A Locus for Autosomal Dominant Hereditary Spastic Ataxia, SAX1, Maps to Chromosome 12p13

The hereditary spastic ataxias (HSA) are a group of clinically heterogeneous neurodegenerative disorders characterized by lower-limb spasticity and generalized ataxia. HSA was diagnosed in three unrelated autosomal dominant families from Newfoundland, who presented mainly with severe leg spasticity, dysarthria, dysphagia, and ocular-movement abnormalities. A genomewide scan was performed on one family, and linkage to a novel locus for HSA on chromosome 12p13, which contains the as-yet-unidentified gene locus SAX1, was identified. Fine mapping confirmed linkage in the two large families, and the third, smaller family showed LOD scores suggestive of linkage. Haplotype construction by use of 13 polymorphic markers revealed that all three families share a disease haplotype, which key recombinants and overlapping haplotypes refine to about 5 cM, flanked by markers D12S93 and GATA151H05. SAX1 is the first locus mapped for autosomal dominant HSA.     

Fine mapping of the Autosomal Dominant Split Hand/Split Foot Locus on Chromosome 7, Band q21.3-q22.1

Split hand/split foot (SHFD) is a human developmental defect characterized by missing digits, fusion of remaining digits, and a deep median cleft in the hands and feet. Cytogenetic studies of deletions and translocations associated with this disorder have indicated that an autosomal dominant split hand/split foot locus (gene SHFD1) maps to 7q21-q22. To characterize the SHFD1 locus, somatic cell hybrid lines were constructed from cytogenetically abnormal individuals with SHFD. Molecular analysis resulted in the localization of 93 DNA markers to one of 10 intervals surrounding the SHFD1 locus. The translocation breakpoints in four SHFD patients were encompassed by the smallest region of overlap among the SHFD-associated deletions. The order of DNA markers in the SHFD1 critical region has been defined as PON–D7S812–SHFD1–D7S811–ASNS. One DNA marker, D7S811, detected altered restriction enzyme fragments in three patients with translocations when examined by pulsed-field gel electro-phoresis (PFGE). These data map SHFD1, a gene that is crucial for human limb differentiation, to a small interval in the q21.3-q22.1 region of human chromosome 7.     

Autosomal Dominant Postaxial Polydactyly, Nail Dystrophy, and Dental Abnormalities Map to Chromosome 4p16, in the Region Containing the Ellis–van Creveld Syndrome Locus

We have studied a four-generation family with features of Weyers acrofacial dysostosis, in which the proband has a more severe phenotype, resembling Ellis-van Creveld syndrome. Weyers acrofacial dysostosis is an autosomal dominant condition with dental anomalies, nail dystrophy, postaxial polydactyly, and mild short stature. Ellis-van Creveld syndrome is a similar condition, with autosomal recessive inheritance and the additional features of disproportionate dwarfism, thoracic dysplasia, and congenital heart disease. Linkage and haplotype analysis determined that the disease locus in this pedigree resides on chromosome 4p16, distal to the genetic marker D4S3007 and within a 17-cM region flanking the genetic locus D4S2366. This region includes the Ellis-van Creveld syndrome locus, which previously was reported to map within a 3-cM region between genetic markers D4S2957 and D4S827. Either the genes for the condition in our family and for Ellis-van Creveld syndrome are near one another or these two conditions are allelic with mutations in the same gene. These data also raise the possibility that Weyers acrofacial dysostosis is the heterozygous expression of a mutation that, in homozygous form, causes the autosomal recessive disorder Ellis-van Creveld syndrome.     

