RAB18 Deficiency

Clinical characteristics.

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

Diagnosis/testing.

The diagnosis of RAB18 deficiency is established in a proband who either has suggestive clinical and neuroimaging findings and biallelic pathogenic variant(s) in RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 identified by molecular genetic testing or meets the clinical diagnostic criteria when molecular genetic testing has not been performed or has not revealed pathogenic variants in one of the four known genes.

Management.

Treatment of manifestations: Treatment is symptomatic and supportive, and is best approached through collaborative multidisciplinary medical specialists and other professionals. Cataracts are usually removed surgically. Management of developmental delay / intellectual disability and feeding difficulties are as per standard practice. Treatment of seizures is by a neurologist based on seizure type. Motor dysfunction due to progressive spasticity may benefit from physical therapy to maximize mobility and use of durable medical equipment. Undescended testes may require surgical correction; hormone supplementation for hypogonadism may occasionally be undertaken.

Surveillance: Routine follow up with an ophthalmologist, neurologist, developmental specialist, feeding team and nutritionist, and endocrinologist is recommended.

Genetic counseling.

RAB18 deficiency is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once the RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible.

Suggestive Findings

RAB18 deficiency should be suspected in individuals with the following clinical and neuroimaging findings.

Note: Findings indicated with an * are the basis of the diagnosis when molecular genetic testing either has not been performed or has not revealed biallelic pathogenic variants in one of the four known genes.

Establishing the Diagnosis

The diagnosis of RAB18 deficiency is established in a proband who EITHER:

• Has suggestive clinical and neuroimaging findings together with biallelic pathogenic variant(s) in RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 identified by molecular genetic testing (see Table 1); OR
• Meets the clinical diagnostic criteria outlined under Suggestive Findings when molecular genetic testing has not been performed or when molecular genetic testing has not revealed pathogenic variants in one of the four known genes.

Note that failure to detect biallelic causative pathogenic variant(s) in one of the four genes known to cause RAB18 deficiency does not necessarily exclude a clinical diagnosis of RAB18 deficiency as additional loci may exist.

Molecular testing approaches can include concurrent or serial single-gene testing, use of a multigene panel, and more comprehensive genomic testing.

Gene-targeted testing requires the clinician to determine which gene(s) are likely involved, whereas genomic testing may not. Persons with the distinctive findings described in Suggestive Findings are likely to be diagnosed using gene-targeted testing (see Option 1), whereas those in whom a specific diagnosis has been elusive are more likely to be diagnosed using genomic testing (see Option 2).

Option 2

When the diagnosis of RAB18 deficiency has not been considered or when gene-targeted testing has not identified pathogenic variant(s), comprehensive genomic testing (typically exome sequencing) is likely to be the diagnostic modality selected.

For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Table 1.

Molecular Genetic Testing Used in Molecularly Confirmed RAB18 Deficiency

Clinical Description

RAB18 deficiency describes the molecular deficit underlying Warburg micro syndrome and Martsolf syndrome. Warburg micro syndrome is characterized by eye, nervous system, and endocrine abnormalities [Warburg et al 1993]; Martsolf syndrome is characterized by similar findings, but with a milder presentation [Martsolf et al 1978]. When first described, Warburg micro syndrome and Martsolf syndrome were considered distinct disorders; however, following discovery of the underlying genetic bases of both phenotypes, it became apparent that these phenotypes are a continuum of clinical manifestations: Warburg micro syndrome on the severe end of the spectrum and Martsolf syndrome at the milder end (see Genotype-Phenotype Correlations).

To date Warburg micro syndrome comprises the majority (>96%) of molecularly confirmed RAB18 deficiency reported [Aligianis et al 2005, Abdel-Salam et al 2007, Yüksel et al 2007, Morris-Rosendahl et al 2010, Bem et al 2011, Borck et al 2011, Dursun et al 2012, Yildirim et al 2012, Handley et al 2013, Liegel et al 2013, Gillespie et al 2014, Picker-Minh et al 2014, Sawyer et al 2014, Imagawa et al 2015, Tasdemir et al 2015, Asahina et al 2016, Gupta et al 2016, Kabzińska et al 2016, Rump et al 2016, Srivastava et al 2016, Trkova et al 2016].

