Exome Sequencing of 47 Chinese Families with Cone-Rod Dystrophy: Mutations in 25 Known Causative Genes

Objective

The goal of this study was to identify mutations in 25 known causative genes in 47 unrelated Chinese families with cone-rod dystrophy (CORD).

Methods

Forty-seven probands from unrelated families with CORD were recruited. Genomic DNA prepared from leukocytes was analyzed by whole exome sequencing. Variants in the 25 genes were selected and then validated by Sanger sequencing.

Results

Fourteen potential pathogenic mutations, including nine novel and five known, were identified in 10 of the 47 families (21.28%). Homozygous, compound heterozygous, and hemizygous mutations were detected in three, four, or three families, respectively. The 14 mutations in the 10 families were distributed among CNGB3 (three families), PDE6C (two families), ABCA4 (one family), RPGRIP1 (one family), RPGR (two families), and CACNA1F (one family).

Conclusions

This study provides a brief view on mutation spectrum of the 25 genes in a Chinese cohort with CORD. Identification of novel mutations enriched our understanding of variations in these genes and their associated phenotypes. To our knowledge, this is the first systemic exome-sequencing analysis of all of the 25 CORD-associated genes.     

IDH1 and IDH2 mutations are frequent in Chinese patients with acute myeloid leukemia but rare in other types of hematological disorders

Frequent mutations in the isocitrate dehydrogenase 1 and 2 genes (IDH1 and IDH2) have been identified in gliomas and acute myeloid leukemia (AML). Our aim is to assess whether IDH mutations were presented in Chinese patients with various hematological disorders. In this study, we screened the IDH1 and IDH2 mutations in a cohort of 456 Chinese patients with various hematological malignancies and disorders. We found three missense (p.R132C, p.R132G, and p.I99M; occurred in five patients) and one silent mutation (c.315C>T; occurred in two patients) in the IDH1 gene and two missense mutations (p.R140Q and p.R172K; occurred in four AML patients) and one silent mutation (c.435G>A) in the IDH2 gene. Except for one non-Hodgkin lymphoma (NHL) patient harboring IDH1 mutation p.R132C, all IDH1 and IDH2 missense mutations were observed in patients with AML. Intriguingly, the IDH2 mutation p.R140Q and novel IDH1 mutation p.I99M co-occurred in a 75-year-old patient with AML developed from myelodysplastic syndromes (MDS). The frequency of IDH1 and IDH2 missense mutations in Chinese AML patients reached 5.9% and 8.3%, respectively. Our results supported the recent findings that IDH gene mutations were common in AML. Conversely, IDH mutations were rather rare in Chinese patients with other types of hematological disorders.     

The MT-ND1 and MT-ND5 genes are mutational hotspots for Chinese families with clinical features of LHON but lacking the three primary mutations

LHON is one of the most common and primary causes of acute blindness in young male adults. Over 95% of LHON cases are caused by one of the three primary mutations (m.11778G>A, m.14484T>C, and m.3460G>A). In contrast to these genetically diagnosed LHON patients, there are many patients with clinical features of LHON but without the three primary mutations, and these patients have been insufficiently analyzed. We reported 10 suspected Chinese LHON families without the three primary mutations. The overall penetrance (53.4%) in these families is significantly higher than in those families with m.11778G>A (33.3%) or m.3460G>A (25.6%). Complete mtDNA genome sequencing of the 10 families showed that they belonged to different haplogroups and all identified variants (excluding m.12332A>G in mt-tRNA^Leu) were previously reported. Eight of 12 private non-synonymous variants in the probands are located in the MT-ND1 and MT-ND5 genes, which is substantially higher than that of individuals from general Chinese populations. Comparison of the private variants in the 10 families and in 10 randomly selected mtDNAs from general Chinese populations using resampling simulation strategy further confirmed this pattern. Our results suggest that the MT-ND1 and MT-ND5 genes are mutational hotspots for Chinese families with suspected LHON lacking the common primary mutations. Variants m.3736G>A (p.V144I) in family Le1235 and m.10680G>A (p.A71T) in Le1107 can be the pathogenic mutations for LHON.     

