Identification of two new members, XPLAC and XTES, of the XK family

VPS13 Forum Proceedings: XK, XK-Related and VPS13 Proteins in Membrane Lipid Dynamics

In 2020, the pandemic interrupted the series of biannual International Neuroacanthocytosis Meetings that brought together clinicians, scientists, and patient groups to share research into a small group of devastating genetic diseases that combine both acanthocytosis (deformed red blood cells) and neurodegeneration with movement disorders. This Meeting Report describes talks at the 5th VPS13 Forum in January 2022, one of a series of online meetings held to fill the gap. The meeting addressed the basic biology of two key proteins implicated in chorea-acanthocytosis (mutations in VPS13A) and McLeod syndrome (mutations in XK). In a remarkable confluence of ideas, the speakers described different aspects of a single functional unit that comprises of VPS13A and XK proteins working together. Conditions caused by VPS13 (A-D) gene family mutations and related genes, such as XK, previously footnote knowledge, seem to turn central for a novel disease paradigm: bulk lipid transfer disorders.

The Clinical Features and Molecular Mechanism of Pituitary Adenoma Associated With Vestibular Schwannoma

OBJECTIVES

To explore the clinical features and mechanism of pituitary adenoma associated with vestibular schwannoma (PAVS).

PATIENTS AND METHODS

The authors retrospectively reviewed pituitary adenoma patients in Beijing Tiantan Hospital from January 1, 2008 to December 31, 2016. A total of two pituitary adenoma samples, 1 vestibular schwannoma sample and one paired pituitary adenoma/blood sample were subjected next-generation sequencing and sanger sequence.

RESULTS

A total of 5675 pituitary adenoma patients from January 1, 2008 to December 31, 2016, were retrospectively analyzed; of these, 4 (7%) patients met the criteria of PAVS. Clinical variable analyses revealed significant correlations between PAVS and older age when compared with sporadic pituitary adenoma (SPA) or sporadic vestibular schwannoma (SVS). The authors found that there were 2 germline mutations of XKR3 in 2/4 PAVS patients. Therefore, the authors speculated that XKR3 might be a genetic predisposition factor. The result also showed that there was no NF2 mutation and NF2-related symptom in the 4 PAVS samples.

CONCLUSIONS

PAVS had a significant correlation with older age when compared with SPA and SVS. XKR3 may be a genetic predisposition factor for PAVS, it represents a therapeutic target for PAVS in the future.

Cryo-EM structures of the caspase-activated protein XKR9 involved in apoptotic lipid scrambling

The exposure of the negatively charged lipid phosphatidylserine on the cell surface, catalyzed by lipid scramblases, is an important signal for the clearance of apoptotic cells by macrophages. The protein XKR9 is a member of a conserved family that has been associated with apoptotic lipid scrambling. Here, we describe structures of full-length and caspase-treated XKR9 from Rattus norvegicus in complex with a synthetic nanobody determined by cryo-electron microscopy. The 43 kDa monomeric membrane protein can be divided into two structurally related repeats, each containing four membrane-spanning segments and a helix that is partly inserted into the lipid bilayer. In the full-length protein, the C-terminus interacts with a hydrophobic pocket located at the intracellular side acting as an inhibitor of protein function. Cleavage by caspase-3 at a specific site releases 16 residues of the C-terminus, thus making the pocket accessible to the cytoplasm. Collectively, the work has revealed the unknown architecture of the XKR family and has provided initial insight into its activation by caspases.

Xkr8 Modulates Bipolar Cell Number in the Mouse Retina

The present study interrogated a quantitative trait locus (QTL) on Chr 4 associated with the population sizes of two types of bipolar cell in the mouse retina. This locus was identified by quantifying the number of rod bipolar cells and Type 2 cone bipolar cells across a panel of recombinant inbred (RI) strains of mice derived from two inbred laboratory strains, C57BL/6J (B6/J) and A/J, and mapping a proportion of that variation in cell number, for each cell type, to this shared locus. There, we identified the candidate gene X Kell blood group precursor related family member 8 homolog (Xkr8). While Xkr8 has no documented role in the retina, we localize robust expression in the mature retina via in situ hybridization, confirm its developmental presence via immunolabeling, and show that it is differentially regulated during the postnatal period between the B6/J and A/J strains using qPCR. Microarray analysis, derived from whole eye mRNA from the entire RI strain set, demonstrates significant negative correlation of Xkr8 expression with the number of each of these two types of bipolar cells, and the variation in Xkr8 expression across the strains maps a cis-eQTL, implicating a regulatory variant discriminating the parental genomes. Xkr8 plasmid electroporation during development yielded a reduction in the number of bipolar cells in the retina, while sequence analysis of Xkr8 in the two parental strain genomes identified a structural variant in the 3^′ UTR that may disrupt mRNA stability, and two SNPs in the promoter that create transcription factor binding sites. We propose that Xkr8, via its participation in mediating cell death, plays a role in the specification of bipolar cell number in the retina.

