Studies of the c-Harvey-Ras gene in psychiatric disorders

Delving into the Genetic Causes of Language Impairment in a Case of Partial Deletion of NRXN1

Introduction

Copy-number variations (CNVs) impacting on small DNA stretches and associated with language deficits provide a unique window to the role played by specific genes in language function.

Methods

We report in detail on the cognitive, language, and genetic features of a girl bearing a small deletion (0.186 Mb) in the 2p16.3 region, arr[hg19] 2p16.3(50761778_50947729)×1, affecting exons 3-7 of NRXN1, a neurexin-coding gene previously related to schizophrenia, autism (ASD), attention deficit hyperactivity disorder (ADHD), mood disorder, and intellectual disability (ID).

Results

The proband exhibits many of the features commonly found in subjects with deletions of NRXN1, like ASD-like traits (including ritualized behaviors, disordered sensory aspects, social disturbances, and impaired theory of mind), ADHD symptoms, moderate ID, and impaired speech and language. Regarding this latter aspect, we observed altered speech production, underdeveloped phonological awareness, minimal syntax, serious shortage of active vocabulary, impaired receptive language, and inappropriate pragmatic behavior (including lack of metapragmatic awareness and communicative use of gaze). Microarray analyses point to the dysregulation of several genes important for language function in the girl compared to her healthy parents.

Discussion

Although some basic cognitive deficit - such as the impairment of executive function - might contribute to the language problems exhibited by the proband, molecular evidence suggests that they might result, to a great extent, from the abnormal expression of genes directly related to language.

Language Impairments in ASD Resulting from a Failed Domestication of the Human Brain

Autism spectrum disorders (ASD) are pervasive neurodevelopmental disorders entailing social and cognitive deficits, including marked problems with language. Numerous genes have been associated with ASD, but it is unclear how language deficits arise from gene mutation or dysregulation. It is also unclear why ASD shows such high prevalence within human populations. Interestingly, the emergence of a modern faculty of language has been hypothesized to be linked to changes in the human brain/skull, but also to the process of self-domestication of the human species. It is our intention to show that people with ASD exhibit less marked domesticated traits at the morphological, physiological, and behavioral levels. We also discuss many ASD candidates represented among the genes known to be involved in the “domestication syndrome” (the constellation of traits exhibited by domesticated mammals, which seemingly results from the hypofunction of the neural crest) and among the set of genes involved in language function closely connected to them. Moreover, many of these genes show altered expression profiles in the brain of autists. In addition, some candidates for domestication and language-readiness show the same expression profile in people with ASD and chimps in different brain areas involved in language processing. Similarities regarding the brain oscillatory behavior of these areas can be expected too. We conclude that ASD may represent an abnormal ontogenetic itinerary for the human faculty of language resulting in part from changes in genes important for the “domestication syndrome” and, ultimately, from the normal functioning of the neural crest.

Perturbed proteostasis in autism spectrum disorders

Dynamic changes in synaptic strength rely on de novo protein synthesis and protein degradation by the ubiquitin proteasome system (UPS). Disruption of either of these cellular processes will result in significant impairments in synaptic plasticity and memory formation. Mutations in several genes encoding regulators of mRNA translation and members of the UPS have been associated with an increased risk for the development of autism spectrum disorders. It is possible that these mutations result in a similar imbalance in protein homeostasis (proteostasis) at the synapse. This review will summarize recent work investigating the role of the UPS in synaptic plasticity at glutamatergic synapses, and propose that dysfunctional proteostasis is a common consequence of several genetic mutations linked to autism spectrum disorders.

