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. 2024 Sep 14;51(1):984. doi: 10.1007/s11033-024-09906-7

Whole genome sequencing in adults with clinical hallmarks of hypophosphatasia negative for ALPL variants

Lothar Seefried 1,, Anna Petryk 2, Guillermo del Angel 2, Felix Reder 3, Peter Bauer 3
PMCID: PMC11401779  PMID: 39276275

Abstract

Background

Hypophosphatasia (HPP) is a rare disease caused by deficient activity of tissue-nonspecific alkaline phosphatase (ALP), encoded by the ALPL gene. The primary objective was to explore novel ALPL variants by whole genome sequencing (WGS) in patients with HPP who previously tested negative by standard methods for ALPL variants. The secondary objective was to search for genes beyond ALPL that may reduce ALP activity or contribute to HPP symptoms.

Methods and results

WGS was performed in 16 patients clinically diagnosed with HPP who had ALP activity below the normal range and tested negative for ALPL variants. Genetic variants in ALPL and genes possibly associated with low ALP activity or phenotypic overlap with HPP were assessed. All 16 patients had ALP activity below the normal range. WGS did not identify any novel disease-causing ALPL variants. Positive findings for other gene variants were identified in 4 patients: 1 patient presented with variants in COL1A1, NLRP12, and SCN9A, coding for collagen, type, I alpha-1 chain, nod-like receptor pyrin domain containing 12, and sodium voltage-gated channel alpha subunit 9, respectively; 1 presented with a heterozygous, likely pathogenic variant in P3H1 coding for prolyl 3 hydroxylase 1; 1 presented with a heterozygous pathogenic variant in SGCE, coding for sarcoglycan epsilon; and 1 presented with a heterozygous variant of uncertain significance in VDR, encoding vitamin D receptor.

Conclusion

Genomic analysis did not identify novel ALPL variants or a pattern of disease-causing variants in genes other than ALPL to explain the HPP phenotype in these patients.

Registration

Clinicaltrials.gov identifier: NCT04925804.

Keywords: Hypophosphatasia, Rare disease, Diagnosis, Genetics, Whole genome sequencing

Introduction

Hypophosphatasia (HPP) is a rare, metabolic disease caused by deficient activity of the enzyme tissue-nonspecific alkaline phosphatase (ALP), encoded by the ALPL gene [1]. Low ALP activity may result in accumulation of its substrates, including inorganic pyrophosphate (PPi) and pyridoxal 5ʹ-phosphate (PLP). The accumulation of PPi, a potent inhibitor of bone mineralization, can cause defective skeletal mineralization, while impaired dephosphorylation of PLP into pyridoxal can result in vitamin B6–responsive seizures [1, 2]. While the clinical spectrum of HPP is extremely broad across different age groups, it is often associated with severe systemic morbidity, particularly due to musculoskeletal deficits (e.g., fractures, muscle weakness) and pain, in patients manifesting the disease later in life [3].

Accurate, timely diagnosis of HPP is important for appropriate disease management [1]. A clinical diagnosis of HPP is based on identification of characteristic manifestations [1]. Differential diagnosis involves the exclusion of other pathologic conditions associated with phenotypic overlap and/or low ALP activity [1]. Sequencing of the ALPL coding region can be used to confirm a diagnosis of HPP [1] and identifies variants in approximately 95% of patients with the disease [4]. To date, more than 400 variants of the ALPL gene, predominantly missense variants, have been identified [5], indicating substantial allelic heterogenicity. However, standard sequencing techniques may fail to detect ALPL variants in some patients because of the presence of new variants not yet classified as pathogenic, variants within introns or within the regulatory sequences of ALPL, large deletions/duplications not typically assessed by standard methods, or variants in other genes involved in the regulation of ALPL [6, 7].

The primary objective of this study was to identify novel ALPL variants by whole gene sequencing (WGS) in patients with HPP who previously tested negative by standard methods for ALPL variants. The secondary objective was to search for genes beyond ALPL that may reduce ALP activity or contribute to HPP symptoms in these patients.

Materials and methods

Objectives

The primary study objective was to compare the results of WGS with those of standard sequencing carried out for detection of ALPL variants. The secondary objective was to search for gene variants within or beyond ALPL that could result in low ALP activity.

Patients

This prospective, observational cohort study (NCT04925804) was conducted between June 2 and December 2, 2021, at a single HPP expert center at University of Würzburg, Germany. Eligible patients had a clinical diagnosis of HPP based on age- and sex-adjusted low serum ALP activity and clinical symptoms. ALP activity was determined on 2 occasions, at least 1 month apart, using a commercially available assay. Patients were required to have an available sequencing report indicating no pathogenic variants, no likely pathogenic variants, or no variants of uncertain significance (VUS) in ALPL. Patients were excluded if they had other potential causes of low ALP, including celiac disease, clofibrate therapy, cleidocranial dysplasia, Cushing’s syndrome, hypothyroidism, massive blood transfusion, milk-alkali syndrome, multiple myeloma, osteogenesis imperfecta type II, pernicious or profound anemia, starvation, vitamin C deficiency, vitamin D intoxication, zinc deficiency, and magnesium deficiency.