Ebola virus requires a host scramblase for externalization of phosphatidylserine on the surface of viral particles

Cell surface receptors for phosphatidylserine contribute to the entry of Ebola virus (EBOV) particles, indicating that the presence of phosphatidylserine in the envelope of EBOV is important for the internalization of EBOV particles. Phosphatidylserine is typically distributed in the inner layer of the plasma membrane in normal cells. Progeny virions bud from the plasma membrane of infected cells, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the plasma membrane in infected cells for EBOV virions to acquire it. Currently, the intracellular dynamics of phosphatidylserine during EBOV infection are poorly understood. Here, we explored the role of XK-related protein (Xkr) 8, which is a scramblase responsible for exposure of phosphatidylserine in the plasma membrane of apoptotic cells, to understand its significance in phosphatidylserine-dependent entry of EBOV. We found that Xkr8 and transiently expressed EBOV glycoprotein GP often co-localized in intracellular vesicles and the plasma membrane. We also found that co-expression of GP and viral major matrix protein VP40 promoted incorporation of Xkr8 into ebolavirus-like particles (VLPs) and exposure of phosphatidylserine on their surface, although only a limited amount of phosphatidylserine was exposed on the surface of the cells expressing GP and/or VP40. Downregulating Xkr8 or blocking caspase-mediated Xkr8 activation did not affect VLP production, but they reduced the amount of phosphatidylserine on the VLPs and their uptake in recipient cells. Taken together, our findings indicate that Xkr8 is trafficked to budding sites via GP-containing vesicles, is incorporated into VLPs, and then promote the entry of the released EBOV to cells in a phosphatidylserine-dependent manner.

Although Ebola virus causes severe hemorrhagic fever with a high mortality rate, there are no approved therapeutics. The viral entry process is one of the targets for antiviral development. Previous studies suggest that binding of phosphatidylserine, a component of the viral envelop, to the receptors promotes the entry of Ebola virus. Ebola virus is released from the surface membrane of infected cells. However, phosphatidylserine normally distributes in the inner layer of the cell surface membrane, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the membrane in infected cells for Ebola virus to acquire it. Because the mechanism by which phosphatidylserine changes its orientation in Ebola virus-infected cells is poorly understood, we studied and identified a cellular enzyme, XK-related protein 8 (Xkr8), as a responsible factor involved in this process. We demonstrated that the Ebola virus glycoprotein promoted the incorporation of Xkr8 in viral particles, which flips phosphatidylserine on their surface, enhancing their entry to cells. Our findings provide new insights into the mechanism of Ebola virus infection, which may be exploited for the development of therapeutics against Ebola virus infection.

Exposure of phosphatidylserine on the cell surface

Phosphatidylserine (PtdSer) is a phospholipid that is abundant in eukaryotic plasma membranes. An ATP-dependent enzyme called flippase normally keeps PtdSer inside the cell, but PtdSer is exposed by the action of scramblase on the cell's surface in biological processes such as apoptosis and platelet activation. Once exposed to the cell surface, PtdSer acts as an 'eat me' signal on dead cells, and creates a scaffold for blood-clotting factors on activated platelets. The molecular identities of the flippase and scramblase that work at plasma membranes have long eluded researchers. Indeed, their identity as well as the mechanism of the PtdSer exposure to the cell surface has only recently been revealed. Here, we describe how PtdSer is exposed in apoptotic cells and in activated platelets, and discuss PtdSer exposure in other biological processes.

