Loss of holocytochrome c-type synthetase causes the male lethality of X-linked dominant micro-phthalmia with linear skin defects (MLS) syndrome

Abstract

Girls with MLS syndrome have microphthalmia with linear skin defects of face and neck, sclerocornea, corpus callosum agenesis and other brain anomalies. This X-linked dominant, male-lethal condition is associated with heterozygous deletions of a critical region in Xp22.31, from the 5' untranslated region of MID1 at the telomeric boundary to the ARHGAP6 gene at the centromeric boundary. HCCS, encoding human holocytochrome c-type synthetase, is the only gene located entirely inside the critical region. Because single gene analysis is not feasible in MLS patients (all have deletions), we generated a deletion of the equivalent region in the mouse to study the molecular basis of this syndrome. This deletion inactivates mouse Hccs, whose homologs in lower organisms (cytochrome c or c1 heme lyases) are essential for function of cytochrome c or c1 in the mitochondrial respiratory chain. Ubiquitous deletions generated in vivo lead to lethality of hemizygous, homozygous and heterozygous embryos early in development. This lethality is rescued by expression of the human HCCS gene from a transgenic BAC, resulting in viable homozygous, heterozygous and hemizygous deleted mice with no apparent phenotype. In the presence of the HCCS transgene, the deletion is easily transmitted to subsequent generations. We did obtain a single heterozygous deleted female that does not express human HCCS, which is analogous to the low prevalence of the heterozygous MLS deletion in humans. Through the study of these genetically engineered mice we demonstrate that loss of HCCS causes the male lethality of MLS syndrome.

Human mitochondrial holocytochrome c synthase’s heme binding, maturation determinants, and complex formation with cytochrome c

“…For example, chromosomal mutations in the gene encoding HCCS can lead to a condition called "microphthalmia with linear skin defects syndrome" (25,26). Additionally, a role for HCCS in apoptosis (separate from that of cytochrome c) has been described in injured motor neurons (27).…”

Microphthalmia with linear skin defects (MLS), Aicardi, and Goltz syndromes: are they related X-linked dominant male-lethal disorders?

“…As expected for a gene subject to XCI, the phenotype of these mice is variable: from apparently unaffected and growth delayed to severely runted, with smaller eyes and brain in one animal. We also reported that when the mice are crossed with mice that are transgenic for a human BAC expressing the HCCS gene from its endogenous genomic locus, hemizygous, homozygous and heterozygous deleted animals survive intact (Prakash et al, 2002). These findings prove that loss of function of HCCS causes male lethality and suggest that heterozygous loss of function of this gene causes most phenotypic features of human MLS syndrome in females.…”

“…No male mice hemizygous for this deletion are born, while heterozygous deleted females only rarely survive to adulthood. We previously reported one such surviving animal (Prakash et al, 2002). Since then, extensive breeding has resulted in an additional three heterozygous deleted females, suggesting that, similar to the human disorder, liveborn affected females are rare.…”

Animal models for human contiguous gene syndromes and other genomic disorders

“…Chromosome engineering and transgenesis (Prakash et al, 2002) Duchenne Muscular Dystrophy (DMD) del(X)(p21.2p21.2) 310200 DMD Mdx mouse model (Sicinski et al, 1989); inducible transgenesis (Ahmad et al, 2000); transgenesis (Cox et al, 1993;Phelps et al, 1995;Rafael et al, 1996;Wells et al, 1995) Hyperglycerolemia (GKD) del(X)(p21p21) 307030 GK Gyk targeted mutagenesis (Huq et al, 1997) usually occurs in most patients because of the molecular mechanism that results in fixed breakpoints reflecting genome architecture. In cases wherein it was not possible to identify patients harboring varying-sized deletions/duplications, or single gene mutations were not found, or a translocation disrupting a particular gene in the critical region was not available, murine models have proven to be extremely valuable for elucidating the predominant gene(s) responsible for the phenotype.…”