must be modified in order to obtain a valid test of familial aggregation. In addition the sample proportions of affected relatives of case and control probands, respectively, are shown to provide unbiased estimates of the expected risk of disease in an offspring given an affected/unaffected parent. We apply these results to methods of sample selection and discuss the practical implications of these findings.     

Maternal factors in onset of Huntington disease.

Analyses of father-offspring and mother-offspring similarity in onset age suggest that nuclear genes account for a significant portion of the modification of onset age in Huntington disease. The effects of non-nuclear modifiers are supported by the finding that the offspring of affected women have significantly older mean ages of onset than offspring of affected men irrespective of the onset age in the parent. The absence of increased father-daughter similarity indicates that modification is not X-linked. The absence of reproductive advantage for late-onset individuals and the absence of a multigenerational maternal-lineage effect suggest that the modifying effect of the sex of the affected parent occurs in a single parental generation. Offspring of affected women with onset between ages 35 and 49 had a significantly older mean onset age than their mothers. This suggests that a protective effect may be conferred upon the offspring of affected women.     

Risk reduction and health promotion behaviors following genetic testing for adult-onset disorders

Although clinical genetic testing is available for over 1,000 inherited disorders, consequences of predictive genetic testing have been most extensively examined for hereditary breast and ovarian cancer (HBOC), hereditary colon cancer, and Huntington disease (HD). These focus primarily on psychological, ethical, legal, and social aspects of genetic testing. Genetic testing may also provide information that can lead to behaviors that promote health and reduce risk for disease, reflecting options available for the disorder for which the person is at risk. However, regardless of condition, people completing a genetic test may inform relatives about the results of the test and implications for their risk to develop the condition. Literature on risk reduction behaviors and communication focuses on families with HBOC or colorectal cancer. Few reports document behaviors for other conditions. This paper presents a systematic review of the research literature on risk reduction and health promotion behaviors following clinical genetic testing for adult onset conditions, primarily HBOC, familial colon cancers, and HD. Insights gleaned from this review are discussed as a basis for planning monitoring of health promotion and risk-reduction behaviors for genetic testing for present and future use.     

Unique spectroscopic properties of synthetic 15-cis beta-carotene,an important compound in photosynthesis,and a medicine for photoprotective function

The (1(1)B(u)+) energy of synthetic 15-cis beta-carotene exhibits a linear dependence on (n(2)-1)/(n(2)+2) in non-polar and polar solvents; in this it is similar to (that of) all-trans beta-carotene. The point of intersection is at (n(2)-1)/(n(2)+2) = 0.3 for both isomers. The microenvironment of 15-cis beta-carotene in the Photosystem II reaction center was established as having a mean refractive index 1.473. Persistent spectral hole burning with a very broad (approximately 30 nm) hole observed around 500 nm (corresponding to an extremely short excited lifetime tau approximately 9 fs) indicates that 15-cis beta-carotene has/displays very efficient photoprotective quenching.     

Improved predictive testing for Huntington disease by using three linked DNA markers.

Eighty-five persons at risk for Huntington disease (HD) have enrolled in a predictive-testing pilot program. Informativeness of the test has been determined for 41 of these candidates by using linked DNA probes. Nine (21.9%) of these persons have been excluded from the test as a result of the unavailability of DNA from crucial family relatives. Homozygosity for all of the three DNA markers (D4S10, D4S62, and D4S95) was not found in any affected parent. Only one (2%) of the 41 test candidates has had an uninformative result. Results have been given to 20 persons, of whom 12 (60%) received a decreased risk and eight (40%) received an increased risk of having inherited the mutant gene for HD. The combined use of three DNA markers significantly increases the informativeness of family structures such that some change in the estimation of genetic risk is now possible for approximately 75% of all persons who request predictive testing.     

Medical geneticists confront ethical dilemmas: cross-cultural comparisons among 18 nations.