IL-17 receptor A and adenosine deaminase 2 deficiency in siblings with recurrent infections and chronic inflammation

BACKGROUND

Data on patients affected by chronic mucocutaneous candidiasis underscore the preponderant role of IL-17 receptor A (IL-17RA) in preserving mucocutaneous immunity. Little is known about the role of adenosine deaminase (ADA) 2 in regulation of immune responses, although recent reports linked ADA2 deficiency with inflammation and vasculitis.

OBJECTIVE

We sought to investigate the mechanisms of chronic inflammation and vasculitis in a child lacking IL-17RA and ADA2 to identify therapeutic targets.

METHODS

We report a family with 2 siblings who have had recurrent mucocutaneous infections with Candida albicans and Staphylococcus aureus and chronic inflammatory disease and vasculitis since early childhood, which were refractory to classical treatments. Array-based comparative genomic hybridization analysis showed that both siblings are homozygous for a 770-kb deletion on chr22q11.1 encompassing both IL17RA and cat eye critical region 1 (CECR1). Immunologic studies were carried out by means of flow cytometry, ELISA, and RIA.

RESULTS

As expected, in the affected child we found a lack of IL-17RA expression, which implies a severe malfunction in the IL-17 signaling pathway, conferring susceptibility to recurrent mucocutaneous infections. Surprisingly, we detected an in vitro and in vivo upregulation of proinflammatory cytokines, notably IL-1β and TNF-α, which is consistent with the persistent systemic inflammation.

CONCLUSIONS

This work emphasizes the utility of whole-genome analyses combined with immunologic investigation in patients with unresolved immunodeficiency. This approach is likely to provide an insight into immunologic pathways and mechanisms of disease. It also provides molecular evidence for more targeted therapies. In addition, our report further corroborates a potential role of ADA2 in modulating immunity and inflammation.

Isoform-Level Gene Expression Profiles of Human Y Chromosome Azoospermia Factor Genes and Their X Chromosome Paralogs in the Testicular Tissue of Non-Obstructive Azoospermia Patients

The human Y chromosome has an inevitable role in male fertility because it contains many genes critical for spermatogenesis and the development of the male gonads. Any genetic variation or epigenetic modification affecting the expression pattern of Y chromosome genes may thus lead to male infertility. In this study, we performed isoform-level gene expression profiling of Y chromosome genes within the azoospermia factor (AZF) regions, their X chromosome counterparts, and few autosomal paralogues in testicular biopsies of 12 men with preserved spermatogenesis and 68 men with nonobstructive azoospermia (NOA) (40 Sertoli-cell-only syndrome (SCOS) and 28 premiotic maturation arrest (MA)). This was undertaken using quantitative real-time PCR (qPCR) at the transcript level and Western blotting (WB) and immunohistochemistry (IHC) at the protein level. We profiled the expression of 41 alternative transcripts encoded by 14 AZFa, AZFb, and AZFc region genes (USP9Y, DDX3Y, XKRY, HSFY1, CYORF15A, CYORF15B, KDM5D, EIF1AY, RPS4Y2, RBMY1A1, PRY, BPY2, DAZ1, and CDY1) as well as their X chromosome homologue transcripts and a few autosomal homologues. Of the 41 transcripts, 18 were significantly down-regulated in men with NOA when compared with those of men with complete spermatogenesis. In contrast, the expression of five transcripts increased significantly in NOA patients. Furthermore, to confirm the qPCR results at the protein level, we performed immunoblotting and IHC experiments (based on 24 commercial and homemade antibodies) that detected 10 AZF-encoded proteins. In addition, their localization in testis cell types and organelles was determined. Interestingly, the two missing proteins, XKRY and CYORF15A, were detected for the first time. Finally, we focused on the expression patterns of the significantly altered genes in 12 MA patients with successful sperm retrieval compared to those of 12 MA patients with failed sperm retrieval to predict the success of sperm retrieval in azoospermic men. We showed that HSFY1-1, HSFY1-3, BPY2-1, KDM5C2, RBMX2, and DAZL1 transcripts could be used as potential molecular markers to predict the presence of spermatozoa in MA patients. In this study, we have identified isoform level signature that can be used to discriminate effectively between MA, SCOS, and normal testicular tissues and suggests the possibility of diagnosing the presence of mature sperm cell in azoospermic men to prevent additional testicular sperm extraction (TESE) surgery.