Investigation of a Family with Autosomal Dominant Dilated Cardiomyopathy Defines a Novel Locus on Chromosome 2q14-q22

Dilated cardiomyopathy (DCM) is a leading cause of heart failure and the most frequent indication for heart transplantation in young patients. Probably >25% of DCM cases are of familial etiology. We report here genetic localization in a three-generation German family with 12 affected individuals with autosomal dominant familial DCM characterized by ventricular dilatation, impaired systolic function, and conduction disease. After exclusion of known DCM loci, we performed a whole-genome screen and detected linkage of DCM to chromosome 2q14-q22. Investigation of only affected individuals defines a 24-cM interval between markers D2S2224 and D2S2324; when unaffected individuals are also included, the critical region decreases to 11 cM between markers D2S2224 and D2S112, with a peak LOD score of 3.73 at recombination fraction 0 at D2S2339. The identification of an additional locus for familial autosomal dominant DCM underlines the genetic heterogeneity and may assist in the elucidation of the causes of this disease.     

A Third Locus for Autosomal Dominant Cerebellar Ataxia Type 1 Maps to Chromosome 14q24.3-qter: Evidence for the Existence of a Fourth Locus

The autosomal dominant cerebellar ataxias (ADCA) type I are a group of neurological disorders that are clinically and genetically heterogeneous. Two genes implicated in the disease, SCA1 (spinal cerebellar ataxia 1) and SCA2, are already localized. We have mapped a third locus to chromosome 14q24.3-qter, by linkage analysis in a non-SCA1/non-SCA2 family and have confirmed its existence in a second such family. We suggest designating this new locus “SCA3.” Combined analysis of the two families restricted the SCA3 locus to a 15-cM interval between markers D14S67 and D14S81. The gene for Machado-Joseph disease (MJD), a clinically different form of ADCA type I, has been recently assigned to chromosome 14q24.3-q32. Although the SCA3 locus is within the MJD region, linkage analyses cannot yet demonstrate whether they result from mutations of the same gene. Linkage to all three loci (SCA1, SCA2, and SCA3) was excluded in another family, which indicates the existence of a fourth ADCA type I locus.     

A High-Resolution Genetic Map of the Nervous Locus on Mouse Chromosome 8

The nervous (nr) mutant mouse displays two gross recessive traits: both an exaggeration of juvenile hyperactivity and a pronounced ataxia become apparent during the third and fourth postnatal weeks. Using an intersubspecific intercross, we have established a high-resolution map of a segment of mouse Chromosome 8 that places thenrlocus in a genomic segment defined byD8Rck1on the centromeric end andD8Mit3on the telomeric end. This map position places thenrlocus within the BALB/cGr congenic region of the C3HeB/FeJ-nrstrain, confirming the accuracy of our study. We used this map position to identify and evaluate three genes—ankyrin 1, cortexin, and farnesyltransferase—as candidates for thenrgene. These three genes were eliminated from consideration but allowed us to establish the conservation of synteny between the region containing thenrlocus and a segment of the short arm of human chromosome 8 (8p21–p11.2). Finally, the incomplete penetrance of thenrphenotype led us to perform a screen for modifier loci, and we present evidence that such a nervous modifier locus may exist on mouse Chromosome 5.     

A Missense Mutation in SLC33A1, which Encodes the Acetyl-CoA Transporter, Causes Autosomal-Dominant Spastic Paraplegia (SPG42)

Hereditary spastic paraplegias (HSPs), characterized by progressive and bilateral spasticity of the legs, are usually caused by developmental failure or degeneration of motor axons in the corticospinal tract. There are considerable interfamilial and intrafamilial variations in age at onset and severity of spasticity. Genetic studies also showed that there are dozens of genetic loci, on multiple chromosomes, that are responsible for HSPs. Through linkage study of a pedigree of HSP with autosomal-dominant inheritance, we mapped the causative gene to 3q24-q26. Screening of candidate genes revealed that the HSP is caused by a missense mutation in the gene for acetyl-CoA transporter (SLC33A1). It is predicted that the missense mutation, causing the change of the highly conserved serine to arginine at the codon 113 (p. S113R), disrupts the second transmembrane domain in the transporter and reverses the orientation of all of the descending domains. Knockdown of Slc33a1 in zebrafish caused a curve-shaped tail and defective axon outgrowth from the spinal cord. Although the wild-type human SLC33A1 was able to rescue the phenotype caused by Slc33a1 knockdown in zebrafish, the mutant SLC33A1 (p.S113R) was not, suggesting that S113R mutation renders SLC33A1 nonfunctional and one that wild-type allele is not sufficient for sustaining the outgrowth and maintenance of long motor axons in human heterozygotes. Thus, our study illustrated a critical role of acetyl-CoA transporter in motor-neuron development and function.     

