Disruption of CCTβ2 Expression Leads to Gonadal Dysfunction

Abstract

There are two mammalian genes that encode isoforms of CTP:phosphocholine cytidylyltransferase (CCT), a key rate-controlling step in membrane phospholipid biogenesis. Quantitative determination of the CCT transcripts reveals that CCTα is ubiquitously expressed and is found at the highest levels in the testis and lung, with lower levels in the liver and ovary. CCTβ2 is a very minor isoform in most tissues but is significantly expressed in the brain, lung, and gonads. CCTβ3 is the third isoform recently discovered in mice and is expressed in the same tissues as CCTβ2, with its highest level in testes. We investigated the role(s) of CCTβ2 by generating knockout mice. The brains and lungs of mice lacking CCTβ2 expression did not exhibit any overt defects. On the other hand, a large percentage of the CCTβ2^−/− females were sterile and their ovaries exhibited defective ovarian follicle development. The proportion of female CCTβ2^−/− mice with defective ovaries increased as the animals aged. The rare litters born from CCTβ2^−/− × CCTβ2^−/0 matings had the normal number of pups. The abnormal ovarian histopathology was characterized by disorganization of the tissue in young adult mice and absence of follicles and ova in older mice, along with interstitial stromal cell hyperplasia which culminated in the emergence of tubulostromal ovarian tumors by 16 months of age. Grossly defective CCTβ2^−/− ovaries were associated with high follicle-stimulating (FSH) and luteinizing (LH) hormone levels. Male CCTβ2^−/0 mice exhibited progressive multifocal testicular degeneration and reduced fertility but had normal FSH and LH levels. Thus, the most notable phenotype of CCTβ2 knockout mice was gonad degeneration and reproductive deficiency. The results indicate that although CCTβ2 is expressed at very low levels compared to the α-isoform, loss of CCTβ2 expression causes a breakdown in the gonadal response to hormonal stimulation.

We thank Kelli Boyd for helpful assistance in analyzing brain pathologies, Andy Rock for the lipid analysis of mouse tissues, and Chuck Rock for critical reading of the manuscript. We also thank Jim Ihle for plasmid constructs and support at the beginning of this project and Peter McKinnon and Gerard Grosveld for advice.

This work was supported by National Institutes of Health grant GM 45737 (to S.J.), Cancer Center (CORE) support grant CA 21765, and the American Lebanese Syrian Associated Charities. This research was also supported by NICHD/NIH through cooperative agreement U54 HD28934 as part of the Specialized Cooperative Centers Program in Reproduction Research.