A functional SNP in the regulatory region of the decay-accelerating factor gene associates with extraocular muscle pareses in myasthenia gravis

Complement activation in myasthenia gravis (MG) may damage muscle endplate and complement regulatory proteins such as decay-accelerating factor (DAF) or CD55 may be protective. We hypothesize that the increased prevalence of severe extraocular muscle (EOM) dysfunction among African MG subjects reported earlier may result from altered DAF expression. To test this hypothesis, we screened the DAF gene sequences relevant to the classical complement pathway and found an association between myasthenics with EOM paresis and the DAF regulatory region c.-198C>G SNP (odds ratio=8.6; P=0.0003). This single nucleotide polymorphism (SNP) results in a twofold activation of a DAF 5′-flanking region luciferase reporter transfected into three different cell lines. Direct matching of the surrounding SNP sequence within the DAF regulatory region with the known transcription factor-binding sites suggests a loss of an Sp1-binding site. This was supported by the observation that the c.-198C>G SNP did not show the normal lipopolysaccharide-induced DAF transcriptional upregulation in lymphoblasts from four patients. Our findings suggest that at critical periods during autoimmune MG, this SNP may result in inadequate DAF upregulation with consequent complement-mediated EOM damage. Susceptible individuals may benefit from anti-complement therapy in addition to immunosuppression.

Stickler-like syndrome due to a dominant negative mutation in the COL2A1 gene

The type II collagenopathies include a wide spectrum of phenotypes ranging from mild spondylo epiphyseal dysplasia (SED) to severe achondrogenesis/hypochondrogenesis. Several attempts have been made at providing phenotype-genotype correlations in this group of disorders. In this report we discuss a South African family in which four members have a phenotype resembling Stickler syndrome type 1. Ocular problems and conductive deafness predominate, while skeletal changes resemble those of a mild form of multiple epiphyseal dysplasia (MED). In distinction to the classical form of Stickler syndrome, the affected persons have stubby digits. DNA analysis of the exons of the COL2A1 gene documented a C-T transversion in exon 39, resulting in an Arg704Cys substitution in the triple helical domain of the type II collagen peptide; this nontermination mutation may be indicative of further heterogeneity in the Stickler group of disorders or of a new syndrome amongst the type II collagenopathies.

Multiple epiphyseal dysplasia, ribbing type: a novel point mutation in the COMP gene in a South African family

Multiple epiphyseal dysplasia is broadly categorised into the more severe Fairbank and the milder Ribbing types. In this paper we document mild MED in a South African kindred, and demonstrate that heterozygosity for a mutation in the cartilage oligomeric matrix protein (COMP) gene causes the condition. The mutation, C1594G, implies a N523K substitution, altering a residue at the carboxyl-terminal end of the calmodulin-like region of COMP. The identification of this mutation demonstrates that the spectrum of manifestations from mild MED through pseudoachondroplasia can all be produced by structural mutations in COMP.

Mseleni joint disease--a molecular genetic approach to defining the aetiology

Mseleni joint disease (MJD) is an unusual form of progressive and widespread degenerative osteoarthropathy that has been identified in several hundred people in the remote Mseleni region of northern Zululand. Affected individuals experience articular discomfort in childhood and may be seriously handicapped as adults, often requiring prosthetic hip joint replacement. Although the condition clusters in families, there is no evidence of Mendelian inheritance and assessment of affected kindreds has not shown any evidence of genomic imprinting. To date our molecular work-up has entailed the study of 47 affected individuals from MJD kindreds to investigate familial predisposition based on the inheritance of a subset of markers and/or genes on the human genome, particularly those associated with the cartilage matrix. In addition, we have collected blood specimens form 111 unaffected but unrelated individuals from the same population group in order to determine whether any relationship exists between genetic components of the human leucocyte antigen (HLA) system and the MJD phenotype. Our investigations show the following: (i) there is no association between MJD and the HLA system which has previously been associated with non-Mendelian genetic conditions; (ii) COL2A1, which has been implicated in some forms of spondylo-epiphyseal dysplasia, may be involved in at least a subset of MJD patients; and (iii) type VI collagen is overabundant in degenerated hip joint cartilage.