A High-Resolution Genetic Map of Mouse Chromosome 5 Encompassing the Reeler (rl) Locus

Using interspecific crosses between BALB/c and Mus spretus (SEG) mice, the murine reeler (rl) gene was mapped to the proximal region of chromosome 5 between the hepatocyte growth factor gene (Hgf) and the D5Mit66 microsatellite. The following order was defined: (centromere)-Cchl2a/Hgf-D5Mit1-D5Nam1/D5Nam2 - rl/D5Mit61 - D5Mit72 - Xmv45 - Htr5a - Peplb - D5Nam3-D5Mit66. Estimated distances between reeler and the nearest flanking markers D5Nam1 and D5Mit72 are 1.5 and 1.0 cM, respectively (95% confidence level), suggesting that the region could be physically mapped using a manageable number of YAC clones.     

Mutations in DDHD2, Encoding an Intracellular Phospholipase A1, Cause a Recessive Form of Complex Hereditary Spastic Paraplegia

We report on four families affected by a clinical presentation of complex hereditary spastic paraplegia (HSP) due to recessive mutations in DDHD2, encoding one of the three mammalian intracellular phospholipases A₁ (iPLA₁). The core phenotype of this HSP syndrome consists of very early-onset (<2 years) spastic paraplegia, intellectual disability, and a specific pattern of brain abnormalities on cerebral imaging. An essential role for DDHD2 in the human CNS, and perhaps more specifically in synaptic functioning, is supported by a reduced number of active zones at synaptic terminals in Ddhd-knockdown Drosophila models. All identified mutations affect the protein’s DDHD domain, which is vital for its phospholipase activity. In line with the function of DDHD2 in lipid metabolism and its role in the CNS, an abnormal lipid peak indicating accumulation of lipids was detected with cerebral magnetic resonance spectroscopy, which provides an applicable diagnostic biomarker that can distinguish the DDHD2 phenotype from other complex HSP phenotypes. We show that mutations in DDHD2 cause a specific complex HSP subtype (SPG54), thereby linking a member of the PLA₁ family to human neurologic disease.     

A Radiation Map of the Short Arm of Porcine Chromosome 2

In recent years, maps of mammalian genomes have been acquiring increasingly higher resolution. Integration of maps of different types has become possible. As a tool in integrating maps of mammalian genomes of different types, high-resolution mapping with radiation-induced hybrids (RH) is used. Here, we present an RH₆₀₀₀ map of the short arm of porcine chromosome 2. The map contains 15 microsatellites and five genes (for parathyroid hormone, lactate dehydrogenase A, myogenic factor, follicle-stimulating hormone beta, and calpain I). The RH panel was obtained on the basis of a hybrid cell line bearing the single porcine chromosome 2 against the background of mink chromosomes. The mean frequency of preserving markers examined in the panel was 18.3%. Integration of four genes in the panel and a comparison of gene order in homeologous regions of human and porcine chromosomes are presented.     

Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Ieukoencephalopathy, Genetic Homogeneity, and Mapping of the Locus within a 2-cM Interval

