Cloning and Characterization of a Putative Human Holocytochromec-Type Synthetase Gene (HCCS) Isolated from the Critical Region for Microphthalmia with Linear Skin Defects (MLS)

Abstract

Microphthalmia with linear skin defects syndrome (MLS) is an X-linked male-lethal disorder associated with X chromosomal rearrangements resulting in monosomy from Xpter to Xp22. Features include micro- phthalmia, sclerocornea, linear skin defects, and agenesis of the corpus callosum. Using a cross-species conservation strategy, an expressed sequence from the 450- to the 550-kb MLS critical region on Xp22 was identified by screening a human embryo cDNA library. Northern analysis revealed a transcript of ∼2.6 kb in all tissues examined, with weaker expression of ∼1.2- and ∼5.2-kb transcripts. The strongest expression was observed in heart and skeletal muscle. Sequence analysis of a 3-kb cDNA contig revealed an 807-bp open reading frame encoding a putative 268-amino-acid protein. Comparison of the sequence with sequences in the databases revealed homology with holocyto- chromec-type synthetases, which catalyze the covalent addition of a heme group ontoc-type cytochromes in the mitochondria. Thec-type cytochromes are required for proper functioning of the electron transport pathway. The human gene (HGMW-approved symbol HCCS) and the corresponding murine gene characterized in this paper are the first mammalian holocyto- chromec-type synthetases to be described in the literature. Because of the lack of a neuromuscular phenotype in MLS, it is uncertain whether the deletion of a mitochondrial holocytochrome synthetase would contribute to the phenotype seen in MLS. The expression pattern of this gene and knowledge about the function of holocytochrome synthetases, however, suggest that it is a good candidate for X-linked encephalomyopathies typically associated with mitochondrial dysfunction.