GUCY2D mutations in retinal guanylyl cyclase 1 provide biochemical reasons for dominant cone–rod dystrophy but not for stationary night blindness

Mutations in the GUCY2D gene coding for the dimeric human retinal membrane guanylyl cyclase (RetGC) isozyme RetGC1 cause various forms of blindness, ranging from rod dysfunction to rod and cone degeneration. We tested how the mutations causing recessive congenital stationary night blindness (CSNB), recessive Leber''s congenital amaurosis (LCA1), and dominant cone–rod dystrophy-6 (CORD6) affected RetGC1 activity and regulation by RetGC-activating proteins (GCAPs) and retinal degeneration-3 protein (RD3). CSNB mutations R666W, R761W, and L911F, as well as LCA1 mutations R768W and G982VfsX39, disabled RetGC1 activation by human GCAP1, -2, and -3. The R666W and R761W substitutions compromised binding of GCAP1 with RetGC1 in HEK293 cells. In contrast, G982VfsX39 and L911F RetGC1 retained the ability to bind GCAP1 in cyto but failed to effectively bind RD3. R768W RetGC1 did not bind either GCAP1 or RD3. The co-expression of GUCY2D allelic combinations linked to CSNB did not restore RetGC1 activity in vitro. The CORD6 mutation R838S in the RetGC1 dimerization domain strongly dominated the Ca²⁺ sensitivity of cyclase regulation by GCAP1 in RetGC1 heterodimer produced by co-expression of WT and the R838S subunits. It required higher Ca²⁺ concentrations to decelerate GCAP-activated RetGC1 heterodimer—6-fold higher than WT and 2-fold higher than the Ser⁸³⁸-harboring homodimer. The heterodimer was also more resistant than homodimers to inhibition by RD3. The observed biochemical changes can explain the dominant CORD6 blindness and recessive LCA1 blindness, both of which affect rods and cones, but they cannot explain the selective loss of rod function in recessive CSNB.     

A study of candidate genes for day blindness in the standard wire haired dachshund

Background

A genetic study was performed to identify candidate genes associated with day blindness in the standard wire haired dachshund. Based on a literature review of diseases in dogs and human with phenotypes similar to day blindness, ten genes were selected and evaluated as potential candidate genes associated with day blindness in the breed.

Results

Three of the genes, CNGB3, CNGA3 and GNAT2, involved in cone degeneration and seven genes and loci, ABCA4, RDH5, CORD8, CORD9, RPGRIP1, GUCY2D and CRX, reported to be involved in cone-rod dystrophies were studied. Polymorphic markers at each of the candidate loci were studied in a family with 36 informative offspring. The study revealed a high frequency of recombinations between the candidate marker alleles and the disease.

Conclusion

Since all of the markers were at the exact position of the candidate loci, and several recombinations were detected for each of the loci, all ten genes were excluded as causal for this canine, early onset cone-rod dystrophy. The described markers may, however, be useful to screen other canine resource families segregating eye diseases for association to the ten genes.

     

A Novel Large In-Frame Deletion within the CACNA1F Gene Associates with a Cone-Rod Dystrophy 3-Like Phenotype

Cone-rod dystrophies (CORDs) represent a heterogeneous group of monogenic diseases leading to early impairment of vision. The majority of CORD entities show autosomal modes of inheritance and X-linked traits are comparably rare. So far, three X-chromosomal entities were reported (CORDX1, -X2 and -X3). In this study, we analysed a large family of German origin with solely affected males over three generations showing a CORDX-like phenotype. Due to the heterogeneity of cone-rod dystrophies, we performed a combined linkage and X-exome sequencing approach and identified a novel large intragenic in-frame deletion encompassing exons 18 to 26 within the CACNA1F gene. CACNA1F is described causative for CORDX3 in a single family originating from Finland and alterations in this gene have not yet been reported in other CORDX pedigrees. Our data independently confirm CACNA1F as the causative gene for CORDX3-like phenotypes and detailed clinical characterization of the family expands the knowledge about the phenotypic spectrum of deleterious CACNA1F alterations.     

Frequency of CHEK2 gene mutations in breast cancer patients from Republic of Bashkortostan