Dynamic changes in synaptic strength rely on de novo protein synthesis and protein degradation by the ubiquitin proteasome system (UPS). Disruption of either of these cellular processes will result in significant impairments in synaptic plasticity and memory formation. Mutations in several genes encoding regulators of mRNA translation (i.e. FMR1) and protein degradation (i.e. UBE3A) have been associated with an increased risk for autism spectrum disorders and intellectual disability (ASD/ID). These mutations similarly disrupt protein homeostasis (proteostasis). Compensatory changes that reset the rate of proteostasis may contribute to the neurological symptoms of ASD/ID. This review summarizes recent work investigating the role of the UPS in synaptic plasticity at glutamatergic synapses, and proposes that dysfunctional proteostasis is a common consequence of several genetic mutations linked to ASD.

This article is part of a mini review series: “Synaptic Function and Dysfunction in Brain Diseases”.

Human iPS Cell-Derived Neurons Uncover the Impact of Increased Ras Signaling in Costello Syndrome

Increasing evidence implicates abnormal Ras signaling as a major contributor in neurodevelopmental disorders, yet how such signaling causes cortical pathogenesis is unknown. We examined the consequences of aberrant Ras signaling in the developing mouse brain and uncovered several critical phenotypes, including increased production of cortical neurons and morphological deficits. To determine whether these phenotypes are recapitulated in humans, we generated induced pluripotent stem (iPS) cell lines from patients with Costello syndrome (CS), a developmental disorder caused by abnormal Ras signaling and characterized by neurodevelopmental abnormalities, such as cognitive impairment and autism. Directed differentiation toward a neuroectodermal fate revealed an extended progenitor phase and subsequent increased production of cortical neurons. Morphological analysis of mature neurons revealed significantly altered neurite length and soma size in CS patients. This study demonstrates the synergy between mouse and human models and validates the use of iPS cells as a platform to study the underlying cellular pathologies resulting from signaling deficits.

SIGNIFICANCE STATEMENT

Increasing evidence implicates Ras signaling dysfunction as a major contributor in psychiatric and neurodevelopmental disorders, such as cognitive impairment and autism, but the underlying cortical cellular pathogenesis remains unclear. This study is the first to reveal human neuronal pathogenesis resulting from abnormal Ras signaling and provides insights into how these phenotypic abnormalities likely contribute to neurodevelopmental disorders. We also demonstrate the synergy between mouse and human models, thereby validating the use of iPS cells as a platform to study underlying cellular pathologies resulting from signaling deficits. Recapitulating human cellular pathologies in vitro facilitates the future high throughput screening of potential therapeutic agents that may reverse phenotypic and behavioral deficits.

The Oscillopathic Nature of Language Deficits in Autism: From Genes to Language Evolution

Autism spectrum disorders (ASD) are pervasive neurodevelopmental disorders involving a number of deficits to linguistic cognition. The gap between genetics and the pathophysiology of ASD remains open, in particular regarding its distinctive linguistic profile. The goal of this article is to attempt to bridge this gap, focusing on how the autistic brain processes language, particularly through the perspective of brain rhythms. Due to the phenomenon of pleiotropy, which may take some decades to overcome, we believe that studies of brain rhythms, which are not faced with problems of this scale, may constitute a more tractable route to interpreting language deficits in ASD and eventually other neurocognitive disorders. Building on recent attempts to link neural oscillations to certain computational primitives of language, we show that interpreting language deficits in ASD as oscillopathic traits is a potentially fruitful way to construct successful endophenotypes of this condition. Additionally, we will show that candidate genes for ASD are overrepresented among the genes that played a role in the evolution of language. These genes include (and are related to) genes involved in brain rhythmicity. We hope that the type of steps taken here will additionally lead to a better understanding of the comorbidity, heterogeneity, and variability of ASD, and may help achieve a better treatment of the affected populations.