To provide a basis for international discussion of ethical problems, we studied responses of medical geneticists in 18 countries to questionnaires about 14 clinical cases and five screening situations. Of 1,053 asked to participate, 677 (64%) responded. There was greater than or equal to 75% consensus on five cases involving (1) disclosure of (1) conflicting diagnostic findings, (2) disclosure of ambiguous results, (3) disclosure of controversial interpretations, (4) protection of mother's confidentiality in cases of false paternity, and (5) nondirective counseling about 45,X and XYY syndrome. A majority (51%-60%) would disclose the diagnosis to relatives at risk for Huntington disease or hemophilia A, against the patient's wishes; would disclose which parent carries a translocation causing Down syndrome; and would disclose XY genotype in a female. As reproductive options for patients with disorders not diagnosable prenatally, 84% would discuss artificial insemination by a donor, 66% would discuss in vitro fertilization with donor egg, and 46% would discuss surrogate motherhood. In all, 85% would perform prenatal diagnosis for (or would refer) parents who refuse abortion, 75% for maternal anxiety, and 42% for selection of fetal sex. Screening questions showed that 72% believed that workplace screening should be voluntary and that results should be confidential.     

The risk of fragile X premutation expansion is lower in carriers detected by general prenatal screening than in carriers from known fragile X families

The Fragile X syndrome is the most common cause of inherited mental retardation. For a female premutation carrier, the risk of having a child with a full mutation is positively correlated with the size of the premutation. The current study was performed to evaluate the risk of premutation expansion in the offspring of average-risk carriers detected by general prenatal screening. Over a 4-year period, 9,660 women underwent DNA screening for FMR1 mutation/premutation at the Tel Aviv Sourasky Medical Center. A premutation was defined as a CGG repeat number >50 in the 5' untranslated region (UTR) of exon 1 in the FMR1 gene. The study included only individuals with no family history of X-linked mental retardation or known FMR1 mutations. A premutation was found in 85 women (1 in 114), 68 of whom consented to have prenatal diagnoses in 74 pregnancies. The abnormal allele was transmitted to the offspring in 44 pregnancies. Of these, no change in allele size was noted in 35 pregnancies (79.6%), and expansion within premutation range was evident in 4 pregnancies (9%). In 5 pregnancies (11.4%), expansion to the full mutation was noted. This occurred only in carriers having more than 90 repeats. We conclude that the likelihood of Fragile X premutation expansion to full mutation is significantly lower in individuals ascertained by general prenatal carrier testing than in those from known Fragile X families.     

Application of a closely linked polymorphism of restriction fragment length to counselling and prenatal testing in families with myotonic dystrophy.

The close genetic linkage between the loci for apolipoprotein CII (ApoC2) and myotonic dystrophy makes ApoC2 the closest fully validated marker for prediction of myotonic dystrophy. Application to genetic counselling and presymptomatic and prenatal prediction is reported in seven families with myotonic dystrophy, including one case in which the disorder was excluded prenatally. Only one of the families did not have members with ApoC2 genotypes that allowed prediction, but careful clinical study of older family members was found to be an important factor. ApoC2 typing of families with myotonic dystrophy should be of practical help both in prediction for asymptomatic relatives and for prenatal diagnosis in pregnancies of an affected parent.     

Effects of sire and dam genotype for complex vertebral malformation (CVM) on risk of return-to-service in Holstein dairy cows and heifers

The emergence of a new hereditary disease, called "Complex vertebral malformation" (CVM), has been described in Holstein cattle population. This paper studied the incidence of mating concerning Holstein dairy cattle with CVM in Brittany and the possible influences of CVM status for sires and dams on return-to-service at different intervals post-service in cows and heifers. It was carried out based on a set of data for first and second inseminations between 1998 and 2001 in cows (n=530,538) and heifers (n=248,140). Incidence of matings between CVM gene carriers, between a carrier bull and a non-carrier cow and between a non-carrier bull and a carrier cow were estimated to be 1.4, 10.6 and 9%, respectively (1.1, 9.3 and 10.9%, respectively in heifers). Compared to CVM-free mating, the relative risk of return-to-service was increased when the sire was a CVM carrier and the dam was at risk of being a carrier, especially for late return (>25 days post-service). When the sire alone was a carrier, the relative risk was increased whenever the return occurred at a low but significant level. Following mating between a non-carrier bull and a dam at risk of being a carrier, the risk of return-to-service decreased at a low but significant level, whatever the interval in the heifers but only for 19-25 days intervals in the cows.     