A number of studies demonstrated that mutations in the CHEK2 gene can increase the risk of oncologic diseases, including breast cancer and that the mutational distribution s depends on the genetic structure of populations. In our study we compared the prevalence of c.1100delC, c.444+1G>A, del5395, p.I157T, and p.R145W CHEK2 mutations in 977 breast cancer patients (Russians, Tatars, Bashkirs, Ukrainians, and individual representatives of other ethnic groups) and in women without any oncologic pathology (n = 1069) from the Republic of Bashkortostan. We found CHEK2 del5395 mutation with a frequency of 1.23% (12/977) in breast-cancer patients, whereas in the control group it frequency was 0.09% (1/1069) (OR: 13.28, CI 95%: 1.72–102.33, p = 0.003). Frequencies of c.1100delC and c.444+1G>A mutations in patients and controls were 0.4%, 0.4% (4/977) and 0.09% (1/1069), 0.2% (2/1069), respectively. The p.I157T substitution in CHEK2 gene was the most widespread variant in two studied cohorts (approximately 5%); however, differences in the frequencies between cases and controls did not reach statistical significance. Truncating mutations were mainly found in women of Slavic origin. All three mutations were found in Russians and Ukrainians. CHEK2 mutations c.1100delC and c.444+1G>A were not found in Bashkirs and Tatars; however, the CHEK2 del5395 deletion was present in Tatars.     

A new ATP7B gene mutation with severe condition in two unrelated Iranian families with Wilson disease

Wilson disease is associated with a defect in copper metabolism and caused by different mutations in ATP7B gene. The aim of this study was to determine mutation frequency of ATP7B exons 8 and 14 in Wilson disease patients from the south of Iran. The exons 8 and 14 of ATP7B gene were analyzed in 65 unrelated Wilson disease patients by Denaturing High Performance Liquid Chromatography, and samples with abnormal peak profile were selected for direct DNA sequencing. Seven out of 65 (10.8%) patients had mutations at exon 14, including c.3061-1G>A in four and c.3207C>A in three patients. In addition, four different mutations were identified at exon 8 of six patients (9.2%). Three of these mutations have been previously reported, including c.2304delC in two patients, c.2293G>A and 2304dupC each in one patient. Furthermore, a novel mutation, c.2335T>G (p.Trp779Gly), was identified in two unrelated patients. The patients with this novel mutation demonstrated severe neuropsychiatric condition. All together, 13 out of 65 (20%) patients had mutations within exons 8 and 14. We also identified a lower frequency of the most common mutations of exons 8 and 14 in the southern Iranian population.     

MFN2 gene analysis in patients with hereditary motor and sensory neuropathy from Bashkortostan Republic

Hereditary motor and sensory neuropathy (HMSN) type IIA is caused by mutations in the mitofusin type-2 (MFN2) gene and represents one of the most common axonal forms of HMSN. We determined the spectrum and frequency of MFN2 gene mutations in patients from the Bashkortostan Republic (BR). Four different mutations were revealed in 5 out of 170 unrelated patients, i.e., c.2113G>A (p.Val705Ile) (1.2% among all types of HMSN in the total sample of patients and 2% among patients of Tatar ethnicity). This mutation was described previously; c.775C>T (p.Arg259Cys) (0.6%, in the total sample of patients and 2% among the patients of Tatar ethnicity); c.776G>A (p.Arg259His) (0.6% in the total sample of patients and 1.5% among the patients of Russians ethnicity); and c.2171T>C (p.Leu724Pro) (1.2% in the total sample of patients and 7.4% among the patients of Bashkirs ethnicity). These are new mutations that were not observed among healthy family members and in control samples of healthy subjects. Five identified nucleotide substitutions represent single nucleotide polymorphisms of the gene, including c.892G>A (p.Gly298Arg), c.957C>T (Gly319Gly), and c1039-222t>c, which were described previously, while c.175+28c>t and c.2204+15t>c represent new nucleotide substitutions in the intron regions of the gene.     

Detection of variants in 15 genes in 87 unrelated Chinese patients with Leber congenital amaurosis

Background

Leber congenital amaurosis (LCA) is the earliest onset and most severe form of hereditary retinal dystrophy. So far, full spectrum of variations in the 15 genes known to cause LCA has not been systemically evaluated in East Asians. Therefore, we performed comprehensive detection of variants in these 15 genes in 87 unrelated Han Chinese patients with LCA.

Methodology/Principal Findings

The 51 most frequently mutated exons and introns in the 15 genes were selected for an initial scan using cycle sequencing. All the remaining exons in 11 of the 15 genes were subsequently sequenced. Fifty-three different variants were identified in 44 of the 87 patients (50.6%), involving 78 of the 88 alleles (11 homozygous and 56 heterozygous variants). Of the 53 variants, 35 (66%) were novel pathogenic mutations. In these Chinese patients, variants in GUCY2D are the most common cause of LCA (16.1% cases), followed by CRB1 (11.5%), RPGRIP1 (8%), RPE65 (5.7%), SPATA7 (4.6%), CEP290 (4.6%), CRX (3.4%), LCA5 (2.3%), MERTK (2.3%), AIPL1 (1.1%), and RDH12 (1.1%). This differs from the variation spectrum described in other populations. An initial scan of 55 of 215 PCR amplicons, including 214 exons and 1 intron, detected 83.3% (65/78) of the mutant alleles ultimately found in these 87 patients. In addition, sequencing only 9 exons would detect over 50% of the identified variants and require less than 5% of the labor and cost of comprehensive sequencing for all exons.

