Mutations for the autosomal recessive and autosomal dominant forms of polycystic kidney disease are not allelic

Summary The autosomal dominant form of polycystic kidney disease (ADPKD) has been linked to the -globin gene locus on 16p. Linkage studies between the autosomal recessive type (ARPKD) and the 3 HVR of the -globin gene cluster showed that the ARPKD and ADPKD are not allelic.     

Clouston syndrome: a rare autosomal dominant trait with palmoplantar hyperkeratosis and alopecia.

A caucasian family is reported in which four males and four females in two generations have exhibited alopecia, dysplastic nails, and hyperkeratosis of palmar and plantar surfaces. This type of ectodermal dysplasia, Clouston syndrome, features normal teeth with severe hair and nail dysplasia.     

Molecular heterogeneity in the mild autosomal dominant forms of osteogenesis imperfecta.

Mild osteogenesis imperfecta (OI type I and OI type IV) is characterized by postnatal onset of fractures, absence of skeletal deformity, presenile hearing loss with or without blue sclerae, and dentinogenesis imperfecta. Using one common DNA polymorphism associated with the pro alpha 2(I) human collagen gene, we found genetic heterogeneity in this disorder. In three families, the OI phenotype segregated independently of the DNA polymorphism, whereas in one family, the OI phenotype cosegregated with a DNA polymorphism in a manner suggesting linkage. Use of DNA polymorphisms associated with both type I procollagen genes should provide a tool to unravel the molecular heterogeneity of various heritable disorders of the connective tissue.     

MRLocus: Identifying causal genes mediating a trait through Bayesian estimation of allelic heterogeneity

Expression quantitative trait loci (eQTL) studies are used to understand the regulatory function of non-coding genome-wide association study (GWAS) risk loci, but colocalization alone does not demonstrate a causal relationship of gene expression affecting a trait. Evidence for mediation, that perturbation of gene expression in a given tissue or developmental context will induce a change in the downstream GWAS trait, can be provided by two-sample Mendelian Randomization (MR). Here, we introduce a new statistical method, MRLocus, for Bayesian estimation of the gene-to-trait effect from eQTL and GWAS summary data for loci with evidence of allelic heterogeneity, that is, containing multiple causal variants. MRLocus makes use of a colocalization step applied to each nearly-LD-independent eQTL, followed by an MR analysis step across eQTLs. Additionally, our method involves estimation of the extent of allelic heterogeneity through a dispersion parameter, indicating variable mediation effects from each individual eQTL on the downstream trait. Our method is evaluated against other state-of-the-art methods for estimation of the gene-to-trait mediation effect, using an existing simulation framework. In simulation, MRLocus often has the highest accuracy among competing methods, and in each case provides more accurate estimation of uncertainty as assessed through interval coverage. MRLocus is then applied to five candidate causal genes for mediation of particular GWAS traits, where gene-to-trait effects are concordant with those previously reported. We find that MRLocus’s estimation of the causal effect across eQTLs within a locus provides useful information for determining how perturbation of gene expression or individual regulatory elements will affect downstream traits. The MRLocus method is implemented as an R package available at https://mikelove.github.io/mrlocus.     

Schilbach-Rott syndrome in a third family: further delineation of an autosomal dominant trait

We describe a father-son Mexican pair with typical features of Schilbach-Rott syndrome (SRS): ocular hypotelorism, cleft palate, hypospadias (only in the child), and microcephaly. This observation documents for the first time a male to male transmission and therefore confirms that the SRS is inherited as an autosomal dominant trait with variable expressivity.     

Early-onset autosomal dominant Alzheimer disease: prevalence, genetic heterogeneity, and mutation spectrum.

To determine the prevalence of early-onset Alzheimer disease (EOAD) and of autosomal dominant forms of EOAD (ADEOAD), we performed a population-based study in the city of Rouen (426,710 residents). EOAD was defined as onset of disease at age <61 years, and ADEOAD was defined as the occurrence of at least three EOAD cases in three generations. Using these stringent criteria, we calculated that the EOAD and ADEOAD prevalences per 100,000 persons at risk were 41.2 and 5.3, respectively. We then performed a mutational analysis of the genes for amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) in 34 families with ADEOAD ascertained in France. In 19 (56%) of these families, we identified 16 distinct PSEN1 missense mutations, including 4 (Thr147Ile, Trp165Cys, Leu173Trp, and Ser390Ile) not reported elsewhere. APP mutations, including a novel mutation located at codon 715, were identified in 5 (15%) of the families. In the 10 remaining ADEOAD families and in 9 additional autosomal dominant Alzheimer disease families that did not fulfill the strict criteria for ADEOAD, no PSEN1, PSEN2, or APP mutation was identified. These results show that (1) PSEN1 and APP mutations account for 71% of ADEOAD families and (2) nonpenetrance at age <61 years is probably infrequent for PSEN1 or APP mutations.     