A genetic model for age at onset in Huntington disease.

Although numerous investigators have confirmed excess paternal transmission among juvenile-onset cases of Huntington disease (HD), there are conflicting reports that the late-onset form is inherited more often from the mother than from the father. Results from a survey of age at onset and age at death in 569 patients corroborate earlier findings of delayed onset of HD among offspring of affected mothers at both ends of the onset-age spectrum: 23 of 28 juvenile-onset offspring had affected fathers, and there were 1.6 times more late-onset offspring born to affected mothers than to affected fathers. These patterns, together with data that link age-at-onset variability to familial longevity trends, suggest a model where age at onset is governed, generally, by a set of independently inherited aging genes, but expression of the HD gene may be significantly delayed in individuals who possess a particular maternally transmitted factor.     

Pregnancy outcomes of rare autosomal trisomies results in non‐invasive prenatal screening: clinical follow‐up data from a single tertiary centre

This study was performed to assess the association between detection of rare autosomal trisomies (RATs) by non‐invasive prenatal screening (NIPS) and adverse pregnancy outcomes. We retrospectively analyzed women with high‐risk RATs results from January 2014 to December 2020. The women''s clinical information was collected, and their pregnancy outcomes were compared with those of women with low‐risk results. In total, 151 (0.24%) RATs results were reported among 62,752 NIPS examinations. Sixty‐five women chose to undergo amniocentesis for confirmation, which revealed 3 cases of true fetal mosaicism for RATs and a positive predictive value of 4.6% (3/65). Among the 139 women with available outcomes, 26 (18.7%) had a preterm birth, 10 (7.2%) underwent pregnancy termination because of fetal defects and 5 (3.6%) had miscarriages. Interestingly, compared with the control group, pregnancies in which NIPS revealed trisomy 16 (T16), T22, T9 and T2 were at higher risk of adverse outcomes, including preterm birth, miscarriage and ultrasound abnormalities. However, the risk of adverse outcomes was comparable between the control group and pregnancies with positive results of T7, T3, T8 and T20. In summary, the risk of adverse pregnancy outcomes was higher in women with specific RATs‐positive NIPS results. Pregnancies with T16, T22, T9 and T2 results, even if false‐positive, should be considered high‐risk pregnancies.     

Attitudes toward genetic testing in patients at risk for HNPCC/FAP and the German population

Adequate knowledge regarding hereditary diseases and genetics, as well as personal attitudes toward gene tests, are major determinants of optimal utilization of genetic testing. In the present study, we aimed to explore the general attitudes toward genetic testing in a sample representative of the German general population (n = 2,076) and to compare the attitudes of persons at risk for hereditary non-polyposis colorectal cancer/familial adenomatous polyposis (HNPCC/FAP) (n = 36) who had attended a university genetic counseling service, with a matched general population sample. We administered a subset of a questionnaire previously used in a Finnish study (Jallinoja et al., 1998). The 12 statements pertain to approval, disapproval, and concern for genetic testing. Overall, the results reveal high approval of genetic testing in the German population and in at-risk persons. In accordance with other studies, we find that the attitudes of individuals for whom hereditary disease is a salient issue of personal relevance and the attitudes of the general public are very similar. Only a few significant differences between these two samples emerged, indicating that at-risk persons hold a more favourable view of the testing. One intriguing finding was the high rate of "don't know" responses, especially in the general population sample. Compared to results from Finland, approval of genetic testing is lower in the German population, and endorsement of "don't knows" is remarkably higher. We argue for increased attention to the issue of attitude change after genetic counseling and for the need of comparative cross-cultural research on attitudes toward gene technology.     