Conclusions/Significance

Our results suggest that specific difference in the variation spectrum found in LCA patients from the Han Chinese and other populations are related by ethnicity. Sequencing exons in order of decreasing risk is a cost-effective way to identify causative mutations responsible for LCA, especially in the context of genetic counseling for individual patients in a clinical setting.     

Gene mapping and mutation screeneng in candidate genes in a Chinese family of autosomal dominant retinitis pigmentosa

We wanted to find the gene defect in a Chinese pedigree with autosomal dominant form of retinitis pigmentosa (ADRP). A small Chinese family with retinitis pigmentosa was collected. The genetic analysis of the family suggested an autosomal dominant pattern. Microsatellite (STR) markers tightly linked to candidate genes for ADRP were selected for linkage analysis. We got a maximum LOD score of 0.87 between markers D19S210 and D19S418. Precursor mRNA-processing factor (PRPF) 31, 3, 8, rhodopsin (RHO), peripherin 2 (PRPH2 or RDS), rod outer segment protein 1 (ROM1), neural retina leucine zipper (NRL), cone-rod homeobox-containing (CRX), inosine-5-prime-monophosphate dehydrogenase, type I (IMPDH1) and retinitis pigmentosa 1 (RP1) were amplified by polymerase chain reaction (PCR) and screened by direct sequencing. One new sequence variation was found. It was the missence mutation c.148G > C (D50H) occurred in exon 1 of RDS gene which existed in all the effected individuals and one unaffected family member. The DNA sequence variation didn’t cosegregate with the RP disease. We considered this transition was one new polymorphism which we speculate involved in the pathogenesis of ADRP and increased the risk of ADRP. Further study should be conducted to confirm the causative gene of this family.     

Recessive Mutations in KCNJ13, Encoding an Inwardly Rectifying Potassium Channel Subunit, Cause Leber Congenital Amaurosis

Inherited retinal degenerations, including retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA), comprise a group of disorders showing high genetic and allelic heterogeneity. The determination of a full catalog of genes that can, when mutated, cause human retinal disease is a powerful means to understand the molecular physiology and pathology of the human retina. As more genes are found, remaining ones are likely to be rarer and/or unexpected candidates. Here, we identify a family in which all known RP/LCA-related genes are unlikely to be associated with their disorder. A combination of homozygosity mapping and exome sequencing identifies a homozygous nonsense mutation, c.496C>T (p.Arg166X), in a gene, KCNJ13, encoding a potassium channel subunit Kir7.1. A screen of a further 333 unrelated individuals with recessive retinal degeneration identified an additional proband, homozygous for a missense mutation, c.722T>C (p.Leu241Pro), in the same gene. The three affected members of the two families have been diagnosed with LCA. All have a distinct and unusual retinal appearance and a similar early onset of visual loss, suggesting both impaired retinal development and progressive retinal degeneration, involving both rod and cone pathways. Examination of heterozygotes revealed no ocular disease. This finding implicates Kir7.1 as having an important role in human retinal development and maintenance. This disorder adds to a small diverse group of diseases consequent upon loss or reduced function of inwardly rectifying potassium channels affecting various organs. The distinct retinal phenotype that results from biallelic mutations in KCNJ13 should facilitate the molecular diagnosis in further families.     

