A genetic linkage study of facioscapulohumeral (Landouzy-Déjérine) disease with 24 polymorphic DNA probes

Molecular genetics of facioscapulohumeral muscular dystrophy (FSHD)

Facioscapulohumeral muscular dystrophy (FSHD; MIM 158900), is an autosomal dominant neuromuscular disorder. The disease is characterized by the weakness of the muscles of the face, upper-arm and shoulder girdle. The gene for FSHD has been mapped to 4q35 (FSHD1A) and is closely linked to D4F1O4S1, which detects two highly polymorphic loci (located at 4q35 and 10q26), with restriction enzyme EcoRI. The polymorphic EcoRI fragment detected with D4F1O4S1 is composed almost entirely of D4Z4 (3.3 kb) tandem repeats. In FSHD patients a deletion of the integral number of D4Z4 repeats generates a fragment which is usually smaller than 35 kb, whereas in normal controls, the size usually ranges from 50 to 300 kb. These 'small' EcoRI fragments segregate with FSHD in families but appear as de novo deletions in the majority of sporadic cases. Each 3.3 kb repeat contains two homeobox domains neither of which has yet been proven to encode a protein. D4Z4 is located adjacent to the 4q telomere and cross hybridizes to several different regions of the genome. Although D4Z4 probably does not encode a protein with any direct association to FSHD, a clear correlation has been shown between the deletion size at this locus and the age at onset of the disease in FSHD patients. In approximately 5-10% of FSHD families the disease locus is unlinked to 4q35 (locus designated FSHD1B), however, none of the non 4q35 loci for FSHD have yet been chromosomally located. Thus so far, only one gene, FRG1 (FSHD region gene 1) has been identified from the FSHD candidate region on 4q35. The apparent low level of expressed sequences from within this region, the integral deletions of D4Z4 repeats observed in FSHD patients and the close proximity of these repeats to the 4q telomere, all suggest that the disease may be the result of position effect variegation. To date, the molecular diagnosis of FSHD with D4F104S1 has been most secure in those families which are linked to other 4q35 markers. Recent studies based on the distinction of 4q35 fragments from those from 10q26 will facilitate molecular diagnosis. The pathophysiology and biochemical defect in FSHD still remains to be elucidated. The identification of the FSHD gene and characterization of the gene product will not only potentiate accurate diagnosis but may also unravel the complexities of the 4q35 FSHD region.

Facioscapulohumeral muscular dystrophy: the impact of genetic research

Recent developments in genetic research have led to the localization and identification of the causative gene defect in a large number of neurological diseases. This paper describes some of the basic principles of molecular genetics and the strategies that have been followed in the search for the gene for facioscapulohumeral muscular dystrophy (FSHD), beginning with the recent localization to chromosome 4q. Many questions remain concerning the pathogenesis and possible genetic heterogeneity of this autosomal dominant myopathy. Hitherto, most evidence favours a genetically homogeneous disorder, but only the isolation and detailed characterization of the FSHD gene will resolve these issues completely.

The mapping of chromosome 4q markers in relation to facioscapulohumeral muscular dystrophy (FSHD)

Four DNA markers on the distal long arm of chromosome 4 have been analyzed for their linkage relationship to facioscapulohumeral muscular dystrophy (FSHD) in a series of 23 families with this disease. Two hypervariable markers, pH30 (D4S139) and EFD 139.1 (D4S184), both show close linkage with the disorder, with a maximum recombination fraction (theta max) of .02 and a maximum lod score (Zmax) of 36.77 and 34.50, respectively; two other markers, the locus for factor XI (F11) and the microsatellite marker Mfd22 (D4S171), both show less close linkage, with respective theta max of .16 (Zmax = 3.40) for F11 and .24 (Zmax = 1.61) for D4S171. While the relative ordering and orientation of the loci on the chromosome remain provisional, analysis of 15 individual recombination events in seven families supports the order D4S171-F11-D4S184-D4S139-FSHD, with the disease locus telomeric to all four markers.