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a recently identified autosomal dominant cerebral arteriopathy characterized by the recurrence of subcortical infarcts leading to dementia. A genetic linkage analysis conducted in two large families recently allowed us to map the affected gene on chromosome 19 in a 12-cM interval bracketed by D19S221 and D19S215. In the present study, these first 2 families and 13 additional ones, including a total of 199 potentially informative meiosis, have been genotyped with eight polymorphic markers located between D19S221 and D19S215. All families were linked to chromosome 19. The highest combined lod score (Z_max = 37.24 at θ = .01) was obtained with marker D19S841, a new CA_n microsatellite marker that we isolated from chromosome 19 cosmids. The recombinant events observed within these families were used to refine the genetic mapping of CADASIL within a 2-cM interval that is now bracketed by D19S226 and D19S199 on 19pl3.1. These data strongly suggest the genetic homogeneity of this recently identified condition and establish the value of its clinical and neuroimaging diagnostic criteria. Besides their importance for the ongoing positional cloning of the CADASIL gene, these data help to refine the genetic mapping of CADASIL relative to familial hemiplegic migraine and hereditary paroxysmal cerebellar ataxia, conditions that we both mapped within the same chromosome 19 region.     

Refinement of the Locus for Autosomal Dominant Hereditary Gingival Fibromatosis (GINGF) to a 3.8-cM Region on 2p21

Hereditary gingival fibromatosis (HGF, MIM 135300; approved gene symbol GINGF) is an oral disease characterized by enlargement of gingiva. Recently, a locus for autosomal dominant HGF has been mapped to an 11-cM region on chromosome 2p21. In the current investigation, we genotyped four Chinese HGF families using polymorphic microsatellite markers on 2p21. The HOMOG test provided evidence for genetic homogeneity, with evidence for linkage in four families (heterogeneity versus homogeneity test HOMOG, χ² = 0.00). A cumulative maximum two-point lod score of 5.04 was produced with marker D2S390 at a recombination frequency of θ = 0 in the four linked families. Haplotype analysis localized the hereditary gingival fibromatosis locus within the region defined by D2S352 and D2S2163. This region overlaps by 3.8 cM with the previously reported HGF region. Single-strand conformation polymorphism and sequence analysis of the coding region of cytochrome P450 1B1 (CYP1B1) excluded it as a likely candidate gene.     

A Genetic Fine Structure Analysis of the Suppressor 3 Locus in Saccharomyces

A meiotic fine structure map of a yeast tyrosine-inserting ochre suppressor, SUP3-omicron, was constructed. This was accomplished by examining ten intragenic suppressor-inactive revertants for their relationship to each other and to the original SUP3-omicron mutation. The second-site revertants map on both sides of the SUP3-omicron mutation. The meiotic map length based on the summation of short intervals is 45+/10(5) asci.     

Lack of Dosage Compensation for an Autosomal Gene Relocated to the X Chromosome in DROSOPHILA MELANOGASTER

Aldehyde oxidase activity has been measured in flies with the structural gene for this enzyme translocated to the X chromosome. These measurements are presented as experimental evidence that, in Drosophila melanogaster, an autosomal gene relocated to the X chromosome is not dosage compensated.     

A Novel Homozygous p.R1105X Mutation of the AP4E1 Gene in Twins with Hereditary Spastic Paraplegia and Mycobacterial Disease

We report identical twins with intellectual disability, progressive spastic paraplegia and short stature, born to a consanguineous family. Intriguingly, both children presented with lymphadenitis caused by the live Bacillus Calmette-Guérin (BCG) vaccine. Two syndromes – hereditary spastic paraplegia (HSP) and mycobacterial disease – thus occurred simultaneously. Whole-exome sequencing (WES) revealed a homozygous nonsense mutation (p.R1105X) of the AP4E1 gene, which was confirmed by Sanger sequencing. The p.R1105X mutation has no effect on AP4E1 mRNA levels, but results in lower levels of AP-4ε protein and of the other components of the AP-4 complex, as shown by western blotting, immunoprecipitation and immunofluorescence. Thus, the C-terminal part of the AP-4ε subunit plays an important role in maintaining the integrity of the AP-4 complex. No abnormalities of the IL-12/IFN-γ axis or oxidative burst pathways were identified. In conclusion, we identified twins with autosomal recessive AP-4 deficiency associated with HSP and mycobacterial disease, suggesting that AP-4 may play important role in the neurological and immunological systems.