Haplotype analysis of markers on chromosome 2q21.2-q22 in nemaline myopathy in the Amish: Evidence for genetic heterogeneity. J.J. Johnston¹, R.I. Kelley^2,3, D.H. Morton^2,3, T.O. Crawford⁴, L.G. Biesecker¹, C.A. Francomano¹. 1) NHGRI, NIH, Bethesda, MD; 2) Department of Pediatrics, Johns Hopkins University, Baltimore, MD; 3) The Clinic for Special Children, Strasburg, PA; 4) Department of Neurology, Johns Hopkins University, Baltimore, MD.

   We have previously described an autosomal-recessive form of nemaline myopathy. This disorder of both muscle and central motor function has an incidence of 1 in 500 among the Lancaster County Old Order Amish. The disease has become known as "chicken breast disease" among the Amish because the affected children develop a severe pectus carinatum preterminally. The first signs of the disease appear at birth with excessive jitteriness, ankle clonus, and multiple large joint contractures, although some mothers report abnormal vibratory fetal movements from the 7th month gestation. The jitteriness resolves as the children lose their deep tendon reflexes over the first two to three months. Thereafter they remain hypotonic, but later they develop a rigid pectus carinatum with progressively restricted thoracic movement and die of pulmonary insufficiency in the second to fourth year. Intelligence throughout is normal. Quadriceps muscle biopsy of three children showed disproportionately small type I myofibers with Z-band streaming and abundant nemaline rods. Nebulin is a skeletal muscle protein that is believed to regulate the assembly of Z disks, and nebulin mutations have been identified in families with recessive nemaline myopathy. We have collected DNA from a large Old Order Amish pedigree including samples from seven affected children to perform linkage analysis. Our initial studies have focused on nebulin as a candidate gene for this disorder. Haplotype analysis of several microsatellite markers from the 4-centimorgan region surrounding the nebulin gene on chromosome 2q21.2-q22 has shown recombination between the markers and the disease, excluding nebulin as a candidate gene for this disorder, and demonstrating genetic heterogeneity among families with autosomal recessive nemaline myopathy. We are now focusing on a genome wide screen to identify the gene which is responsible for this disorder in the Amish population.