Linkage localization of facioscapulohumeral muscular dystrophy (FSHD) in 4q35

Fasioscapulohumeral muscular dystrophy (FSHD) has recently been localized to 4q35. We have studied four families with FSHD. Linkage to the 4q35 probes D4S163, D4S139, and D4S171 was confirmed. We found no recombinants helpful in detailed localization of the FSHD gene. Two of our families include males with a rapidly progressive muscle disease that had been diagnosed, on the basis of clinical features, as Duchenne muscular dystrophy. One of these males is available for linkage study and shares the haplotype of his FSHD-affected aunt and cousin.

A closely linked DNA marker for facioscapulohumeral disease on chromosome 4q

Close linkage of a hypervariable DNA probe on chromosome 4q (pH30, locus D4S139) has been found with the locus for facioscapulohumeral disease. Three recombinants were identified in a total of 140 meioses, giving a maximum lod score of 36.77 at a recombination fraction of 0.02. All but two of the families studied proved informative with this probe; all informative families showed evidence of linkage (except one family with a single scorable meiosis), making genetic heterogeneity unlikely from our data. The close linkage and highly informative nature of the probe will make it suitable for clinical application in presymptomatic and prenatal diagnosis. We have also confirmed loose linkage with the marker (Mfd22, locus D4S171) used to establish the initial assignment of the disorder to chromosome 4.

Deficiency of complex III of the mitochondrial respiratory chain in a patient with facioscapulohumeral disease

Facioscapulohumeral disease (FSHD), an inherited neuromuscular disorder, is characterized by progressive wasting of specific muscle groups, particularly the proximal musculature of the upper limbs; the primary defect in this disorder is unknown. We studied a patient with FSHD to determine whether the mitochondrial respiratory chain was functionally abnormal. Muscle biopsy revealed fiber atrophy with patchy staining for oxidative enzymes. Electron microscopy of a liver section showed many enlarged mitochondria with paracrystalline inclusions. Decreased oxidation of the respiratory substrates-alanine and succinate-in skin fibroblasts suggested a deficiency of complex III of the electron-transport chain; cytochrome c oxidase activity (complex IV) was in the normal range. Biochemical analysis of liver supported the fibroblast data, since succinate oxidase activity (electron-transport activity through complexes II-IV) was reduced, whereas complex IV activity was normal. Furthermore, analysis of the cytochrome spectrum in liver revealed typical peaks for cytochromes cc1 and aa3, whereas cytochrome b (a component of complex III) was undetectable. Southern blot analysis of fibroblast mtDNA revealed no major deletions or rearrangements. Our study provides the first documentation of a specific enzyme-complex deficiency associated with FSHD.

A clinically homogeneous group of families with facioscapulohumeral (Landouzy-Déjérine) muscular dystrophy: linkage analysis of six autosomes

Facioscapulohumeral muscular dystrophy (FSHMD) is a neuromuscular disorder characterized by autosomal dominant inheritance and clinical onset in the muscles of the face and shoulder girdle. Using a set of RFLP markers spaced at approximately 20 centimorgans, we have begun a systematic search for markers linked to the disease. A total of 81 RFLP loci on six autosomes (1, 2, 5, 7, 10, and 16) have been examined for linkage to FSHMD in 13 families. With the computer program CRI-MAP, two-point and multipoint analyses have not resulted in any LOD score indicative of linkage to FSHMD. However, these analyses have allowed us to exclude 909 centimorgans (sex average) of our genetic maps in intervals where the LOD score is less than -2.0. We estimate our data have excluded 23% of the human genome.

An exclusion map for facioscapulohumeral (Landouzy-Déjérine) disease

By using the genetic linkage data between the facioscapulohumeral muscular dystrophy (FSHD) gene and 57 markers on various autosomes, we have constructed an exclusion map for this disorder. The maximum likelihood location of the FSHD gene and the percentage of the excluded areas on each chromosome are presented here. This exclusion map shows that more than 80% of the genome has been excluded as a likely location of any locus responsible for FSHD in the majority of families. Chromosomes 3, 5, 10, 11, 15, and 19 remain largely unexcluded. Concentration on the highlighted areas of the genome should facilitate the identification of the site of the FSHD gene.