Targeted Next-Generation Sequencing Reveals Novel USH2A Mutations Associated with Diverse Disease Phenotypes: Implications for Clinical and Molecular Diagnosis

USH2A mutations have been implicated in the disease etiology of several inherited diseases, including Usher syndrome type 2 (USH2), nonsyndromic retinitis pigmentosa (RP), and nonsyndromic deafness. The complex genetic and phenotypic spectrums relevant to USH2A defects make it difficult to manage patients with such mutations. In the present study, we aim to determine the genetic etiology and to characterize the correlated clinical phenotypes for three Chinese pedigrees with nonsyndromic RP, one with RP sine pigmento (RPSP), and one with USH2. Family histories and clinical details for all included patients were reviewed. Ophthalmic examinations included best corrected visual acuities, visual field measurements, funduscopy, and electroretinography. Targeted next-generation sequencing (NGS) was applied using two sequence capture arrays to reveal the disease causative mutations for each family. Genotype-phenotype correlations were also annotated. Seven USH2A mutations, including four missense substitutions (p.P2762A, p.G3320C, p.R3719H, and p.G4763R), two splice site variants (c.8223+1G>A and c.8559-2T>C), and a nonsense mutation (p.Y3745*), were identified as disease causative in the five investigated families, of which three reported to have consanguineous marriage. Among all seven mutations, six were novel, and one was recurrent. Two homozygous missense mutations (p.P2762A and p.G3320C) were found in one individual family suggesting a potential double hit effect. Significant phenotypic divergences were revealed among the five families. Three families of the five families were affected with early, moderated, or late onset RP, one with RPSP, and the other one with USH2. Our study expands the genotypic and phenotypic variability relevant to USH2A mutations, which would help with a clear insight into the complex genetic and phenotypic spectrums relevant to USH2A defects, and is complementary for a better management of patients with such mutations. We have also demonstrated that a targeted NGS approach is a valuable tool for the genetic diagnosis of USH2 and RP.     

Mutations in a BTB-Kelch Protein, KLHL7, Cause Autosomal-Dominant Retinitis Pigmentosa

Retinitis pigmentosa (RP) refers to a genetically heterogeneous group of progressive neurodegenerative diseases that result in dysfunction and/or death of rod and cone photoreceptors in the retina. So far, 18 genes have been identified for autosomal-dominant (ad) RP. Here, we describe an adRP locus (RP42) at chromosome 7p15 through linkage analysis in a six-generation Scandinavian family and identify a disease-causing mutation, c.449G→A (p.S150N), in exon 6 of the KLHL7 gene. Mutation screening of KLHL7 in 502 retinopathy probands has revealed three different missense mutations in six independent families. KLHL7 is widely expressed, including expression in rod photoreceptors, and encodes a 75 kDa protein of the BTB-Kelch subfamily within the BTB superfamily. BTB-Kelch proteins have been implicated in ubiquitination through Cullin E3 ligases. Notably, all three putative disease-causing KLHL7 mutations are within a conserved BACK domain; homology modeling suggests that mutant amino acid side chains can potentially fill the cleft between two helices, thereby affecting the ubiquitination complexes. Mutations in an identical region of another BTB-Kelch protein, gigaxonin, have previously been associated with giant axonal neuropathy. Our studies suggest an additional role of the ubiquitin-proteasome protein-degradation pathway in maintaining neuronal health and in disease.     

Union Makes Strength: A Worldwide Collaborative Genetic and Clinical Study to Provide a Comprehensive Survey of RD3 Mutations and Delineate the Associated Phenotype

Leber congenital amaurosis (LCA) is the earliest and most severe retinal degeneration (RD), and the most common cause of incurable blindness diagnosed in children. It is occasionally the presenting symptom of multisystemic ciliopathies which diagnosis will require a specific care of patients. Nineteen LCA genes are currently identified and three of them account for both non-syndromic and syndromic forms of the disease. RD3 (LCA12) was implicated as a LCA gene based on the identification of homozygous truncating mutations in two LCA families despite the screening of large cohorts of patients. Here we provide a comprehensive survey of RD3 mutations and of their clinical expression through the screening of a cohort of 852 patients originating worldwide affected with LCA or early-onset and severe RD. We identified three RD3 mutations in seven unrelated consanguineous LCA families - i.e., a 2 bp deletion and two nonsense mutations – predicted to cause complete loss of function. Five families originating from the Southern Shores of the Mediterranean segregated a similar mutation (c.112C>T, p.R38*) suggesting that this change may have resulted from an ancient founder effect. Considering the low frequency of RD3 carriers, the recurrence risk for LCA in non-consanguineous unions is negligible for both heterozygote and homozygote RD3 individuals. The LCA12 phenotype in our patients is highly similar to those of patients with mutant photoreceptor-specific guanylate cyclase (GUCY2D/LCA1). This observation is consistent with the report of the role of RD3 in trafficking of GUCYs and gives further support to a common mechanism of photoreceptor degeneration in LCA12 and LCA1, i.e., inability to increase cytoplasmic cGMP concentration in outer segments and thus to recover the dark-state. Similar to LCA1, LCA12 patients have no extraocular symptoms despite complete inactivation of both RD3 alleles, supporting the view that extraocular investigations in LCA infants with RD3 mutations should be avoided.     