Validation of dHPLC for molecular diagnosis of beta-thalassemia in Southern Italy

Beta-thalassemia, the most common hereditary anemia in the Mediterranean area, results from over 200 causative mutations in the beta-globin locus. The aim of this study was to validate a denaturing high-performance liquid chromatography (dHPLC)-based assay for postnatal and prenatal molecular diagnosis of beta-thalassemia in Southern Italy. Sixty beta-thalassemic patients, affected either by thalassemia intermedia or thalassemia major, were analyzed in a blind study. We also carried out prenatal molecular diagnosis in 12 couples at-risk for having affected offspring. Chorionic villi samples were subjected to dHPLC analysis upon molecular characterization of the parental beta-globin alleles. Direct sequence analysis was used to validate each result, showing an accuracy rate of 100% for dHPLC. Overall, our protocol was able to identify the responsible mutations in all 96 analyzed subjects (including 12 prenatals in at-risk pregnancies), detecting the eight most common mutations in Southern Italy. Three rare mutations (one of which, reported here for the first time) that standard mutation detection methods failed to reveal, were also identified. dHPLC assay proved to be a reliable, rapid, and sensitive method for detecting both common and rare mutations within the beta-globin gene. Because of this property our protocol has the potential to be implemented for mutational screening in different areas of high prevalence for beta-thalassemia.     

Complex segregation analysis of Parkinson's disease in the Finnish population

The risk of Parkinson's disease (PD) is higher among relatives of affected individuals than among other members of the population, and most family studies have suggested autosomal dominant inheritance, although both autosomal dominant and recessive susceptibility genes have recently been identified. We carried out a complex segregation analysis with POINTER to assess the mode of inheritance of PD in the population of northern Finland. Nuclear families (n=265) were identified through a proband with idiopathic PD. The analysis was first carried out for the total data set, and then the heterogeneity between early-onset (proband under 55 years at onset) and late-onset families was examined. Finally, families with more than one affected individual were analyzed separately. The sporadic model was rejected (P<0.0001). Significant heterogeneity was found between the early-onset and late-onset families, suggesting that major genes have a greater role in early-onset PD than in late-onset PD and that the etiology of idiopathic PD is heterogeneous, even in the Finnish population, which has evolved from a small group of founders. The analysis of familial PD supported the hypothesis that a major locus was present in this subset, but it was not possible to distinguish between a recessive model with a high penetrance and a dominant model with lower penetrance.     

Study of the duplication of 17p11.2-12 chromosome region in the patients with hereditary motor and sensory neuropathy type 1A

Charcot-Marie-Tooth neuropathy (CMT) is one of the most common hereditary disorders, affecting 1:2500 individuals. CMT is a heterogeneous group of disorders characterized by chronic peripheral motor and sensory neuropathy. We have performed the detection of 1.5 Mb CMT1A tandem duplication in 17p11.2-12 chromosome region for autosome-dominant CMT1 patients and their relatives using the analysis of two (CA)n polymorphic microsatellite loci: 17S921 and 17S1358 localised in the duplication region. CMT1A duplication was found in three of five autosome-dominant CMT1 families. It has been shown that CMT1A duplication analysis is important for early differential diagnosis of CMT including prenatal diagnosis and genetic consulting in high risk families.     

Factors determining dissemination of results and uptake of genetic testing in families with known BRCA1/2 mutations

BACKGROUND: Uptake of genetic testing remains low, even in families with known BRCA1 and BRCA2 (BRCA1/2) mutations, despite effective interventions to reduce risk. We report disclosure and uptake patterns by BRCA1/2-positive individuals to at-risk relatives, in the setting of no-cost genetic counseling and testing. METHODS: Relatives of BRCA1/2-positive individuals were offered cost-free and confidential genetic counseling and testing. If positive for a BRCA1/2 mutation, participants were eligible to complete a survey about their disclosure of mutation status and the subsequent uptake of genetic testing by at-risk family members. RESULTS: One hundred and fifteen of 142 eligible individuals responded to the survey (81%). Eighty-eight (77%) of those surveyed disclosed results to all at-risk relatives. Disclosure to first-degree relatives (FDRs) was higher than to second-degree relatives (SDRs) and third-degree relatives (TDR) (95% vs. 78%; p < 0.01). Disclosure rates to male versus female relatives were similar, but reported completion of genetic testing was higher among female versus male FDRs (73% vs. 49%; p < 0.01) and SDRs (68% vs. 43%; p < 0.01), and among members of maternal versus paternal lineages (63% vs. 0%; p < 0.01). Men were more likely than women to express general difficulty discussing positive BCRA1/2 results with at-risk family members (90% vs. 70%; p = 0.03), while women reported more emotional distress associated with disclosure than men (48% vs. 13%; p < 0.01). DISCUSSION: We report a very high rate of disclosure of genetic testing information to at-risk relatives. However, uptake of genetic testing among at-risk individuals was low despite cost-free testing services, particularly in men, SDRs, and members of paternal lineages. The complete lack of testing among paternally related at-risk individuals and the lower testing uptake among men signify a significant barrier to testing and a challenge for genetic counselors and physicians working with high-risk groups. Further research is necessary to ensure that family members understand their risk and the potential benefits of genetic counseling.     

