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Leber Hereditary Optic Neuropathy (LHON) is a maternally inherited blinding disease caused by missense mutations in the mitochondrial DNA (mtDNA). However, incomplete penetrance and a predominance of male patients presenting with vision loss suggest that modifying factors play an important role in the development of the disease. Evidence from several studies suggests that both nuclear modifier genes and environmental factors may be necessary to trigger the optic neuropathy in individuals harboring an LHON-causing mtDNA mutation. Recently, an optic neuropathy susceptibility locus at Xp21-Xq21 has been reported. In this study, we performed X-chromosomal linkage analysis in a large Brazilian family harboring a homoplasmic G11778A mtDNA mutation on a haplogroup J background. We report the identification of a novel LHON susceptibility locus on chromosome Xq25-27.2, with multipoint non-parametric linkage scores of > 5.00 (P = 0.005) and a maximum two-point non-parametric linkage score of 10.12, (P = 0.003) for marker DXS984 (Xq27.1). These results suggest genetic heterogeneity for X-linked modifiers of LHON.

Purpose: Axenfeld-Rieger syndrome is a genetically heterogeneous, autosomal dominant disorder that is characterized by anterior segment defects, glaucoma, and extraocular anomalies. This study examined the two genes known to cause Rieger syndrome, PITX2 and FOXC1, for mutations in five Brazilian families with Axenfeld-Rieger syndrome., Methods: Five families with a total of 23 persons affected by Axenfeld-Rieger syndrome were recruited for this study. A sequencing-based mutation screen was undertaken for the PITX2 and FOXC1 genes. Linkage analysis was used to study one large family for which no mutations were detected in the PITX2 or FOXC1 genes., Results: Two of the five families harbored mutations in the PITX2 gene, but none of the families had a detectable FOXC1 mutation. Haplotypic analysis of three Rieger syndrome regions in a large family with Axenfeld-Rieger syndrome excluded linkage to the 4q25 (PITX2), 6p25 (FOXC1), and 13q14 (RIEG2) regions., Conclusions: It appears that the PITX2 gene is responsible for a significant portion of Axenfeld-Rieger syndrome in the Brazilian population. Furthermore, there is also evidence for the presence of genetic heterogeneity of the disorder within the Brazilian population. Finally, a large family with Axenfeld-Rieger syndrome has been identified that does not appear to harbor any of the three known loci. Axenfeld-Rieger syndrome gene segregation in this family likely represents a novel locus.