Mutations in signal transduction pathways and inherited diseases Paolo Sassone-Corsi and Emiliana Borrelli CNRS-INSERM, Strasbourg, France Intracellular signal transduction pathways have central roles in processes such as growth, differentiation, neurotransmission and development. The aberrant expression of components of various signal transduction pathways has profound consequences for cellular functions. Recent findings indicate that many cases of neoplasia and inherited diseases have, at their roots, mutations in key steps of signalling pathways. Current Opinion in Genetics and Development 1992, 2:455-458

Introduction

Troubles in the nucleus

A prerequisite for normal cell growth and differentiation is that each cell must be able to receive, interpret and respond appropriately to signals sent both from other cells and from their environment. The plasma membrane is the external interface of the cell that bears many elements required for the primary analysis of such signals. Among these elements are membrane receptors that are able to interact specifically with a variety of compounds. Binding of a ligand to its receptor initiates a cascade of events that modulate a variety of cellular functions including the control of gene expression. By altering the spectrum of genes expressed, the cell appropriately adapts its physiology to a given stimulus. Aberrations in this process can lead to deregulated cell proliferation and ultimately tumourigenesis. Two major signal transduction pathways, which use cyclic adenosine monophosphate (cAMP) and diacylglycerol (DAG) as secondary messengers, exist [1,2,3]. Each pathway is characterized by its specific protein kinase, protein kinase A (PKA) and protein kinase C (PKC), respectively, although a large number of kinases has been identified to date [4]. A myriad of interactions between membrane receptors and cytoplasmic and nuclear components are responsible for the fine modulation of gene expression [5°]. Significantly, oncogenic products occupy pivotal positions in all the pathways of intracellular signalling [6"], graphically demonstrating that aberrations in transducing systems can have dramatic consequences for cellular physiology and metabolism. The past year has seen many important advances in our understanding of the links between defective signal transduction and disease. In this review we will focus on some key examples that represent paradigms of this phenomenon.

In the nucleus, many transcription factors such as AP-1 [7"], NF-×B [8], CREM [9"] and SRF [10] have been identiffed as the final targets of signal transduction pathways. Some of them are constituted by oncoproteins, like Fos and Jun for AP-1 [7"], and Rel for NF-×B [11]. Yet other oncogenic products have been found to have important functions at the level of gene regulation, as in the cases of Myc, Myb and ErbA [12.]. Correct transcription initiation relies upon cooperation between these various factors [13"]. In addition, proteins that negatively modulate the regulatory activity of nuclear oncoproteins have been found to respond to various signalling systems [14",15], and as such could be considered as transcriptional antioncogenes. A striking example of how inherited diseases and neoplasia can be associated with a specific transcription factor defect concerns the disease Wilm's tumour [16.]. In the USA, among children under 16 years of age, there are 350 new cases of Wilms' tumour diagnosed every year. Congenital abnormalities associated with Wilms' tumour include aniridia, hemihypertrophy, malformation of the genitalia and neurofibromatosis (NF). The mode of inheritance is autosomal dominant with variable penetrance [17]. In the past, this kind of neoplasia has been explained by a 'two-hit' mutation model, in which the loss of a functional gene product resulted from mutation at homologous loci. Recently the w t l gene, which is located at the Wilms' tumour locus (11p13), has been cloned and characterized. It is postulated to be a tumour suppressor gene and encodes a nuclear protein that interacts with specific DNA promoter elements by means of a zinc finger-containing domain [16.]. The Wilrns'

Abbreviations cAMP--cyclic adenosine monophosphate; CF---cystic fibrosis; CFI'R--CF transmembrane regulator; CRE---cAMP-responsiveelements; DAG~iacylglycerol; GAP--GTPase-activatingprotein; G-protein--GTP-binding protein; NF---neurofibromatosis; Pl~--protein kinase A; PKC--protein kinase C; WT--Wilms' tumour. (~) Current Biology Ltd ISSN 0959-437X

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Mammaliangenetics tumour (WT)-I protein binds to the same promoter element as EGR-1, a mitogen-inducible, immediate-early gene product that functions as a transcriptional activator. WT1 acts as a repressor of EGR-1 and is thus likely to modulate the expression of several genes [16"]. This finding could represent an inlportant step forward in the understanding of the Wilms' tumour, although it is still unclear whether w t l is frequently inactivated in Wilms' patients.