Evidence for locus heterogeneity in human autosomal dominant split hand/split foot malformation.

Split hand/split foot (SHSF; also known as ectrodactyly) is a human developmental disorder characterized by missing central digits and other distal limb malformations. An association between SHSF and cytogenetically visible rearrangements of chromosome 7 at bands q21-q22 provides compelling evidence for the location of a causative gene at this location, and the locus has been designated SHFD1. In the present study, marker loci were localized to the SHFD1 critical region through the analysis of somatic cell hybrids derived from individuals with SHSF and cytogenetic abnormalities involving the 7q21-q22 region. Combined genetic and physical data suggest that the order of markers in the SHFD1 critical region is cen-D7S492-D7S527-(D7S479-D7S491)-SHFD1-++ +D7S554-D7S518-qter. Dinucleotide repeat polymorphisms at three of these loci were used to test for linkage of SHSF to this region in a large pedigree that demonstrates autosomal dominant SHSF. Evidence against linkage of the SHSF gene to 7q21-q22 was obtained in this pedigree. Therefore, combined molecular and genetic data provide evidence for locus heterogeneity in autosomal dominant SHSF. We propose the name SHSF2 for this second locus.     

Linkage to chromosome 2q36.1 in autosomal dominant Dandy-Walker malformation with occipital cephalocele and evidence for genetic heterogeneity

We previously reported a Vietnamese-American family with isolated autosomal dominant occipital cephalocele. Upon further neuroimaging studies, we have recharacterized this condition as autosomal dominant Dandy-Walker with occipital cephalocele (ADDWOC). A similar ADDWOC family from Brazil was also recently described. To determine the genetic etiology of ADDWOC, we performed genome-wide linkage analysis on members of the Vietnamese-American and Brazilian pedigrees. Linkage analysis of the Vietnamese-American family identified the ADDWOC causative locus on chromosome 2q36.1 with a multipoint parametric LOD score of 3.3, while haplotype analysis refined the locus to 1.1 Mb. Sequencing of the five known genes in this locus did not identify any protein-altering mutations. However, a terminal deletion of chromosome 2 in a patient with an isolated case of Dandy-Walker malformation also encompassed the 2q36.1 chromosomal region. The Brazilian pedigree did not show linkage to this 2q36.1 region. Taken together, these results demonstrate a locus for ADDWOC on 2q36.1 and also suggest locus heterogeneity for ADDWOC.     

Familial high myopia: evidence of an autosomal dominant mode of inheritance and genetic heterogeneity.

High myopia, defined as a refractive error inferior to -6 diopters, often appears as a familial disease. In order to precise its genetic background, we performed a segregation analysis on 32 French families (320 subjects including 120 individuals with clinical data) containing at least one high myopic person in their genealogy. Under the assumption of a two-alleles single gene model, the autosomal dominant transmission mode showed a much greater likelihood than the autosomal recessive mode, which therefore was rejected. From the segregation model obtained, a two-point linkage analysis was made on 18 families (107 subjects), among the 32 used for the segregation analysis. Different candidate loci were tested: collagen genes including Stickler syndrome types 1 and 2, proteoglycan genes, Marfan 1 syndrome and a Marfan like disorder localised in 3p24.2-p25. No evidence of linkage was found with any of the studied markers. In addition, the absence of linkage with chromosome 18p11.31 markers, a locus linked to familial high myopia in 6 North American families and 1 family of Chinese descent, demonstrated the genetic heterogeneity of the disease.     