Genetically defined Martsolf syndrome has been described in only four families (8 individuals), whose countries of origin are Pakistan, Mexico, Gambia, and Egypt [Aligianis et al 2006, Handley et al 2013].

Other

RAB18 deficiency is often – but not always – associated with short stature (height <-2 SD).

Dysmorphic features associated with RAB18 deficiency are mild but form a recognizable pattern, including deep-set eyes, a wide nasal bridge and prominent nasal root, relatively narrow mouth, and proportionately large anteverted ears (Figure 1).

Figure 1.

Photographs of individuals with Warburg micro and Martsolf syndromes Brothers K2.1 (age 15 years) and K2.2 (age 13 years) with Warburg micro syndrome are homozygous for the RAB3GAP1 p.Thr18Pro variant.

Mild micrognathia, a high-arched palate, and delayed dentition may also be seen [Handley et al 2013, Aligianis & Handley 2016].

Mild hypertrichosis has been reported; see for example Yüksel et al [2007], Morris-Rosendahl et al [2010], Picker-Minh et al [2014], and Mandarano et al [2017].

Osteopenia has been reported as potentially a primary manifestation of Warburg micro syndrome in one family [Picker-Minh et al 2014].

Although secondary complications from RAB18 deficiency can be life-threatening, RAB18 deficiency is not known to directly affect life expectancy.

Phenotype Correlations by Gene

Clinical observations cannot be used to distinguish the genetic basis of RAB18 deficiency.

The strongest suggestion of phenotype correlation by gene comes from comparative analysis of brain MRIs in which it appears that biallelic loss-of-function RAB3GAP2 variants may result in milder malformations than biallelic variants in either RAB3GAP1 or RAB18 [Handley et al 2013].

Of note, several individuals with biallelic loss-of-function pathogenic variants in TBC1D20 have developed glaucoma [Liegel et al 2013], an uncommon finding in RAB18 deficiency caused by biallelic variants in the other three genes (RAB3GAP1, RAB3GAP2, or RAB18).

Genotype-Phenotype Correlations

RAB18 deficiency results from biallelic pathogenic variants in RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 [Aligianis et al 2005, Aligianis et al 2006, Bem et al 2011, Borck et al 2011, Liegel et al 2013]. Biallelic loss-of-function variants in any one of these genes cause Warburg micro syndrome, which corresponds to the severe end of the phenotypic spectrum. Pathogenic variants that diminish but do not completely nullify gene expression or function can cause or contribute to Martsolf syndrome, which corresponds to the milder end of the phenotypic spectrum.

Warburg micro syndrome comprises the majority (98/102 families) of molecularly confirmed RAB18 deficiency reported [Aligianis et al 2005, Abdel-Salam et al 2007, Yüksel et al 2007, Morris-Rosendahl et al 2010, Bem et al 2011, Borck et al 2011, Dursun et al 2012, Yildirim et al 2012, Handley et al 2013, Liegel et al 2013, Gillespie et al 2014, Picker-Minh et al 2014, Sawyer et al 2014, Tasdemir et al 2015, Asahina et al 2016, Gupta et al 2016, Imagawa et al 2015, Kabzińska et al 2016, Rump et al 2016, Srivastava et al 2016, Trkova et al 2016].

• RAB18 deficiency usually results from pathogenic variants likely to completely nullify gene expression (nonsense, frameshift, intragenic deletion or consensus splice sites). The clinical presentation in these individuals is highly consistent, in keeping with the likelihood that the clinical consequences of these loss-of-function variants are equivalent.
• Other variants associated with Warburg micro syndrome that are likely to abrogate the function of the encoded protein (see Molecular Genetics) include:
  • RAB3GAP1: Three missense variants [Handley et al 2013, Asahina et al 2016].
  • RAB18: Two missense variants, an in-frame deletion, and an extension variant [Bem et al 2011, Handley et al 2013].