Abnormal red cell features associated with hereditary neurodegenerative disorders: the neuroacanthocytosis syndromes

PURPOSE OF REVIEW

This review discusses the mechanisms involved in the generation of thorny red blood cells (RBCs), known as acanthocytes, in patients with neuroacanthocytosis, a heterogenous group of neurodegenerative hereditary disorders that include chorea-acanthocytosis (ChAc) and McLeod syndrome (MLS).

RECENT FINDINGS

Although molecular defects associated with neuroacanthocytosis have been identified recently, their pathophysiology and the related RBC abnormalities are largely unknown. Studies in ChAc RBCs have shown an altered association between the cytoskeleton and the integral membrane protein compartment in the absence of major changes in RBC membrane composition. In ChAc RBCs, abnormal Lyn kinase activation in a Syk-independent fashion has been reported recently, resulting in increased band 3 tyrosine phosphorylation and perturbation of the stability of the multiprotein band 3-based complexes bridging the membrane to the spectrin-based membrane skeleton. Similarly, in MLS, the absence of XK-protein, which is associated with the spectrin-actin-4.1 junctional complex, is associated with an abnormal membrane protein phosphorylation state, with destabilization of the membrane skeletal network resulting in generation of acanthocytes.

SUMMARY

A novel mechanism in generation of acanthocytes involving abnormal Lyn activation, identified in ChAc, expands the acanthocytosis phenomenon toward protein-protein interactions, controlled by phosphorylation-related abnormal signaling.

Identification of novel single nucleotide polymorphisms associated with acute respiratory distress syndrome by exome-seq

Acute respiratory distress syndrome (ARDS) is a lung condition characterized by impaired gas exchange with systemic release of inflammatory mediators, causing pulmonary inflammation, vascular leak and hypoxemia. Existing biomarkers have limited effectiveness as diagnostic and therapeutic targets. To identify disease-associating variants in ARDS patients, whole-exome sequencing was performed on 96 ARDS patients, detecting 1,382,399 SNPs. By comparing these exome data to those of the 1000 Genomes Project, we identified a number of single nucleotide polymorphisms (SNP) which are potentially associated with ARDS. 50,190SNPs were found in all case subgroups and controls, of which89 SNPs were associated with susceptibility. We validated three SNPs (rs78142040, rs9605146 and rs3848719) in additional ARDS patients to substantiate their associations with susceptibility, severity and outcome of ARDS. rs78142040 (C>T) occurs within a histone mark (intron 6) of the Arylsulfatase D gene. rs9605146 (G>A) causes a deleterious coding change (proline to leucine) in the XK, Kell blood group complex subunit-related family, member 3 gene. rs3848719 (G>A) is a synonymous SNP in the Zinc-Finger/Leucine-Zipper Co-Transducer NIF1 gene. rs78142040, rs9605146, and rs3848719 are associated significantly with susceptibility to ARDS. rs3848719 is associated with APACHE II score quartile. rs78142040 is associated with 60-day mortality in the overall ARDS patient population. Exome-seq is a powerful tool to identify potential new biomarkers for ARDS. We selectively validated three SNPs which have not been previously associated with ARDS and represent potential new genetic biomarkers for ARDS. Additional validation in larger patient populations and further exploration of underlying molecular mechanisms are warranted.

Exposure of phosphatidylserine by Xk-related protein family members during apoptosis

Apoptotic cells expose phosphatidylserine (PtdSer) on their surface as an "eat me" signal. Mammalian Xk-related (Xkr) protein 8, which is predicted to contain six transmembrane regions, and its Caenorhabditis elegans homolog CED-8 promote apoptotic PtdSer exposure. The mouse and human Xkr families consist of eight and nine members, respectively. Here, we found that mouse Xkr family members, with the exception of Xkr2, are localized to the plasma membrane. When Xkr8-deficient cells, which do not expose PtdSer during apoptosis, were transformed by Xkr family members, the transformants expressing Xkr4, Xkr8, or Xkr9 responded to apoptotic stimuli by exposing cell surface PtdSer and were efficiently engulfed by macrophages. Like Xkr8, Xkr4 and Xkr9 were found to possess a caspase recognition site in the C-terminal region and to require its direct cleavage by caspases for their function. Site-directed mutagenesis of the amino acid residues conserved among CED-8, Xkr4, Xkr8, and Xkr9 identified several essential residues in the second transmembrane and second cytoplasmic regions. Real time PCR analysis indicated that unlike Xkr8, which is ubiquitously expressed, Xkr4 and Xkr9 expression is tissue-specific.

