Mice deficient for the RB gene (RB-/-), prior to death at embryonic day 14.5, show increased cell death in all tissues that normally express RBI: the nervous system, liver, lens, and skeletal muscle precursor cells. We have generated transgenic mice (RBlox) that express low levels of pRb, driven by an RB1 minigene. RBIox/RB -/-mutant fetuses die at birth with specific skeletal muscle defects, including increased cell death prior to myoblast fusion, shorter myotubes with fewer myofibrils, reduced muscle fibers, accumulation of elongated nuclei that actively synthesized DNA within the myotubes, and reduction in expression of the late muscle-specific genes MCK and MRF4. Thus, insufficient pRb results in failure of myogenesis in vivo, manifest in two ways. First, the massive apoptosis of myoblasts implicates a role of pRb in cell survival. Second, surviving myotubes failed to develop normally and accumulated large polyploid nuclei, implicating pRb in permanent withdrawal from the cell cycle. These results demonstrate a role for pRb during terminal differentiation of skeletal muscles in vivo and place pRb at a nodal point that controls cell proliferation, differentiation, and death.[Key Words: Retinoblastoma gene; differentiation; myogenesis; cell cycle control; apoptosis] Received July 25, 1996; revised version accepted October 16, 1996.Terminal differentiation is a dynamic process coupled to cell cycle arrest that requires continuous active control (Blau 1993). The retinoblastoma gene (RB1) product (pRB) has been implicated in cell cycle exit and terminal differentiation. For example, SV40 large T antigen, which binds and inactivates pRb, can stimulate differentiated myotubes in culture to reenter the cell cycle (Gu et al. 1993). RB1 is a tumor suppressor gene, absence of which predisposes individuals to retinoblastoma in infancy and, to a lesser extent, osteosarcoma in the second decade of life, at times when these tissues normally undergo terminal differentiation (for review, see Zacksenhaus et al. 1993a). Inactivation of RB1 also contributes to the malignant progression of a wide spectrum of tumors including breast, prostate, lung, and bladder. Moreover, tumors with apparently normal RB1 frequently contain mutations in the pathway that regulates pRb function, 4These authors contributed equally to this work. 5Corresponding authors. resulting in inactivation of pRb (for review, see Weinberg 1995).pRb is a member of a family of proteins including p107 (Ewen et al. 1991) and p130 (Harmon et al. 1993;Li et al. 1993) that interact with transcription factors and viral oncoproteins through shared conserved domains. The RB family of proteins exerts a negative effect on cell proliferation by modulating the activity of certain transcription factors (Defeo-Jones et al.

Global Retinoblastoma Study Group IMPORTANCE Early diagnosis of retinoblastoma, the most common intraocular cancer, can save both a child's life and vision. However, anecdotal evidence suggests that many children across the world are diagnosed late. To our knowledge, the clinical presentation of retinoblastoma has never been assessed on a global scale.OBJECTIVES To report the retinoblastoma stage at diagnosis in patients across the world during a single year, to investigate associations between clinical variables and national income level, and to investigate risk factors for advanced disease at diagnosis. DESIGN, SETTING, AND PARTICIPANTSA total of 278 retinoblastoma treatment centers were recruited from June 2017 through December 2018 to participate in a cross-sectional analysis of treatment-naive patients with retinoblastoma who were diagnosed in 2017. MAIN OUTCOMES AND MEASURESAge at presentation, proportion of familial history of retinoblastoma, and tumor stage and metastasis. RESULTSThe cohort included 4351 new patients from 153 countries; the median age at diagnosis was 30.5 (interquartile range, 18.3-45.9) months, and 1976 patients (45.4%) were female. Most patients (n = 3685 [84.7%]) were from low-and middle-income countries (LMICs). Globally, the most common indication for referral was leukocoria (n = 2638 [62.8%]), followed by strabismus (n = 429 [10.2%]) and proptosis (n = 309 [7.4%]). Patients from high-income countries (HICs) were diagnosed at a median age of 14.1 months, with 656 of 666 (98.5%) patients having intraocular retinoblastoma and 2 (0.3%) having metastasis. Patients from low-income countries were diagnosed at a median age of 30.5 months, with 256 of 521 (49.1%) having extraocular retinoblastoma and 94 of 498 (18.9%) having metastasis. Lower national income level was associated with older presentation age, higher proportion of locally advanced disease and distant metastasis, and smaller proportion of familial history of retinoblastoma. Advanced disease at diagnosis was more common in LMICs even after adjusting for age (odds ratio for low-income countries vs upper-middle-income countries and HICs, 17.92 [95% CI,, and for lower-middle-income countries vs upper-middle-income countries and HICs, 5.74 [95% CI,). CONCLUSIONS AND RELEVANCEThis study is estimated to have included more than half of all new retinoblastoma cases worldwide in 2017. Children from LMICs, where the main global retinoblastoma burden lies, presented at an older age with more advanced disease and demonstrated a smaller proportion of familial history of retinoblastoma, likely because many do not reach a childbearing age. Given that retinoblastoma is curable, these data are concerning and mandate intervention at national and international levels. Further studies are needed to investigate factors, other than age at presentation, that may be associated with advanced disease in LMICs.

Hereditary retinoblastoma is an autosomal dominant disorder caused by mutations in the RB1 gene. Analysis of this rare condition has helped to elucidate the mechanisms underlying hereditary cancer predisposition in general. As identification of RB1 gene mutations has become a part of clinical management of patients with retinoblastoma, there is now a wealth of data. In this article, we summarize the current knowledge on the relations between the genotype and phenotypic expression. Moreover, detailed analysis of genotype-phenotype relations shows that hereditary retinoblastoma has features of a complex trait.

No abstract

Studies of retinoblastoma clearly identify mutation of the RB1 gene on chromosome 13 as the primary cause of this cancer. However, all retinoblastoma tumors have an abnormal karyotype (1, 2) indicating the presence of additional mutations and suggesting that mutation of both RB1 alleles is insufficient for development of retinoblastoma. In addition, analysis of RB1 expression and of RB1 mutations in different tumors leads to the following dilemma: while the RB1 gene product, p110RB1, is expressed in most dividing cells, germline mutations inactivating the function of p110RB1 predispose primarily to retinoblastoma and to a lesser extent to osteosarcoma, but do not predispose to cancer in general. However, many tumors contain somatic mutations that disrupt RB1 function. Thus, we are faced with the unusual situation in which germline mutations in the RB1 gene predispose to a very limited set of cancers, but somatic mutations in RB1 appear to contribute to malignancy in many tissues. We propose that the role of the RB1 gene is to maintain the cells in a stable, quiescent state required for terminal differentiation and that the effect of RB1 mutations in different tissues depends on the pattern of differentiation in that tissue. In tissues where differentiation follows a linear process from undifferentiated precursors to fully differentiated cells, loss of RB1 function during early stages of differentiation may lead to uncontrolled growth and the development of cancer. On the other hand, in cell renewal systems where cell number is usually maintained by a process of programmed cell death (PCD) or apoptosis, loss of RB1 function may lead to cell death.