Martsolf syndrome. The three pathogenic variants reported in four families with Martsolf syndrome [Aligianis et al 2006, Handley et al 2013] are as follows:

• RAB3GAP1. A homozygous frameshift variant, c.9delC, identified in affected sibs, inactivates the normal RAB3GAP1 transcript but also expresses a novel transcript whose protein product may provide sufficient residual function to result in the milder phenotype of Martsolf syndrome [Handley et al 2013] (see details in Molecular Genetics).
• RAB3GAP2
  • The homozygous missense variant c.3154G>T (Gly1052Cys) was identified in three affected individuals from one family [Aligianis et al 2006]. In lymphocyte RNA from two of the affected individuals, this variant was found to promote exon-skipping and to introduce a frameshift into the shortened transcript; however, some full-length transcript was also found in each case, compatible with residual full-length protein expression.
  • The homozygous missense variant c.1276C>T (p.Arg426Cys) affects a conserved amino acid residue, and the altered protein may retain some functional activity [Handley et al 2013].

Nomenclature

Although the following naming system has been used – in some instances – to designate the causative gene for Warburg micro syndrome, no clinical purpose is served by this naming system as the causative gene does not influence phenotype.

• Warburg micro syndrome type 1: RAB3GAP1
• Warburg micro syndrome type 2: RAB3GAP2
• Warburg micro syndrome type 3: RAB18
• Warburg micro syndrome type 4: TBC1D20

Prevalence

Data on the prevalence of RAB18 deficiency are limited.

As expected for an autosomal recessive disorder, incidence is known to be higher in isolated communities and in communities with a high rate of consanguinity [Bem et al 2011, Handley et al 2013].

Incidence may be higher in populations in which pathogenic founder variants are present at a high frequency. Notably, loss of function of an essential splice site variant in RAB3GAP1, c.748+1G>A, has been identified as a founder allele in 11 families of Turkish origin [Aligianis et al 2005, Yüksel et al 2007, Dursun et al 2012, Yildirim et al 2012, Handley et al 2013, Tasdemir et al 2015].

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with RAB18 deficiency, the evaluations in Table 3 (if not performed as part of the evaluation that led to diagnosis) are recommended.

Table 3.

Recommended Evaluations Following Initial Diagnosis of RAB18 Deficiency

Treatment of Manifestations

Treatment is symptomatic and supportive. It is best approached through collaborative care involving medical specialists, and is best coordinated by a pediatrician who is aware of the child's general health and development. Psychosocial support for families is an important component of effective care.

Table 4.

Treatment of Manifestations in Individuals with RAB18 Deficiency

Prevention of Secondary Complications

Respiratory infections. Individuals with Warburg micro syndrome are prone to respiratory infection because of reduced mobility, aspiration, and motor dysfunction. These can be life threatening and should be treated accordingly.

Risk of injury. Reduced mobility, hormonal imbalance, and treatment with anticonvulsants are risk factors for the development of osteoporosis. Affected individuals are at increased risk of broken limbs and dislocations and may be unable to effectively communicate the cause of the resulting distress. Consideration should be given to ongoing monitoring of bone densities and vitamin D levels, and to orthopedic assessment in case of acute admissions.

Surveillance

No formal surveillance guidelines exist for RAB18 deficiency. Suggested surveillance includes routine follow up with

• An ophthalmologist
• A neurologist
• A developmental specialist, such as a developmental pediatrician
• A feeding team and nutritionist
• An endocrinologist, especially in infancy to assess degree of hypogonadism and at the age that puberty typically would occur
• Hearing tests

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Mode of Inheritance

RAB18 deficiency is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• The parents of an affected child are obligate heterozygotes (i.e., carriers of one RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 pathogenic variant).
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

• At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Other family members. Each sib of the proband's parents is at a 50% risk of being a carrier of a RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 pathogenic variant.