The McCune-Albright syndrome is characterized by polyostoic fibrous dyslasia, car# a u lair pigmentation, and in female patients, endocrine dysfunction with precocious puberty; additional abnormalities have also been described. These results constitute a good example of how abnormal signal transduction arising from somatic mutations might be the molecular basis of man), complex pathological phenotypes [22].

The GAP-RAS conspiracy When G-proteins turn bad The levels of intracellular GTP are crucial for the transduction of signals from the membrane into the cytoplasm. A large number of GTP-binding proteins (G-proteins) are associated with membrane receptors. Depending on the various kinds of G-proteins and receptors, a specific signalling pathway can be activated or repressed [3]. The pivotal role that G-proteins play in the cellular physiology is hinted at, for example, by the fact that the oncogenic p21 Ras proteins have a G-protein-like function [6°]. Recently, the results obtained by tandis et al. [18] studying human pituitary tumours have demonstrated that G-proteins can play a direct role in the generation of neoplastic growth. The role of cAMP as an intracellular second messenger for several trophic hormones, and its ability to stimulate growth of certain cultured cells, led to the prediction that oncogenic mutations would be found in genes encoding proteins that control its synthesis. The authors identified mutations that result in autonomous cAMP synthesis in four growth homlone-secreting pituitary tumours [18]. The mutations cause constitutive activation of ms, which is the GTP-binding subunit of the stimulatory regulator of adenylyl cyclase, by inhibiting its GTPase activity. Thus, a subset of pituitary tumours with constitutively active adenylyl c3,clase carry oncogenic mutations in the ms-encoding gene. The resulting activation of the cyclase bypasses the cells' normal requirement for trophic hormone. An oncogene, gap, which encodes the ms protein, is thus postulated. Amino acids substitutions in gap identify a domain in the G-protein cz-chain that is required for the intrinsic ability of the protein to hydrolyze GTP. In a second group of human endocrine tumours, somatic mutations in the 0t subunit of the inhibitory G-protein Gi2 replace a residue equivalent to one of those affected by gap mutations. Thus, the mutations convert the 0ti2 gene into a dominantly acting oncogene called gip2, which encodes mutant 0ti2 subunits that are constitutively active [ 19]. Interestingly, the putative oncogenic mutations in 0ti2 also consistutively activate the protein's ability to inhibit cAMP accumulation [20°]. Additional evidence for the crucial role played by/G-proteins in neoplasia and inherited diseases has trecently been obtained by Weinstein et al. [21..]. ~ctivating mutations in the Gs0t gene have been found in abnormal tissues from patients with the McCune-Albright syndrome. It is proposed that somatic mutations of this gene could underlie the clinical manifestations of the disease.

The p21"as GTPAse-activating protein (GAP) is a cytosolic protein that catalyzes conversions of the active GTPbound form of t)21"as to the inactive GDP-bound form. Oncogenic mutants of p21 ras, such as those that occur in human turnouts at significant frequencies, escape downregulation by GAP and thus constitutively remain in the GTP-bound foml. In addition, GAP has been implicated as a component of signalling complexes that include activated platelet-derived growth factor receptors and other tyrosine substrates [23,24]. It is thus clear that GAP plays an important role in various signalling pathways. Recent important developments in the field have come with the discovery that the neurofibromatosis gene, NF1, from the von Recklinghausen neurofibromatosis locus, encodes a GAP-like protein [25,26,27]. The NFl-encoded product can interact with RAS proteins and shows structural and functional similarities to GAP. Neurofibromatosis is one of the most common autosomal dominant disorders, with an incidence of about 1/3000 individuals in all ethnic groups. The clinical features of the disease include the characteristic caf# au lait spots and in some cases serious compLications: including learning disabilities; major orthopaedic abnormalities; and the development of malignanw, especially neurofibrosarcoma and optic glioma. NFI is located in the qll.2 region of the long arm of chromosome 17. More recently, it has been shown that more than one GAP regulates p21 ras in the same cell, and that NF1-GAP is very efficient since it binds to p21"as 300-fold more tightly than, for instance, p120GAP [28,]. In conclusion, these findings clearly implicate signal transduction systems as important factors in some common inherited diseases.