The shape of the human language-ready brain

Our core hypothesis is that the emergence of our species-specific language-ready brain ought to be understood in light of the developmental changes expressed at the levels of brain morphology and neural connectivity that occurred in our species after the split from Neanderthals–Denisovans and that gave us a more globular braincase configuration. In addition to changes at the cortical level, we hypothesize that the anatomical shift that led to globularity also entailed significant changes at the subcortical level. We claim that the functional consequences of such changes must also be taken into account to gain a fuller understanding of our linguistic capacity. Here we focus on the thalamus, which we argue is central to language and human cognition, as it modulates fronto-parietal activity. With this new neurobiological perspective in place, we examine its possible molecular basis. We construct a candidate gene set whose members are involved in the development and connectivity of the thalamus, in the evolution of the human head, and are known to give rise to language-associated cognitive disorders. We submit that the new gene candidate set opens up new windows into our understanding of the genetic basis of our linguistic capacity. Thus, our hypothesis aims at generating new testing grounds concerning core aspects of language ontogeny and phylogeny.

Autism traits in the RASopathies

BACKGROUND

Mutations in Ras/mitogen-activated protein kinase (Ras/MAPK) pathway genes lead to a class of disorders known as RASopathies, including neurofibromatosis type 1 (NF1), Noonan syndrome (NS), Costello syndrome (CS), and cardio-facio-cutaneous syndrome (CFC). Previous work has suggested potential genetic and phenotypic overlap between dysregulation of Ras/MAPK signalling and autism spectrum disorders (ASD). Although the literature offers conflicting evidence for association of NF1 and autism, there has been no systematic evaluation of autism traits in the RASopathies as a class to support a role for germline Ras/MAPK activation in ASDs.

METHODS

We examined the association of autism traits with NF1, NS, CS and CFC, comparing affected probands with unaffected sibling controls and subjects with idiopathic ASDs using the qualitative Social Communication Questionnaire (SCQ) and the quantitative Social Responsiveness Scale (SRS).

RESULTS

Each of the four major RASopathies showed evidence for increased qualitative and quantitative autism traits compared with sibling controls. Further, each RASopathy exhibited a distinct distribution of quantitative social impairment. Levels of social responsiveness show some evidence of correlation between sibling pairs, and autism-like impairment showed a male bias similar to idiopathic ASDs.

CONCLUSIONS

Higher prevalence and severity of autism traits in RASopathies compared to unaffected siblings suggests that dysregulation of Ras/MAPK signalling during development may be implicated in ASD risk. Evidence for sex bias and potential sibling correlation suggests that autism traits in the RASopathies share characteristics with autism traits in the general population and clinical ASD population and can shed light on idiopathic ASDs.

Strong synaptic transmission impact by copy number variations in schizophrenia

Schizophrenia is a psychiatric disorder with onset in late adolescence and unclear etiology characterized by both positive and negative symptoms, as well as cognitive deficits. To identify copy number variations (CNVs) that increase the risk of schizophrenia, we performed a whole-genome CNV analysis on a cohort of 977 schizophrenia cases and 2,000 healthy adults of European ancestry who were genotyped with 1.7 million probes. Positive findings were evaluated in an independent cohort of 758 schizophrenia cases and 1,485 controls. The Gene Ontology synaptic transmission family of genes was notably enriched for CNVs in the cases (P = 1.5 x 10(-7)). Among these, CACNA1B and DOC2A, both calcium-signaling genes responsible for neuronal excitation, were deleted in 16 cases and duplicated in 10 cases, respectively. In addition, RET and RIT2, both ras-related genes important for neural crest development, were significantly affected by CNVs. RET deletion was exclusive to seven cases, and RIT2 deletions were overrepresented common variant CNVs in the schizophrenia cases. Our results suggest that novel variations involving the processes of synaptic transmission contribute to the genetic susceptibility of schizophrenia.

Synaptic plasticity, AMPA-R trafficking, and Ras-MAPK signaling

Synaptic modification of transmission is a general phenomenon expressed at almost every excitatory synapse in the mammalian brain. Over the last three decades, much has been discovered about the cellular, synaptic, molecular, and signaling mechanisms responsible for controlling synaptic transmission and plasticity. Here, we present a brief review of these mechanisms with emphasis on the current understanding of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPA-R) trafficking and Ras-mitogen-activated protein kinase (MAPK) signaling events involved in controlling synaptic transmission.