Carrier (Heterozygote) Detection

Carrier testing for at-risk relatives requires prior identification of the RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 pathogenic variants in the family.

Related Genetic Counseling Issues

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are carriers or are at risk of being carriers.

DNA banking. Because it is likely that testing methodology and our understanding of genes, pathogenic mechanisms, and diseases will improve in the future, consideration should be given to banking DNA from probands in whom a molecular diagnosis has not been confirmed (i.e., the causative pathogenic mechanism is unknown).

Prenatal Testing and Preimplantation Genetic Testing

Once the RAB3GAP1, RAB3GAP2, RAB18, or TBC1D20 pathogenic variants have been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing for RAB18 deficiency are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. While most centers would consider use of prenatal testing to be a personal decision, discussion of these issues may be helpful.

Molecular Pathogenesis

The molecular pathology of Warburg micro syndrome and Martsolf syndrome arises either from the absence of Ras-related protein Rab-18 (RAB18) protein or from its functional absence as a result of dysregulation. Because RAB3GAP1, RAB3GAP2, and TBC1D20 are each essential for the regulation of RAB18, biallelic loss-of-function variants in the genes that encode these proteins lead to clinically indistinguishable phenotypes.

RAB18 encodes a highly conserved member of the RAB subfamily of the RAS superfamily of small GTPases [Handley 2017]. Different RAB protein isoforms function to regulate discrete steps in trafficking between cellular membrane compartments. RAB18 is proposed to have roles in regulation of lipid droplets, lipolysis, and lipogenesis [Martin et al 2005, Ozeki et al 2005, Pulido et al 2011], trafficking between the Golgi and endoplasmic reticulum (ER) [Dejgaard et al 2008, Handley et al 2015], ER structure [Gerondopoulos et al 2014], exocytosis [Vazquez-Martinez et al 2007], and autophagy [Feldmann et al 2017]. The specific cellular deficit(s) that underlie the pathology of Warburg micro syndrome and Martsolf syndrome are not yet known.

In common with other small GTPases, RAB proteins function as "molecular switches." They can bind to GDP or GTP and adopt different conformations according to which nucleotide is bound. These different conformations are in turn associated with altered protein-binding characteristics that affect interactions with regulators and with the mediators of their downstream cellular functions. Switching between GDP- and GTP-bound conformations is tightly regulated by two classes of protein, guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs mediate the exchange of bound GDP for GTP. GAPs stimulate the intrinsic GTPase activity of the RAB proteins, thereby mediating hydrolysis of bound GTP into GDP.

RAB3GAP1 and RAB3GAP2 each encode essential subunits of a binary complex with GEF activity toward RAB18 [Gerondopoulos et al 2014]. This complex is necessary to mediate GDP-GTP exchange and the associated RAB18 conformational change.

TBC1D20 encodes a RAB-GAP with modest in vitro GAP activity toward RAB18. Several lines of evidence indicate that it functions as a RAB18-GAP physiologically [Haas et al 2007, Handley et al 2015]. The regulation of RAB18 by TBC1D20 opposes that of the RAB3GAP complex, promoting its GDP- rather than the GTP-bound conformation. However, both regulators are required for RAB18 to function appropriately, in a spatiotemporally restricted manner.

RAB3GAP1

Gene structure. The canonic RAB3GAP1 transcript, NM_012233.2, comprises 24 coding exons and encodes a 981-amino acid protein.

An additional in-frame microexon of 21 nucleotides constitutes exon 24 that is incorporated into the 25-exon alternative transcript, NM_001172435; this microexon may be differentially expressed in the brain [Irimia et al 2014]. For a detailed summary of gene and protein information, see Table A, Gene; a summary of transcripts is in Ensemble.

Pathogenic variants. Biallelic pathogenic variants in RAB3GAP1 are the most frequently reported cause of RAB18 deficiency. Pathogenic variants are found throughout the gene and in most cases are likely to compromise gene expression.