Linkage of autosomal dominant common variable immunodeficiency to chromosome 5p and evidence for locus heterogeneity

Common variable immunodeficiency (CVID, OMIM 240500) and selective immunoglobulin A deficiency (IgAD) are the most frequent primary immunodeficiencies in humans. Of the cases with CVID/IgAD, 20%-25% are familial, but the only previous claims of linkage or association are to the HLA region on chromosome 6p. We report the results of a genome-wide scan in three multiplex families with CVID, IgAD, and dysgammaglobulinemia, where affection is inherited in an autosomal dominant pattern. Two of the families are consistent with linkage to the telomeric region of chromosome 5p, whereas the third is consistent with linkage to the HLA region. Using a locus heterogeneity model and a conservative penetrance model, we obtained a LOD score of 3.35 for the 5p region. We sequenced the exons of one promising candidate gene within this region (PDCD6, also known as ALG-2) but found no causative mutation.     

Phenotypic heterogeneity in the stargazin allelic series

The stargazer mutant mouse is characterized by its ataxic gait, head tossing, and absence seizures. The mutation was identified in the gamma2 subunit gene of the high voltage-dependent calcium channel, Cacng2. Subsequently, two allelic variants of stargazer have arisen, waggler and stargazer 3J. In this study, we have compared these new alleles to the original stargazer allele. All three mutations affect the Cacng2 mRNA levels as they all arise from disruptions within the introns of this gene. Our results show that the mutations cause reduced Cacng2 mRNA and protein levels. Stargazer and waggler mice have the least amount of mRNA and undetectable protein, whereas stargazer 3J appears to be the mildest allele, both in terms of the phenotype and protein expression. Electroencephalographic (EEG) analysis confirmed that stargazer has frequent spike-wave discharges (SWDs); the average duration of each discharge burst is 5 seconds and recurs every minute. The waggler allele causes a greater variation in SWD activity depending on the individual mouse, and the stargazer 3J mouse has no SWDs. The preliminary characterization of this heterogeneous allelic series provides a basis to explore more biochemical and physiological parameters relating to the role of the Cacng2 product in calcium channel activity, AMPA receptor localization, and cerebellar disturbances.     

Centronuclear myopathy with autosomal dominant inheritance(author's transl)]

For the first time in Germany cases of a "centronuclear myopathy" are described in a 14-year-old boy and his 18-year-old sister. First symptoms in both patients appeared at 4 to 5 years of age with a "sleepy facial expression", clumsy gait and rapid fatigue. Within few years the disease progressed to generalized muscle weakness and atrophy, ptosis, ophthalmoplegia externa and areflexia. Weakness and atrophy were most pronounced in the distal muscles of the lower extremities. Both patients were free of epilepsy and the EEG recordings were normal. Motor and sensory nerve conduction velocities were normal. Repetitive stimulation of nerves revealed a normal transmission from nerve to muscle. Muscle biopsy showed a type I muscle fiber hypotrophy and a type II muscle fibre hypertrophy in addition to a predominance of type I fibres. Both fibre types showed central nuclei, sometimes appearing as chains in longitudinal sections. In most cells with central nuclei there persists a very small pericentral zone free of myofibrils but with increased activity of oxidative enzymes and phosphorylase. 2--3% of muscle fibres in cross sections showed a decreased of absent enzyme activity in the most peripheral fibre zone. Electron microscopy showed evidence of a centrally distinct myofibrillar disintegration. The father of both children had a ptosis at least from the 20th year of age. 5 years later generalized progressive muscle atrophy was recorded. Aged 51 years he died of pneumonia. Though not proved most probably the father suffered from the same disease as the children, pointing to an autosomal dominant inheritance in this family. The disease, according to the literature, seems to be genetically heterogeneous. The clinical picture seems to be independent of the mode of inheritance. Our patients showed a relatively rapid progression of symptoms. Pathogenetically the "centronuclear myopathy" may result from a disturbance of correlated nerve-muscle structures starting during early fetal life.     

Epigenetics and autosomal dominant polycystic kidney disease

The roles of epigenetic modulation of gene expression and protein functions in autosomal dominant polycystic kidney disease (ADPKD) have recently become the focus of scientific investigation. Evidence generated to date indicates that one of the epigenetic modifiers, histone deacetylases (HDACs), are important regulators of ADPKD. HDACs are involved in regulating the expression of the Pkd1 gene and are the target of fluid flow-induced calcium signal in kidney epithelial cells. Pharmacological inhibition of HDAC activity has been found to reduce the progression of cyst formation and slow the decline of kidney function in Pkd1 conditional knockout mice and Pkd2 knockout mice, respectively, implicating the potential clinical application of HDAC inhibitors on ADPKD. Since the expression of HDAC6 is upregulated in cystic epithelial cells, the potential roles of HDAC6 in regulating cilia resorption and epidermal growth factor receptor (EGFR) trafficking through deacetylating α-tubulin and regulating Wnt signaling through deacetylating β-catenin are also discussed. This article is part of a Special Issue entitled: Polycystic Kidney Disease.     