State-dependent Ras signaling and AMPA receptor trafficking

Synaptic trafficking of AMPA-Rs, controlled by small GTPase Ras signaling, plays a key role in synaptic plasticity. However, how Ras signals synaptic AMPA-R trafficking is unknown. Here we show that low levels of Ras activity stimulate extracellular signal-regulated kinase kinase (MEK)-p42/44 MAPK (extracellular signal-regulated kinase [ERK]) signaling, whereas high levels of Ras activity stimulate additional Pi3 kinase (Pi3K)-protein kinase B (PKB) signaling, each accounting for approximately 50% of the potentiation during long-term potentiation (LTP). Spontaneous neural activity stimulates the Ras-MEK-ERK pathway that drives GluR2L into synapses. In the presence of neuromodulator agonists, neural activity also stimulates the Ras-Pi3K-PKB pathway that drives GluR1 into synapses. Neuromodulator release increases with increases of vigilance. Correspondingly, Ras-MEK-ERK activity in sleeping animals is sufficient to deliver GluR2L into synapses, while additional increased Ras-Pi3K-PKB activity in awake animals delivers GluR1 into synapses. Thus, state-dependent Ras signaling, which specifies downstream MEK-ERK and Pi3K-PKB pathways, differentially control GluR2L- and GluR1-dependent synaptic plasticity.

Association between the neurofibromatosis-1 (NF1) locus and autism in the Japanese population

Autistic patients have a 100 to 190-fold increased risk of neurofibromatosis compared to the general population. This suggests that the two diseases may share a common etiological background. Recently, a new allele (or the six-repeat allele) of the (AAAT)(n) repeat polymorphism in an Alu sequence in the neurofibromatosis-1 (NF1) gene was observed exclusively in severe autistic patients, not in controls, in Caucasians of French ancestry. This suggests a role of the NF1 gene in the development of autism. We investigated three microsatellite polymorphisms within the intron-27b and intron-38 of the NF1 region, including the (AAAT)(n) and two (CA)n repeat polymorphisms, in Japanese subjects with autism (n = 74) and controls (n = 122). The six-repeat allele of the (AAAT)(n) polymorphism was not found either in patients or controls, possibly indicating an ethnic difference in the polymorphism. However, significant differences were observed in the allele distributions of the (AAAT)(n) and a (CA)(n), which were located at intron-27b, between patients and controls, although an association was not significant between autism and another polymorphism at intron-38. This may suggest an involvement of the NF1 locus in susceptibility to autism, although further investigations are recommended.

Lack of evidence for an association between WNT2 and RELN polymorphisms and autism

Autism is a pervasive neurodevelopmental disorder characterized by deficits in language development and social interaction, as well as stereotypical, repetitive behaviors. The etiology of autism is largely unknown. Family and twin studies have provided compelling evidence for a strong genetic component in most idiopathic cases. Several recent candidate gene studies have suggested that alleles of WNT2 and the reelin gene (RELN), two genes involved in distinct aspects of neurodevelopment, confer greater susceptibility to autism. We screened WNT2 for DNA polymorphisms by sequencing all exons and adjacent intronic regions in 24 autistic patients, and identified not only the WNT2 variants reported previously (two common single-nucleotide polymorphisms (SNPs) in the 5' upstream region and the 3' untranslated region (UTR), respectively), but also two new SNPs in its 3' UTR. We genotyped all four WNT2 polymorphisms and a polymorphic trinucleotide repeat in the 5' UTR of RELN in 107 families with multiple autistic children, and evaluated evidence for association between these variants and autism by the transmission disequilibrium test (TDT). Our results revealed no deviation from the null hypothesis of no association. Our interpretation of these findings is that it is unlikely that DNA variations in RELN and WNT2 play a significant role in the genetic predisposition to autism.

A dyslexia susceptibility locus (DYX7) linked to dopamine D4 receptor (DRD4) region on chromosome 11p15.5

Dyslexia is a disability in acquiring reading and spelling skills that is independent of general intelligence and educational opportunity, and is highly heritable. It is known that dyslexia often co-occurs with attention deficit hyperactivity disorder (ADHD), and the 7-repeat allele of the 48-bp tandem repeat in exon 3 of the dopamine D4 receptor (DRD4) has been implicated in ADHD. We, therefore, investigated DRD4 as a candidate gene for dyslexia by testing for linkage and association with 14 markers at and around the DRD4 locus on chromosome 11p15.5. Using 100 families having at least two siblings affected with dyslexia, model-free linkage analysis revealed evidence for linkage to the DRD4-exon 3 repeat (two-point MFLOD = 2.27, P = 0.001) and to HRAS located just proximal to DRD4 (two-point MFLOD = 2.68, P = 0.0004). Evidence for linkage was maximal between DRD4 and HRAS (three-point MFLOD = 3.57, P = 0.00005). However, linkage disequilibrium analysis showed no significant evidence for association between dyslexia and DRD4 or HRAS. In particular, dyslexic subjects showed no significant increase of the DRD4 7-repeat allele associated with ADHD. It is possible that other DRD4 variants, not in strong linkage disequilibrium with the exon 3 repeat polymorphism, or alternatively, another gene very closely linked to DRD4, may influence susceptibility to dyslexia.

Disease susceptibility genes for autism

Autism is a complex neurodevelopmental disorder characterized by impairment in social interaction accompanied by a delay or lack of language, restricted interests, stereotyped behavior, and repetitive movement. Genetic predisposition to autism is evident from family and twin studies, and heritability in idiopathic autism is estimated at over 90%. Frequency of the disorder is approximately 1:2000 with a male to female ratio of 4:1. Affected individuals look normal at birth, and the symptoms manifest at the first 2-3 years of life. The spectrum of clinical symptoms and the severity of the disorder are variable even among siblings. Family studies and several genome-wide linkage analyses support the hypothesis of complex inheritance with involvement of as many as 10-100 genes of moderate effect. Identification of genes responsible for the phenotype would help to understand the molecular mechanisms of the disorder. Several genes have been proposed to play a role in susceptibility to autism, and this paper will overview those genes and their potential role in the disorder.

Ras and Rap control AMPA receptor trafficking during synaptic plasticity

Recent studies show that AMPA receptor (-R) trafficking is important in synaptic plasticity. However, the signaling controlling this trafficking is poorly understood. Small GTPases have diverse neuronal functions and their perturbation is responsible for several mental disorders. Here, we examine the small GTPases Ras and Rap in the postsynaptic signaling underlying synaptic plasticity. We show that Ras relays the NMDA-R and CaMKII signaling that drives synaptic delivery of AMPA-Rs during long-term potentiation. In contrast, Rap mediates NMDA-R-dependent removal of synaptic AMPA-Rs that occurs during long-term depression. Ras and Rap exert their effects on AMPA-Rs that contain different subunit composition. Thus, Ras and Rap, whose activity can be controlled by postsynaptic enzymes, serve as independent regulators for potentiating and depressing central synapses.

The human secretin gene: fine structure in 11p15.5 and sequence variation in patients with autism

Secretin is a peptide hormone involved in digestion that has been studied as a potential therapeutic agent in patients with autism. We characterized the human secretin locus to determine whether mutations in this gene might play a role in a fraction of autism patients. While the secretin gene (SCT) was not found to be mutated in the majority of autistic patients, rare heterozygous sequence variants were identified in three patients. We also investigated length variation in a variable number of tandem repeats (VNTR) immediately upstream of SCT and found no significant differences in length between patients with autism and normal controls. SCT is located on 11p15.5, adjacent to DRD4 and HRAS. This region has been reported to be associated with both autism and attention deficit hyperactivity disorder (ADHD). Although imprinting is a characteristic of some genes in the vicinity, we could find no evidence for methylation of SCT in lymphoblast cells from patients or control individuals.