• Homozygous pathogenic variants were identified in 67 families: 19 were splice site, 18 nonsense, 20 frameshift, six missense, and four intragenic deletions [Aligianis et al 2005, Abdel-Salam et al 2007, Yüksel et al 2007, Morris-Rosendahl et al 2010, Dursun et al 2012, Yildirim et al 2012, Handley et al 2013, Gillespie et al 2014, Picker-Minh et al 2014, Sawyer et al 2014, Imagawa et al 2015, Tasdemir et al 2015, Gupta et al 2016, Kabzińska et al 2016, Rump et al 2016, Srivastava et al 2016, Patel et al 2017].
• Compound heterozygous pathogenic variants were identified in ten families: 12 were nonsense, three splice site, three frameshift, one missense, and one was unidentified [Handley et al 2013, Asahina et al 2016, Trkova et al 2016].

Pathogenic variants identified in RAB3GAP1 in more than one family include:

• In individuals of Turkish ethnic origin: the splice site variant c.748+1G>A [Aligianis et al 2005, Yüksel et al 2007, Dursun et al 2012, Yildirim et al 2012, Handley et al 2013, Tasdemir et al 2015].
• In individuals of Pakistani ethnic origin: the splice site variant c.649-2A>G and a missense variant, c.52A>C [Aligianis et al 2005, Handley et al 2013].

Recurrent pathogenic variants identified in individuals with different ethnic backgrounds include c.899+1G>A, c.1039C>T, and c.2801delC.

The apparent frameshift variant c.9delC was identified in sibs with Martsolf syndrome [Handley et al 2013] (see Genotype-Phenotype Correlations). Analysis of lymphoblast RNA from an affected child detected expression of an alternate RAB3GAP1 transcript, which may be sufficient to explain the milder Martsolf syndrome phenotype in this family.

Table 5.

RAB3GAP1 Pathogenic Variants Discussed in This GeneReview

Normal gene product. Functional characterization of RAB3GAP1 protein domains is limited. Biochemical evidence suggests that GTPase-activating protein ("GAP") activity specific for RAB3 isoforms resides in the C-terminal portion of the protein between amino acid residues 600 and 981 and identifies Arg728 as a critical residue in this activity [Clabecq et al 2000]. No evidence to date has definitively implicated loss of RAB3-GAP activity in, or excluded it from, a role in disease pathogenesis.

Abnormal gene product. Biochemical characterization of several of the disease-associated variants in RAB3GAP1 has been carried out [Gerondopoulos et al 2014].

Two of three pathogenic missense variants in RAB3GAP1, resulting in p.Thr18Pro and p.Glu24Val substitutions, disrupt the in vitro RAB18-GEF activity of the RAB3GAP1-RAB3GAP2 complex but do not affect the in vitro RAB3-GAP activity of RAB3GAP1. These data suggest that loss of RAB18-GEF activity is responsible for disease pathogenesis.

A third pathogenic missense variant, c.560G>C, p.Arg187Pro, has not been characterized at a molecular level [Asahina et al 2016].

The location of the loss-of-function missense variants in RAB3GAP1 may indicate that the N-terminal region of RAB3GAP1 is critical for its RAB18-GEF activity. However, the association of a c.9delC variant with Martsolf syndrome rather than Warburg micro syndrome [Handley et al 2013] suggests that the extreme N-terminus may be dispensable for the protein to retain some functional activity.

Two of the pathogenic variants in RAB3GAP1, c.2801delC and c.2865_2866insTTCT, affect the last exon of the gene and are therefore unlikely to reduce protein expression as a result of nonsense-mediated decay of the transcript [Aligianis et al 2005, Handley et al 2013]. This suggests that residues at the C-terminus of the protein, amino acids 934-981, are functionally important.

RAB3GAP2

Gene structure. The canonic RAB3GAP2 transcript, NM_012414, comprises 35 coding exons and encodes a 1,393-amino acid protein. For a detailed summary of gene and protein information, see Table A, Gene.

