Mutations in the small GTPase gene RAB39B are responsible for X-linked mental retardation associated with autism, epilepsy, and macrocephaly.

Human Mental Retardation (MR) is a common and highly heterogeneous pediatric disorder affecting around 3% of the general population; at least 215 X-linked MR (XLMR) conditions have been described, and mutations have been identified in 83 different genes, encoding proteins with a variety of function, such as chromatin remodeling, synaptic function, and intracellular trafficking. The small GTPases of the RAB family, which play an essential role in intracellular vesicular trafficking, have been shown to be involved in MR. We report here the identification of mutations in the small GTPase RAB39B gene in two male patients. One mutation in family X (D-23) introduced a stop codon seven amino acids after the start codon (c.21C > A; p.Y7X). A second mutation, in the MRX72 family, altered the 5' splice site (c.215+1G > A) and normal splicing. Neither instance produced a protein. Mutations segregate with the disease in the families, and in some family members intellectual disabilities were associated with autism spectrum disorder, epileptic seizures, and macrocephaly. We show that RAB39B, a novel RAB GTPase of unknown function, is a neuronal-specific protein that is localized to the Golgi compartment. Its downregulation leads to an alteration in the number and morphology of neurite growth cones and a significant reduction in presynaptic buttons, suggesting that RAB39B is required for synapse formation and maintenance. Our results demonstrate developmental and functional neuronal alteration as a consequence of downregulation of RAB39B and emphasize the critical role of vesicular trafficking in the development of neurons and human intellectual abilities.

Quantification of Self-renewal in Murine Mammosphere Cultures.

The mammary gland is characterized by extensive regeneration capacity, as it goes through massive hormonal changes throughout the life cycle of a female. The role of mammary stem cells (MaSCs) is widely studied both in the physiological/developmental context and with regards to breast carcinogenesis. In this aspect, ex vivo studies focused on MaSC properties are highly sought after. Mammosphere cultures represent a surrogate of organ formation and have become a valuable tool for both basic and translational research. Here, we present a detailed protocol for the generation of murine primary mammosphere cultures and the quantitation of MaSC growth properties. The protocol includes mammary gland collection and digestion, isolation of primary mammary epithelial cells (MECs), establishment of primary mammosphere cultures, serial passaging, quantitation of mammosphere growth parameters and interpretation of the results. As an example, we present the effect of low-level constitutive Myc expression on normal MECs leading to increased self-renewal and proliferation.

p53 Loss in Breast Cancer Leads to Myc Activation, Increased Cell Plasticity, and Expression of a Mitotic Signature with Prognostic Value.

Loss of p53 function is invariably associated with cancer. Its role in tumor growth was recently linked to its effects on cancer stem cells (CSCs), although the underlying molecular mechanisms remain unknown. Here, we show that c-myc is a transcriptional target of p53 in mammary stem cells (MaSCs) and is activated in breast tumors as a consequence of p53 loss. Constitutive Myc expression in normal mammary cells leads to increased frequency of MaSC symmetric divisions, extended MaSC replicative-potential, and MaSC-reprogramming of progenitors, whereas Myc activation in breast cancer is necessary and sufficient to maintain the expanding pool of CSCs. Concomitant p53 loss and Myc activation trigger the expression of 189 mitotic genes, which identify patients at high risk of mortality and relapse, independently of other risk factors. Altogether, deregulation of the p53:Myc axis in mammary tumors increases CSC content and plasticity and is a critical determinant of tumor growth and clinical aggressiveness.