A risk-factor model of epistatic interaction, focusing on autism

Research to date on the genetics of autism has not uncovered a major susceptibility locus and indications are that a number of genes, perhaps as many as 15-20, may play detectable but minor roles in the etiology of the condition. To cope with this situation, a risk-factor model based on standard epidemiologic designs is proposed. The model supposes that adding a factor to a fixed set of existing factors always increases the total risk. Thus, according to the model genetic contributions cumulate but are not necessarily additive. A threshold, hence, epistasis is required. The model is applied to several conditions in which the risk of autism is elevated, some genetic (fragile X, tuberous sclerosis) and some exogenous (rubella and thalidomide embryopathies). Male gender is discussed as a risk factor. This approach is contrasted primarily with Gillberg and Coleman's view of autism as "a syndrome or series of syndromes caused by many different separate individual diseases." The principal point of difference is whether the effects of different causes cumulate or do not cumulate. In the present approach they do, in Gillberg and Coleman's they do not.

Lack of association between HoxA1 and HoxB1 gene variants and autism in 110 multiplex families

A recent report suggested that the HoxA1 and/or HoxB1 genes play a role in susceptibility to autism. To determine whether these findings could be confirmed, we screened these genes for DNA polymorphisms by sequencing all exons in 24 individuals with autism. We identified the same sequence variants in the genes that appeared in this report, which include one single-base substitution variant in HoxA1 and a common haplotype in HoxB1. We performed an association study by applying the transmission disequilibrium test to detect possible association of these variants to autism in 110 multiplex families. Our results demonstrated no deviation from the null hypothesis of no association. We have also separately examined transmissions within individual mating types, for paternal versus maternal alleles, to affected versus unaffected children, and for transmission to affected boys versus girls. None of these subsets revealed significant deviation from the null expectation. Our interpretation of these findings is that it is unlikely that HoxA1 and HoxB1 play a significant role in the genetic predisposition to autism.

Molecular genetics and animal models in autistic disorder

Autistic disorder is a behavioural syndrome beginning before the age of 3 years and lasting over the whole lifetime. It is characterised by impaired communication, impaired social interactions, and repetitive interests and behaviour. The prevalence is about 7/10,000 taking a restrictive definition and more than 1/500 with a broader definition, including all the pervasive developmental disorders. The importance of genetic factors has been highlighted by epidemiological studies showing that autistic disorder is one of the most genetic neuropsychiatric diseases. The relative risk of first relatives is about 100-fold higher than the risk in the normal population and the concordance in monozygotic twin is about 60%. Different strategies have been applied on the track of susceptibility genes. The systematic search of linked loci led to contradictory results, in part due to the heterogeneity of the clinical definitions, to the differences in the DNA markers, and to the different methods of analysis used. An oversimplification of the inferred model is probably also cause of our disappointment. More work is necessary to give a clearer picture. One region emerges more frequently: the long arm of chromosome 7. Several candidate genes have been studied and some gave indications of association: the Reelin gene and the Wnt2 gene. Cytogenetical abnormalities are frequent at 15q11-13, the region of the Angelman and Prader-Willi syndrome. Imprinting plays an important role in this region, no candidate gene has been identified in autism. Biochemical abnormalities have been found in the serotonin system. Association and linkage studies gave no consistent results with some serotonin receptors and in the transporter, although it seems interesting to go further in the biochemical characterisation of the serotonin transporter activity, particularly in platelets, easily accessible. Two monogenic diseases have been associated with autistic disorder: tuberous sclerosis and fragile X. A better knowledge of the pathophysiology of these disorders can help to understand autism. Different other candidate genes have been tested, positive results await replications in other samples. Animal models have been developed, generally by knocking out the different candidate genes. Behaviour studies have mainly focused on anxiety and learning paradigms. Another group of models results from surgical or toxic lesions of candidate regions in the brain, in general during development. The tools to analyse these animals are not yet standardised, and an important effort needs to be undertaken.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.