Autosomal dominant progressive external ophthalmoplegia is a rare human disease that shows a Mendelian inheritance pattern, but is characterized by large-scale mitochondrial DNA (mtDNA) deletions. We have identified two heterozygous missense mutations in the nuclear gene encoding the heart/skeletal muscle isoform of the adenine nucleotide translocator (ANT1) in five families and one sporadic patient. The familial mutation substitutes a proline for a highly conserved alanine at position 114 in the ANT1 protein. The analogous mutation in yeast caused a respiratory defect. These results indicate that ANT has a role in mtDNA maintenance and that a mitochondrial disease can be caused by a dominant mechanism.

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The molecular mechanisms by which the nuclear genome regulates the biosynthesis of mitochondrial DNA (mtDNA) are only beginning to be unravelled. A naturally occurring in vivo model for a defect in this cross-talk of two physically separate genomes is a human disease, an autosomal dominant progressive external ophthalmoplegia, in which multiple deletions of mtDNA accumulate in the patients' tissues. The assignment of this disease locus to 10q 23.3-24.3 is the first direct evidence for involvement of both nuclear and mitochondrial genomes in a single disorder.

Diseases caused by nuclear genes that affect mitochondrial DNA (mtDNA) stability are an interesting group of mitochondrial disorders, involving both cellular genomes. In these disorders, a primary nuclear gene defect causes secondary mtDNA loss or deletion formation, which leads to tissue dysfunction. Therefore, the diseases clinically resemble those caused by mtDNA mutations, but follow a Mendelian inheritance pattern. Several clinical entities associated with multiple mtDNA deletions have been characterized, the most frequently described being autosomal dominant progressive external ophthalmoplegia (adPEO). MtDNA depletion syndrome (MDS) is a severe disease of childhood, in which tissue-specific loss of mtDNA is seen. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients may have multiple mtDNA deletions and/or mtDNA depletion. Recent reports of thymidine phosphorylase mutations in MNGIE and adenine nucleotide translocator mutations in adPEO have given new insights into the mechanisms of mtDNA maintenance in mammals. The common mechanism underlying both of these gene defects could be disturbed mitochondrial nucleoside pools, the building blocks of mtDNA. Future studies on MNGIE and adPEO pathogenesis, and identification of additional gene defects in adPEO and MDS will provide further understanding about the mammalian mtDNA maintenance and the crosstalk between the nuclear and mitochondrial genomes.

Mutations of UFD1L Are Not Responsible for the Majority of Cases of DiGeorge Syndrome/ Velocardiofacial Syndrome without Deletions within Chromosome 22q11 To the Editor: Deletions of chromosome 22q11 are associated with a wide spectrum of congenital malformation, encompassed by the acronym "CATCH22" (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate, and hypocalcemia on chromosome 22), including velocardiofacial syndrome (VCFS; MIM 192430), DiGeorge syndrome (DGS; MIM 188400), and conotruncal-anomaly face (Emanuel et al. 1998). The major anomalies include outflow-tract congenital heart defects, hypoplasia of the parathyroids and thymus, craniofacial dysmorphism, and learning/behavioral problems (Ryan et al. 1997). Many of these are thought to be due to a defective neural-crest contribution during development. The DiGeorge chromosomal region (DGCR) is entirely cloned (Carlson et al. 1997) and sequenced, and several genes have been reported mapping to the region. Mutation screens of genes mapping to the proximal end of this region, termed the "minimal DiGeorge chromosomal region" (MDGCR; Gong et al. 1996), have been negative (Wadey et al. 1995; Gong et al. 1997; Gottlieb et al. 1997; Lindsay et al. 1998). Attention therefore has turned to the regions adjacent and distal to the MDGCR. Recently, the gene UFD1L was proposed as the major gene haploinsufficient in this group of syndromes (Yamagishi et al. 1999). UFD1L is downstream of dHAND, a gene known to be involved in control of the development of structures affected in DGS, and Ufd1l is expressed in the branchial arches, frontonasal mass, and outflow tract. In addition, a single patient has been reported with a de novo deletion affecting UFD1L and the neighboring gene, CDC45L2 (Yamagishi et al. 1999). CDC45 is required for initiation of DNA replication in yeast, and CDC45 mutants are nonviable. However, CDC45L2 expression is not altered in d-HAND Ϫ/Ϫ embryos. On the basis of these findings, Yamagishi and colleagues concluded that UFD1L hap-conditions can be obtained at the e-mail addresses that follow: