CARF, also known as CDKN2AIP and Collaborator of ARF, is a multi-module regulator with a significant impact on cell proliferation. It functions as a key regulator in the p53-HDM2-p21WAF1 axis, interacting with ARF, p53, and HDM2 proteins [1,2,4,5]. It stabilizes the p53-tumor suppressor protein either in an ARF-dependent or-independent manner, transcriptionally represses HDM2, and is in turn degraded by HDM2, forming a self-regulatory loop. CARF is involved in regulating DNA damage response, cell cycle checkpoints, cell survival, and death signaling pathways, and is crucial for determining cell fate in processes like senescence and malignant transformation [1,2,4].

CARF knockout mouse models have revealed its important physiological functions. Male CARF-/-mice show impaired fertility and Sertoli-cell-only (SCO) syndrome phenotypes. Loss of CARF in Sertoli cells reduces GDNF expression, hindering spermatogonial stem cells (SSCs) self-renewal, and its loss in undifferentiated spermatogonia impairs their proliferation, both of which lead to SCO syndrome phenotypes. These phenotypes can be rescued by reactivating the Wnt signaling pathway, indicating CARF promotes spermatogonial self-renewal and proliferation through this pathway [3]. Additionally, male mice deficient in Carf expression present impaired spermatogenesis and fertility. CARF interacts with key splicing factors like PABPC1, targets mRNAs in spermatocytes, and its deficiency leads to aberrant splicing patterns. It also affects PIWIL1 expression and localization, influencing the ratio of pachytene-piRNA, suggesting its importance in regulating alternative splicing in spermatogenesis [6].

In conclusion, CARF plays essential roles in regulating cell fate, especially in processes related to cell proliferation, senescence, and spermatogenesis. The use of CARF knockout mouse models has been instrumental in uncovering its functions in male infertility-related conditions, providing insights into the underlying molecular mechanisms and potential therapeutic targets for such diseases.