Problems at the surface An indication that the normal functioning of membrane proteins might be crucial for maintenance of the correct physiology of cells is obtained from the classical studies involving the serotonin receptor. Overexpression of this receptor causes transformation of cultured cells [29]. There are, however, examples of disturbances occurring at the level of membrane proteins that are more directly linked to inherited diseases. Cystic fibrosis (CF) is the most common inherited disease in Caucasian populations (about 1/2000) and is one of the commonest causes of death in childhood. The

Mutations in signal transduction p a t h w a y s and inherited d i s e a s e s Sassone-Corsi, Borrelli

clinical features o f cystic fibrosis are the result o f a b n o r malities o f all the e x o c r i n e glands, a n d a susceptibility to respiratory infection. T h e g e n e t i c s o f the c o n d i t i o n sugg e s t e d that t h e s e a b n o r m a l i t i e s s h o u l d all b e attributable to a single a b n o r m a l protein. T h e g e n e carrying the mutation that causes CF has b e e n identified. T h e p r o d u c t e n c o d e d has b e e n n a m e d CF t r a n s m e m b r a n e regulator (CFTR), an A T P - d e p e n d e n t , t r a n s m e m b r a n e transp o r t protein, w h i c h a p p e a r s to b e involved in regulating the c h l o r i d e flux [30,31,32]. T h e findings indicate that in CF a large n u m b e r o f m u t a n t c h r o m o s o m e s carry a d e l e t i o n o f 3 b p in the c o d i n g s e q u e n c e , resulting in the loss o f a phenylalanine r e s i d u e at a m i n o acid p o s i t i o n 508 o f the protein. Thus, CF is a s s o c i a t e d with defective regulation o f a p i c a l - m e m b r a n e c h l o r i d e channels. T h e s e c h a n n e l s are normally activated by c A M P - d e p e n d e n t PKA and PKC, b u t in CF t h e s e kinases fail to activate o t h e r w i s e n o r m a l c h l o r i d e channels. M o r e r e c e n t findings d e s c r i b e that the c h l o r i d e c h a n n e l s can also b e activated b y raising the intracellular levels o f calcium a n d by the c a l m o d u l i n d e p e n d e n t p r o t e i n kinase [33"']. This finding is o f interest b e c a u s e calcium c a l m o d u l i n - a c t i v a t i o n o f c h l o r i d e c h a n n e l s c o u l d r e p r e s e n t a potentially t h e r a p e u t i c r o u t e for c i r c u m v e n t i n g defective regulation o f the c h a n n e l s in CF. Thus, the case o f CF illustrates h o w a d e f e c t i v e c o m p o n e n t o f a signal t r a n s d u c t i o n p a t h w a y can a c c o u n t for a c o m m o n inherited disease with p r o f o u n d p a t h o l o g i c a l features.

2.

BERRBX;EMJ: Inositol Triphosphate and Diacylglycerol: Two Interacting Second Messengers. Ann Rev BioctJem 1987, 56:159-193.

3.

BORREIM E, MONTMAYEURJP, FOULKESNS, SASSONE-CORSIP: Signal Transduction and Gene Control: The cAMP Pathway. CRC Rev Oncogenesis 1992, in press.

4.

HUNTERT: A Thousand and One Protein Kinases. Cell 1987, 50:823--829.