Duchenne and Becker muscular dystrophy prevalence in South Africa and molecular findings in 128 persons affected

A genetic service for Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) was initiated in Cape Town in 1987. Of the 143 DMD patients diagnosed during the period 1987-1992, 66 had a familial pattern of inheritance and 77 were apparently sporadic. Twenty BMD patients were identified, of whom 12 had other affected relatives and 8 were sporadic. Overall minimum prevalence rates of 1/100,000 for DMD and 1/755,000 for BMD were calculated. A markedly low DMD prevalence in the indigenous black population (1/250,000) contributed to the overall low DMD prevalence in South Africa when compared with that in the UK (1/40,000). By means of molecular methods, the diagnosis in 42% of the affected DMD males was confirmed by detection of deletions in the dystrophin gene. Deletions were identified in 50% of Indian, white and mixed ancestry patients. In contrast, only 22% of blacks had identifiable deletions. DMD appears to be underrepresented in the black population; the low deletion frequency in this group suggests that unique mutations not detectable by methods used in this study may be more frequent in these patients than in the other populations. The increased DMD frequency in Indians corroborates findings reported from the UK.

Duchenne muscular dystrophy--a molecular service

In 1987 a carrier detection and prenatal diagnostic service for Duchenne muscular dystrophy using molecular technology was instituted at the Department of Human Genetics, University of Cape Town, to serve affected families in southern Africa. DNA samples from 100 affected male subjects and 350 of their relatives from a total of 110 families have been banked. To date restriction fragment length polymorphism (RFLP) analysis and deletion screening has been performed on the DNA of 60 male patients and 116 female relatives at risk of being carriers of the faulty gene. The DNA probes used were pERT 87-1 (MspI polymorphism) pERT 87-15, pXJ1.1, pXJ2.3 (TaqI, polymorphism), pXJ1.2 (BclI polymorphism), P20 (MspI and EcoRV polymorphism) and the cDNA probes. DNA deletions have been detected in 30 of the 60 affected boys and the carrier risks of 49 women have been determined by RFLP analyses. In those families where the risks were uncertain because the affected males had died, prenatal exclusion testing was offered to potential carriers. Two pregnancies were terminated when male fetuses were shown to be affected, since they had the same deletion as that observed in the proband.

X-linked spastic paraplegia: evidence for homogeneity with a variable phenotype

Hereditary spastic paraplegia (HSP) is rarely inherited in an X-linked recessive mode in pure and complicated forms. Recently, molecular linkage studies have suggested that these variant X-linked HSP conditions result from locus heterogeneity. In this paper we report on the clinical and linkage analysis of a kindred with complicated X-linked HSP. The finding in this family of a map location of the putative HSP gene in the same region as the documented for the pure HSP gene provides evidence that allelic mutations might also be responsible for the variable phenotype encountered in these X-linked disorders.

X-linked mixed deafness with stapes fixation in a Mauritian kindred: linkage to Xq probe pDP34

Molecular linkage analysis was undertaken on a large Mauritian kindred with X-linked mixed deafness, stapes fixation, and perilymphatic gusher (X-LDSF). DNA probe pDP34 (DXYS1) was tightly linked to the disorder, with a lod score of 6.32 at zero recombination. This observation indicates that the gene for this form of deafness maps to the Xq13-q21.1 region and has important implications for carrier screening and antenatal diagnosis.

Duchenne muscular dystrophy in South Africa. Prevention by molecular techniques

The availability of DNA markers closely linked to the Duchenne muscular dystrophy (DMD) gene locus has facilitated carrier detection and prenatal diagnosis of this condition. More than 50 affected South African kindreds are being studied using DNA probes within and flanking the DMD region of the X chromosome in order to ascertain the nature of the molecular defects in affected males and to investigate the feasibility of genetic management by means of these techniques. The results of this study and the implications of this new molecular technology for DMD patients in South Africa are reviewed and discussed.