Exome Sequencing Reveals Novel and Recurrent Mutations with Clinical Significance in Inherited Retinal Dystrophies

This study aimed to identify the underlying molecular genetic cause in four Spanish families clinically diagnosed of Retinitis Pigmentosa (RP), comprising one autosomal dominant RP (adRP), two autosomal recessive RP (arRP) and one with two possible modes of inheritance: arRP or X-Linked RP (XLRP). We performed whole exome sequencing (WES) using NimbleGen SeqCap EZ Exome V3 sample preparation kit and SOLID 5500xl platform. All variants passing filter criteria were validated by Sanger sequencing to confirm familial segregation and the absence in local control population. This strategy allowed the detection of: (i) one novel heterozygous splice-site deletion in RHO, c.937-2_944del, (ii) one rare homozygous mutation in C2orf71, c.1795T>C; p.Cys599Arg, not previously associated with the disease, (iii) two heterozygous null mutations in ABCA4, c.2041C>T; p.R681* and c.6088C>T; p.R2030*, and (iv) one mutation, c.2405-2406delAG; p.Glu802Glyfs*31 in the ORF15 of RPGR. The molecular findings for RHO and C2orf71 confirmed the initial diagnosis of adRP and arRP, respectively, while patients with the two ABCA4 mutations, both previously associated with Stargardt disease, presented symptoms of RP with early macular involvement. Finally, the X-Linked inheritance was confirmed for the family with the RPGR mutation. This latter finding allowed the inclusion of carrier sisters in our preimplantational genetic diagnosis program.     

Determining mutations in G6PC and SLC37A4 genes in a sample of Brazilian patients with glycogen storage disease types Ia and Ib

Glycogen storage disease (GSD) comprises a group of autosomal recessive disorders characterized by deficiency of the enzymes that regulate the synthesis or degradation of glycogen. Types Ia and Ib are the most prevalent; while the former is caused by deficiency of glucose-6-phosphatase (G6Pase), the latter is associated with impaired glucose-6-phosphate transporter, where the catalytic unit of G6Pase is located. Over 85 mutations have been reported since the cloning of G6PC and SLC37A4 genes. In this study, twelve unrelated patients with clinical symptoms suggestive of GSDIa and Ib were investigated by using genetic sequencing of G6PC and SLC37A4 genes, being three confirmed as having GSD Ia, and two with GSD Ib. In seven of these patients no mutations were detected in any of the genes. Five changes were detected in G6PC, including three known point mutations (p.G68R, p.R83C and p.Q347X) and two neutral mutations (c.432G > A and c.1176T > C). Four changes were found in SLC37A4: a known point mutation (p.G149E), a novel frameshift insertion (c.1338_1339insT), and two neutral mutations (c.1287G > A and c.1076-28C > T). The frequency of mutations in our population was similar to that observed in the literature, in which the mutation p.R83C is also the most frequent one. Analysis of both genes should be considered in the investigation of this condition. An alternative explanation to the negative results in this molecular study is the possibility of a misdiagnosis. Even with a careful evaluation based on laboratory and clinical findings, overlap with other types of GSD is possible, and further molecular studies should be indicated.     

Infantile-onset ascending hereditary spastic paraplegia with bulbar involvement due to the novel ALS2 mutation c.2761C > T

Recessive mutations in the alsin gene cause three clinically distinct motor neuron diseases: juvenile amyotrophic lateral sclerosis (ALS2), juvenile primary lateral sclerosis (JPLS) and infantile-onset ascending hereditary spastic paraplegia (IAHSP). A total of 23 different ALS2 mutations have been described for the three disorders so far. Most of these mutations result in a frameshift leading to a premature truncation of the alsin protein. We report the novel ALS2 truncating mutation c.2761C > T; p.R921X detected by homozygosity mapping and sequencing in two infants affected by IAHSP with bulbar involvement. The mutation c.2761C > T resides in the pleckstrin domain, a characteristic segment of guanine nucleotide exchange factors of the Rho GTPase family, which is involved in the overall neuronal development or maintenance. This study highlights the importance of using homozygosity mapping combined with candidate gene analysis to identify the underlying genetic defect as in this Saudi consanguineous family.