Penetrance rate estimation in autosomal dominant conditions

Accurate estimates of the penetrance rate of autosomal dominant conditions are important, among other issues, for optimizing recurrence risks in genetic counseling. The present work on penetrance rate estimation from pedigree data considers the following situations: 1) estimation of the penetrance rate K (brief review of the method); 2) construction of exact credible intervals for K estimates; 3) specificity and heterogeneity issues; 4) penetrance rate estimates obtained through molecular testing of families; 5) lack of information about the phenotype of the pedigree generator; 6) genealogies containing grouped parent-offspring information; 7) ascertainment issues responsible for the inflation of K estimates.     

Mapping quantitative trait loci with dominant markers in four-way crosses

 A common problem in mapping quantitative trait loci (QTLs) is that marker data are often incomplete. This includes missing data, dominant markers, and partially informative markers, arising in outbred populations. Here we briefly present an iteratively re-weighted least square method (IRWLS) to incorporate dominant and missing markers for mapping QTLs in four-way crosses under a heterogeneous variance model. The algorithm uses information from all markers in a linkage group to infer the QTL genotype. Monte Carlo simulations indicate that with half dominant markers, QTL detection is almost as efficient as with all co-dominant markers. However, the precision of the estimated QTL parameters generally decreases as more markers become missing or dominant. Notable differences are observed on the standard deviation of the estimated QTL position for varying levels of marker information content. The method is relatively simple so that more complex models including multiple QTLs or fixed effects can be fitted. Finally, the method can be readily extended to QTL mapping in full-sib families. Received: 16 June 1998 / Accepted: 29 September 1998     

Confirmation of genetic heterogeneity in limb-girdle muscular dystrophy: Linkage of an autosomal dominant form to chromosome 5q

Limb-girdle muscular dystrophy (LGMD) is a clinically and genetically heterogenous group of disorders, with both recessive and dominant forms reported. Recently, a series of recessive LGMD families were linked to chromosome 15q. We report herein the results of our linkage studies in a previously reported large autosomal dominant family. The LGMD gene in this family was localized to chromosome 5q22.3-31.3 by using a series of CA(n) microsatellite repeat markers. Linkage to 15q was excluded. These findings confirm genetic heterogeneity in this clinically diverse syndrome.     

The autosomal dominant trait of obesity, acanthosis nigricans, hypertension, ischemic heart disease and diabetes type 2.

OBJECTIVE: Among obese subjects, acanthosis nigricans in both males and females is not as uncommon as previously thought. Whereas this finding was extensively evaluated in females, mostly in the context of polycystic ovaries syndrome, little attention has been paid to obese males with acanthosis nigricans. As acanthosis seems to be a marker for insulin resistance, the present study was designed to evaluate the hypothesis that the clinical syndrome of obesity and acanthosis would take a different clinical course than that of simple obesity. METHODS: To characterize the course of acanthosis nigricans and obesity in males, we examined 22 children and adolescents with this complex, together with their parents and grandparents and found them to follow a detrimental sequence of the metabolic syndrome. We compared the findings to 13 age-matched males with obesity but no clinical apparent acanthosis nigricans. We analyzed the clinical course, fat distribution, glucose, insulin and C-peptide and lipoproteins. RESULTS: Onset of obesity in the metabolic syndrome group was at a mean age of 6.4 years, as compared to 2.3 years in the controls. The metabolic syndrome patients had a truncal (android) distribution of fat and their fasting blood glucose was significantly higher. HDL/total cholesterol was lower. Examination of the pedigrees suggested autosomal dominant inheritance of the obesity and acanthosis nigricans complex, extending to hypertension and ischemic heart disease in the parents' generation, and further extending to include diabetes type 2 in the grandparents' generation. CONCLUSIONS: This metabolic syndrome is inherited as an autosomal dominant trait, with onset of truncal obesity at age 6-7 years, acanthosis nigricans during childhood or adolescence, extending to hypertension and ischemic heart disease during young adulthood, and further extending to include diabetes type 2 in late adulthood. It is recommended that such children should be followed up as an 'at-risk' group, and would probably benefit from intensive weight reduction, which may prevent the later manifestations of the syndrome.