MONTMAYEUR JP, BOPmELU E: Transcription Mediated by a cAMP-responsive Promoter Element is Reduced Upon Activation of Dopamine D2 Receptors. Proc Nail Acad Sci USA 1991, 88:3135-3139. This paper presents a demonstration of the physiological transduction existing between the activation of a specific signal transduction pathway and gene regulation. Into cultured cells, the authors introduced vectors expressing different G-protein-coupled membrane receptors. Some of them (e.g. ]32.adrenergic) are known to activate the cAMP pathway; others (e.g. D2 dopamine) are known to down-regulate the cAMP pathway. The effects of the activated receptors can be scored at the nuclear level by using reporters containing a cAMP-responsive promoter element. 5.



6.

CANTLEYLC, AUGERKR, CARPENTERC, DUCKWORTHB, GRAZlAM



A, KAPELLERR, SOLTOFFS: Oncogenes and Signal Transduction. Cell 1991, 64:281-302.

The authors of this review successfully describe the links between signal transduction and oncogenesis. The products of most oncogenes occupy key positions in intracellular signalling. The work focuses particularly on protein tyrosine kinases. 7. .

Conclusion E x a m p l e s o f inherited d i s e a s e s b e i n g attributable to mutations in signal t r a n s d u c t i o n p a t h w a y s are b e i n g unc o v e r e d with i n c r e a s e d frequency. In this review, w e have r e p o r t e d select e x a m p l e s f r o m a m o n g s t several. T h e links b e t w e e n g e n e t i c i n h e r i t e d diseases, neoplasia and signal t r a n s d u c t i o n are evident. T h e c o m p l e x i t y o f the signalling pathways, w h i c h o f t e n s h o w cell- and developmental-specificity, w o u l d s e e m to a c c o u n t for the c o m p l e x nature o f m a n y i n h e r i t e d diseases. As exemplified by CF, o u r p r o g r e s s i v e u n d e r s t a n d i n g o f t h e s e defective pathways gives h o p e for the a p p l i c a t i o n o f m o r e effective therapies in the future.

Acknowledgements We thank NS Foulkes, J-L Mandel and JP Montmayeur for discussions and reading of the manuscript. The work carried out in our laboratories is supported by CNRS, INSERM, ARC and Rhone-Poulenc-Rorer.

ANGELP, KARh'~M: The Role of Jun, Fos and the AP-1 Complex in Cell-proliferation and Transformation. Biocbem Bic~ ploys Acta 1991, 1072:129-157. This review article is the most recent on the subject and summarizes succinctly a large amount of data that have been collected over the past few years. The nuclear oncoproteins Fos and Jun associate to constitute the transcription factor AP-1. The activity of this regulatory factor is modulated by other oncogenes, carcinogens and turnout promoters. SEN R, BALTIMORED: Inducibility of x Immunoglobulin Enhancer Protein NF-×B by a Post-translational Mechanism. Cell 1986, 47:921-928. 9. •

FOULKESNS, BORRELLI E, SASSONE-CORSl-P: CREM Gene: Use of Alternative DNA-binding Domains Generates Multiple Antagonists of cAMP-induced Transcription. Cell 1991, 64:739-749. This paper describes a new member of the family of transcription factors that bind to cyclic AMP-responsive elements. The CREMgene has several striking features. By the dynamic use of alternative splicing, it encodes both an activator and also transcriptional antagonists of cAMPinduced transcription. Its expression is cell- and tissue.specific and the various isoforms are differentially distributed. Regulation of splicing in appears to be under the control both of various hormones and of signal transduction mechanisms. 10.

PRYWESR, DUTrAA, CROMLISHJA, ROEDERRG: Phosphorylation of Serum Response Factor, a Factor that Binds to the Serum Response Element in the c-fos Promoter. Proc Natl Acad Sci USA 1988, 85:7206--7210.

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KIERANM, BL~m

Mutations in signal transduction pathways and inherited diseases.

Intracellular signal transduction pathways have central roles in processes such as growth, differentiation, neurotransmission and development. The abe...
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