NUP214 fusion genes in acute leukemia (Review)

Nucleoporin 214 (NUP214), previously termed CAN, is required for cell cycle and nucleocytoplasmic transport. The genetic features and clinical implications of five NUP214-associated fusion genes are described in this review. SET-NUP214 was most frequently observed in T-cell acute lymphoblastic leukemia (T-ALL), concomitant with the elevated expression of HOXA cluster genes. Furthermore, the fusion transcript may be regarded as a potential minimal residual disease marker for SET-NUP214-positive patients. Episomal amplifications of NUP214-ABL1 are specific to T-ALL patients. The NUP214-ABL1 gene is observed in ~6% of T-ALL, in children and adults. Targeted tyrosine kinase inhibitors plus standard chemotherapy appear to present a promising treatment strategy. DEK-NUP214 is formed by the fusion of exon 2 of DEK and exon 6 of NUP214. Achieving molecular negativity of DEK-NUP214 is of great importance for individual management. SQSTM1-NUP214 and NUP214-XKR3 were only identified in one T-ALL patient and one cell line, respectively. The NUP214 fusions have significant diagnostic and therapeutic implications for leukemia patients. Additional NUP214-associated fusions require identification in future studies.

Genome-wide association study identifies a novel susceptibility gene for serum TSH levels in Chinese populations

Thyroid-stimulating hormone (TSH) is a sensitive indicator of thyroid function. High and low TSH levels reflect hypothyroidism and hyperthyroidism, respectively. Even within the normal range, small differences in TSH levels, on the order of 0.5-1.0 mU/l, are associated with significant differences in blood pressure, BMI, dyslipidemia, risk of atrial fibrillation and atherosclerosis. Most of the variance in TSH levels is thought to be genetically influenced. We conducted a genome-wide association study of TSH levels in 1346 Chinese Han individuals. In the replication study, we genotyped four candidate SNPs with the top association signals in an independent isolated Chinese She cohort (n = 3235). We identified a novel serum TSH susceptibility locus within XKR4 at 8q12.1 (rs2622590, Pcombined = 2.21 × 10(-10)), and we confirmed two previously reported TSH susceptibility loci near FOXE1 at 9q22.33 and near CAPZB at 1p36.13, respectively. The rs2622590_T allele at XKR4 and the rs925489_C allele near FOXE1 were correlated with low TSH levels and were found to be nominally associated to patients with papillary thyroid carcinoma (PTC) (OR = 1.41, P= 0.014 for rs2622590_T, and OR = 1.61, P= 0.030 for rs925489_C). The rs2622590 and rs925489 genotypes were also correlated with the expression levels of FOXE1 and XKR4, respectively, in PTC tissues (P = 2.41 × 10(-4) and P= 0.02). Our findings suggest that the SNPs in XKR4 and near FOXE1 are involved in the regulation of TSH levels.

Giant axon formation in mice lacking Kell, XK, or Kell and XK: animal models of McLeod neuroacanthocytosis syndrome

McLeod neuroacanthocytosis syndrome (MLS) is a rare X-linked multisystem disease caused by XK gene mutations and characterized by hematological and neurological abnormalities. XK, a putative membrane transporter, is expressed ubiquitously and is covalently linked to Kell, an endothelin-3-converting enzyme (ECE-3). Absence of XK results in reduction of Kell at sites where both proteins are coexpressed. To elucidate the functional roles of XK, Kell, and the XK-Kell complex associated with pathogenesis in MLS, we studied the pathology of the spinal cord, anterior roots, sciatic nerve, and skeletal muscle from knockout mouse models, using Kel(-/-), Xk(-/-), Kel(-/-)Xk(-/-), and wild-type mice aged 6 to 18 months. A striking finding was that giant axons were frequently associated with paranodal demyelination. The pathology suggests probable anterograde progression from the spinal cord to the sciatic nerve. The neuropathological abnormalities were found in all three genotypes, but were more marked in the double-knockout Kel(-/-)Xk(-/-) mice than in either Kel(-/-) or Xk(-/-) mice. Skeletal muscles from Xk(-/-) and Kel(-/-)Xk(-/-) mice showed mild abnormalities, but those from Kel(-/-) mice were similar to the wild type. The more marked neuropathological abnormalities in Kel(-/-)Xk(-/-) mice suggest a possible functional association between XK and Kell in nonerythroid tissues.