Pathogenic variants. In all affected individuals in the 11 families reported to date with RAB3GAP2 pathogenic variants, the variants have been homozygous [Aligianis et al 2006, Borck et al 2011, Handley et al 2013].

Nonsense variants were identified in five families, frameshift variants in two families, and missense variants in two families. An in-frame deletion and a variant affecting splicing were each identified in single families. One pathogenic missense RAB3GAP2 variant, c.1276C>T, p.Arg426Cys, was associated with Martsolf syndrome in two families of different ethnic origins.

Table 6.

RAB3GAP2 Pathogenic Variants Discussed in This GeneReview

Normal gene product. Functional characterization of RAB3GAP2 protein domains is limited.

Abnormal gene product. Characterization of the pathogenic p.Arg426Cys variant in RAB3GAP2 shows that this substitution disrupts the in vitro RAB18-GEF activity of the RAB3GAP1-RAB3GAP2 complex [Gerondopoulos et al 2014].

An in-frame deletion associated with Warburg micro syndrome, c.499_507del, removes conserved residues Phe167, Tyr168, and Thr169 from the protein, suggesting that these residues are also functionally important [Borck et al 2011].

RAB18

Gene structure. The canonic RAB18 transcript, NM_021252.4, comprises seven coding exons and encodes a 206-amino acid protein. Multiple poorly characterized alternatively-spliced RAB18 transcripts have been identified. One such transcript, NM_001256410, incorporates an additional exon encoding 29 additional amino acids, but this transcript has not been associated with disease. For a detailed summary of gene and protein information, see Table A, Gene.

Pathogenic variants. In nine families in which pathogenic variants have been reported to date, the variants were homozygous in eight and compound heterozygous in one [Bem et al 2011, Handley et al 2013, Gillespie et al 2014, Mandarano et al 2017].

A pathogenic founder variant, c.71T>A, is responsible for RAB18 deficiency in five families of ethnic Pakistani origin [Bem et al 2011, Gillespie et al 2014]. Other identified pathogenic variants are in Table 7.

Table 7.

RAB18 Pathogenic Variants Discussed in This GeneReview

Normal gene product. A crystal structure of RAB18 has been generated (Protein Data Bank reference: 1X3S). In this structure, residues Gly15-Ser23, Phe33-Thr40, Asp63-Gln67, Asn122-Lys126, and Ala150-Lys153 are each in close proximity to bound nucleotide. Residues Lys21-Ser22, Ala38-Thr40, and Asp63-Thr64 are each in close proximity to bound magnesium. RAB18 has a C-terminal Cys-Ser-Val-Leu motif (amino acids 203-206) that is subject to post-translational modification. The Cys residue is geranylgeranylated, the terminal Ser-Val-Leu is then cleaved and the Cys residue is methylated [Leung et al 2007]. The geranylgeranyl-modification is important for the interactions of RAB18 with regulatory molecules and with membranes.

Abnormal gene product. A comparable clinical picture in individuals with a nonsense variant, a deletion of exon 2, and other pathogenic variants in this gene suggests that each of these variants completely abrogates protein function [Bem et al 2011, Mandarano et al 2017].

In vitro analyses of recombinant RAB18 p.Leu24Gln and p.Arg93del proteins shows that these variants completely abolish nucleotide binding [Bem et al 2011].

A c.284C>G, p.Thr95Arg variant has not been functionally characterized, but the proximity of Thr95 to Arg93 suggests that this substitution has a similar effect [Handley et al 2013].

A disease-associated extension variant, c.619T>C, is unlikely to impair nucleotide binding but is likely to compromise the post-translational modification of RAB18 and its association with membranes [Bem et al 2011].

Together, these data suggest that both nucleotide binding and post-translational modification are essential for RAB18 to function.

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Acknowledgments

We thank the patients, their families, and clinicians for their collaboration and contribution to our knowledge about Warburg micro syndrome and Martsolf syndrome.

Revision History

• 4 January 2018 (bp) Review posted live
• 2 May 2017 (mh) Original submission