Linkage disequilibrium, cystic fibrosis, and genetic counseling.

Strong linkage disequilibrium occurs between the cystic fibrosis (CF) locus and polymorphisms detected with the DNA probes XV-2c and KM-19. In a North American population, 86% of CF chromosomes occur with a haplotype which occurs on only 14% of normal chromosomes. An individual homozygous for the highest-risk haplotype has an 81-fold greater probability of carrying a CF allele than does an individual homozygous for the lowest-risk haplotype. The linkage-disequilibrium data can be used for prenatal diagnosis and genetic counseling in CF families. The data are useful in 1-in-4-risk pregnancies when DNA is not available from the propositus and in counseling close relatives of CF families. Serious problems arise with some pregnancies which remain at intermediate risks after analysis, and families are left with difficult decisions. It is not clear that genetic testing for couples at less than 1-in-4 risk is cost-effective or standard care, but use of linkage-disequilibrium data will provide more accurate risk probabilities in a substantial proportion of cases if such testing is carried out. Our results emphasize the need for a specific biological or molecular carrier test. This experience in using linkage-disequilibrium and linkage data in combination for genetic counseling provides a model system for the diagnosis of other disorders.     

Homozygote for Huntington disease.

Four offspring of three different Huntington disease (HD) affected x affected matings were assessed by genetic linkage analysis for possible homozygosity. One individual was found to have a 95% likelihood of being an HD homozygote. The homozygote individual had an age at onset and symptoms which were similar to those of affected HD heterozygote relatives, including some with younger onset. This confirms the observation of Wexler et al. that in HD the homozygote is not more severely afflicted than the heterozygote.     

The use of association data to identify family members at high risk for marker-linked diseases.

The study of genetic markers linked and associated with disease has provided important evidence of a genetic contribution to numerous diseases and has helped to establish their modes of inheritance. However, this information has not been fully utilized in counseling individuals at risk for these disorders. In the case of recessive, marker-linked diseases, such as idiopathic hemochromatosis linked to HLA in family studies and associated with specific HLA alleles in population surveys, the only current clinical application has been to identify siblings who share both HLA-marker haplotypes with the affected proband. They are considered to be presymptomatically affected, and more definitive invasive investigations are considered appropriate. All other relatives, including parents, offspring, and other siblings, who share only one marker with the proband, have been counseled only that their risk is equivalent to the gene frequency of the disease allele, for example, 3%-6% for hemochromatosis. We have developed a generally applicable method to utilize population association data to derive more specific and accurate risk figures for these other relatives of patients with marker-linked and associated diseases. We have applied this method to idiopathic hemochromatosis. If the offspring of a patient with hemochromatosis lacks A3, B7, and B14, the risk to that offspring for developing hemochromatosis is less than 2%. On the other hand, if they receive HLA A3 from their unaffected parent, their risk climbs to 9%-10%; if they receive an A3-B14 haplotype, their risk increases to virtually 100%. As demonstrated by our example, the application of association data to family members already at a basal increased risk for marker-linked disease can significantly refine the disease risk estimates given to those relatives. This information can be utilized to select individuals in whom invasive diagnostic testing or preventative intervention is indicated.