No evidence for a second evolutionary stratum during the early evolution of mammalian sex chromosomes

Mammalian sex chromosomes originated from a pair of autosomes, and homologous genes on the sex chromosomes (gametologs) differentiated through recombination arrest between the chromosomes. It was hypothesized that this differentiation in eutherians took place in a stepwise fashion and left a footprint on the X chromosome termed "evolutionary strata." The evolutionary stratum hypothesis claims that strata 1 and 2 (which correspond to the first two steps of chromosomal differentiation) were generated in the stem lineage of Theria or before the divergence between eutherians and marsupials. However, this prediction relied solely on the molecular clock hypothesis between pairs of human gametologs, and molecular evolution of marsupial sex chromosomal genes has not yet been investigated. In this study, we analyzed the following 7 pairs of marsupial gametologs, together with their eutherian orthologs that reside in stratum 1 or 2: SOX3/SRY, RBMX/Y, RPS4X/Y, HSFX/Y, XKRX/Y, SMCX/Y (KDM5C/D, JARID1C/D), and UBE1X/Y (UBA1/UBA1Y). Phylogenetic analyses and estimated divergence time of these gametologs reveal that they all differentiated at the same time in the therian ancestor. We have also provided strong evidence for gene conversion that occurred in the 3' region of the eutherian stratum 2 genes (SMCX/Y and UBE1X/Y). The results of the present study show that (1) there is no compelling evidence for the second stratum in the stem lineage of Theria; (2) gene conversion, which may have occurred between SMCX/Y and UBE1X/Y in the eutherian lineage, potentially accounts for their apparently lower degree of overall divergence.

Association of the NPAS3 gene and five other loci with response to the antipsychotic iloperidone identified in a whole genome association study

A whole genome association study was performed in a phase 3 clinical trial conducted to evaluate a novel antipsychotic, iloperidone, administered to treat patients with schizophrenia. Genotypes of 407 patients were analyzed for 334,563 single nucleotide polymorphisms (SNPs). SNPs associated with iloperidone efficacy were identified within the neuronal PAS domain protein 3 gene (NPAS3), close to a translocation breakpoint site previously observed in a family with schizophrenia. Five other loci were identified that include the XK, Kell blood group complex subunit-related family, member 4 gene (XKR4), the tenascin-R gene (TNR), the glutamate receptor, inotropic, AMPA 4 gene (GRIA4), the glial cell line-derived neurotrophic factor receptor-alpha2 gene (GFRA2), and the NUDT9P1 pseudogene located in the chromosomal region of the serotonin receptor 7 gene (HTR7). The study of these polymorphisms and genes may lead to a better understanding of the etiology of schizophrenia and of its treatment. These results provide new insight into response to iloperidone, developed with the ultimate goal of directing therapy to patients with the highest benefit-to-risk ratio.

McLeod syndrome: a neurohaematological disorder

The X-linked McLeod syndrome is defined by absent Kx red blood cell antigen and weak expression of Kell antigens, and this constellation may be accidentally detected in routine screening of apparently healthy blood donors. Most carriers of this McLeod blood group phenotype have acanthocytosis and elevated serum creatine kinase levels and are prone to develop a severe neurological disorder resembling Huntington's disease. Onset of neurological symptoms ranges between 25 and 60 years, and the penetrance of the disorder appears to be high. Additional symptoms of the McLeod neuroacanthocytosis syndrome that warrant therapeutic and diagnostic considerations include generalized seizures, neuromuscular symptoms leading to weakness and atrophy, and cardiopathy mainly manifesting with atrial fibrillation, malignant arrhythmias and dilated cardiomyopathy. Therefore, asymptomatic carriers of the McLeod blood group phenotype should have a careful genetic counseling, neurological examination and a cardiologic evaluation for the presence of a treatable cardiomyopathy.

