Evidence for genetic heterogeneity in families with congenital motor nystagmus (CN).

Congenital motor nystagmus (CN) is a relatively common genetic disorder (approximately 1 in 1500) characterized by bilateral involuntary ocular oscillations, with onset occurring within the first six months of life. To date, three loci associated with CN have been mapped to chromosomes 6p12, Xp11.4-p11.3, and Xq26-q27. We analyzed five pedigrees segregating for CN. Mapping studies using markers in these three regions showed that only one pedigree exhibited suggestive linkage with a lod score of 2.08, straight theta=0.0, at chromosome Xp11. This pedigree had both affected male and female members, with two unaffected obligate female carriers. The remaining four pedigrees did not exhibit evidence of linkage for any of the three chromosome locations. Three of the pedigrees, Pedigrees 2, 4, and 5, exhibited several instances of male-to-male transmission, excluding X-linkage, and exhibited a lod score of -3.82, straight theta=0.0, for marker D6S459 located at 6p12, thus excluding the chromosome 6 locus. This provides evidence for at least a fourth locus associated with CN.