McLeod phenotype without the McLeod syndrome

BACKGROUND

McLeod neuroacanthocytosis syndrome is a late-onset X-linked multisystem disorder affecting the peripheral and central nervous systems, red blood cells (RBCs), and internal organs. A variety of mutations have been found in the responsible gene (XK) including single nonsense and missense mutations, nucleotide mutations at or near the splice junctions of introns of XK, and different deletion mutations. To date no clear phenotype-genotype correlation is apparent. The clinical details of one case of McLeod phenotype without apparent neuromuscular abnormalities have been reported. Here the clinical details of two additional cases are presented, of which the genetic details have previously been published.

STUDY DESIGN AND METHODS

Two asymptomatic or minimally symptomatic cases at ages expected to manifest the McLeod syndrome (MLS) were evaluated. The first case had been authenticated as a genuine McLeod both by serology and by genotyping (R222G missense mutation) and the second case had a mutation in XK (IVS2+5G>A) and by serology exhibited very weak Kx antigen and no detectable Kell antigens, except extremely low k antigen by adsorption-elution technique. The patients were examined for hematologic, neurologic, and other clinical abnormalities.

RESULTS

Despite documented McLeod phenotype on RBCs, and identified mutations of XK, neurologic and other clinical findings were minimal at ages expected to manifest MLS.

CONCLUSIONS

The different XK mutations may have different effects upon the XK gene product and thus may account for the variable phenotype.

Insights into extensive deletions around the XK locus associated with McLeod phenotype and characterization of two novel cases

The McLeod phenotype is derived from various forms of XK gene defects that result in the absence of XK protein, and is defined hematologically by the absence of Kx antigen, weakening of Kell system antigens, and red cell acanthocytosis. Individuals with the McLeod phenotype usually develop late-onset neuromuscular abnormalities known as the McLeod syndrome (MLS). MLS is an X-linked multi-system disorder caused by absence of XK alone, or when the disorder is caused by large deletions, it may be accompanied with Duchenne muscular dystrophy (DMD), chronic granulomatous disease (CYBB), retinitis pigmentosa (RPGR), and ornithine transcarbamylase deficiency (OTC). XK defects derived from a large deletion at the XK locus (Xp21.1) have not been characterized at the molecular level. In this study, the deletion breakpoints of two novel cases of McLeod phenotype with extensive deletions are reported. Case 1 has greater than 1.12 million base-pairs (mb) deletion around the XK locus with 7 genes affected. Case 2 has greater than 5.65 mb deletion from TCTE1L to DMD encompassing 20 genes. Phylogenetic analyses demonstrated that DMD, XK and CYBB have close paralogs, some of which may partially substitute for the functions of their counterparts. The loci around XK are highly conserved from fish to human; however, the disorders are probably specific to mammals, and may coincide with the translocation of the loci to the X chromosome after the speciation in birds. The non-synonymous to synonymous nucleotide substitution rate ratio (omega=dN/dS) in these genes was examined. CYBB and RPGR show evidence of positive selection, whereas DMD, XK and OTC are subject to selective constraint.

Expression profiles of mouse Kell, XK, and XPLAC mRNA

Kell and XK are related because in red cells they exist as a disulfide-bonded complex. Kell is an endothelin-3-converting enzyme, and XK is predicted to be a transporter. Absence of XK, which is accompanied by reduced Kell on red cells, results in acanthocytosis and late-onset forms of central nervous system and neuromuscular abnormalities that characterize the McLeod syndrome. In this study, expression of mouse XK, XPLAC, a homolog of XK, and Kell were compared by in situ hybridization histochemistry (ISHH) and RT-PCR. ISHH showed that Kell and XK are coexpressed in erythroid tissues. ISHH detected XK, but not Kell, mRNA in testis, but RT-PCR indicated that both Kell and XK are coexpressed. XK, but not Kell, was significantly expressed in brain, spinal cord, small intestine, heart, stomach, bladder, and kidney. ISHH did not detect XK in skeletal muscle but RT-PCR did. In brain, XK was predominantly expressed in neuronal rather than in supportive cells. By contrast, XPLAC was predominantly expressed in the thymus. Coexpression of Kell and XK in erythroid tissues and the different expressions in non-erythroid tissues suggest that XK may have a complementary hematological function with Kell and a separate role in other tissues.