For each patient, medical history was obtained using a standardized questionnaire indicating specified signs and symptoms as present or absent, and entering the history and date of use of specified medications. Data were collated by means of an electronic case report form. Patient-level data were de-identified by assigning project-specific codes (i.e., patient numbers) to the clinical data and biologic specimens. The protocol and an amendment were approved by the ethics committee of the medical faculty of the University of Rostock (Vote number A2020-0169) as well as by the ethics committee of the University of Würzburg (Vote number 221/20). The study was conducted in compliance with guidelines for Good Clinical Practice and the ethical principles of the Declaration of Helsinki. Written informed consent was obtained from patients prior to initiation of study procedures.

WGS

Kits containing the Centogene GmbH (Rostock, Germany) proprietary dry blood spot card (CentoCard) were used to collect blood samples from patients and were returned to Centogene for analysis. Genomic DNA was isolated from the CentoCard using a standard protocol developed by Centogene. After fragmentation of genomic DNA by sonication, Illumina adapters were ligated to generate fragments for subsequent sequencing on the HiSeqX platform (Illumina, Inc., San Diego, CA, USA) to yield an average coverage depth of more than 30X.

Genetic testing was performed to detect ALPL variants that are pathogenic/likely pathogenic/VUS and rare in the general population. The ALPL gene (including exons, introns, and regulatory sequences) was evaluated, as well as a panel of genes (Table 1) directly or indirectly related to ALPL, ALPL regulation, or the HPP phenotype based on available evidence in the literature [1, 8, 9]. Variant calls included single nucleotide variants (SNVs; deep intronic variants with or without splice predictions were also considered), copy number variants (CNVs), and any other rearrangements (e.g., inversions, gross insertions, translocations). All potentially relevant variants detected in ALPL or any additionally paneled genes are reported herein, including variants found in reference population datasets (e.g., gnomAD).

Table 1.

Candidate genes that may regulate ALP activity or modify the HPP phenotype

Diseases with Phenotypic Overlap
Disease Expected ALP Enzyme Activity Gene(s)
Osteogenesis imperfecta Normal or elevated [1] BMP1, COL1A1, COL1A2, CREB3L1, IFITM5, MBTPS2, SERPINH1, SP7, SPARC, TENT5A, TMEM38B, WNT1
Cole-Carpenter syndrome Normal P4HB, SEC24D
Dentinogenesis imperfecta Normal DSPP
Vitamin D-dependent rickets Elevated CYP27B1, VDR
Tumoral calcinosis Elevated FGF23
Campomelic dysplasia Normal SOX9
Hadju-Cheney syndrome Normal NOTCH2
Cleidocranial dysplasia Normal or reduced [25] RUNX2
Hypophosphatemic rickets Elevated PHEX, SLC37A3
Stüve-Wiedemann syndrome Normal LIFR
Modifier Genes of HPP
Study population Gene(s)
46 patients with HPP, ALP not systematically available [8] ANKH, ENPP1, FGFR3, PHOSPHO1, PTH1R, PTH2R, SPP1, TNFRSF11A
Population-based GWAS (various populations) [9] GPLD1
Gene Panels
Panel Genes
Blueprint Genetics (Skeletal Dysplasia with Abnormal Mineralization panel) ALPL, ANKH, B4GALT7, CASR, CLCN5, COL1A1, COL1A2, COL3A1, COL5A1, COL5A2, CRTAP, CYP27B1, ENPP1, FBN1, FGF23, FKBP10, GALNT3, MGP, P3H1, PHEX, PLOD2, PLS3, PPIB, PTDSS1, SERPINF1, SLC34A3, SLC39A13, SNX10, SOX9, TNFRSF11A, TNFRSF11B, VDR
GeneDx (Abnormal Mineralization panel) ALPL, ANKH, AP2S1, CASR, CLCN5, CYP27B1, CYP2R1, DMP1, ENPP1, FAH, FGF23, PHEX, SLC34A1, SLC34A3, SLC9A3R1, VDR

ALP, alkaline phosphatase; GWAS, genome-wide association study; HPP, hypophosphatasia

Genes: ALPL, alkaline phosphatase, tissue-nonspecific; ANKH, ANKH inorganic pyrophosphate transport regulator; AP2S1, adaptor-related protein complex 2, sigma-1 subunit; B4GALT7, beta-1,4-galactosyltransferase 7; BMP1, bone morphogenetic protein 1; CASR, calcium-sensing receptor; CLCN5, chloride voltage-gated channel 5; COL1A1, collagen, type I, alpha-1 chain; COL1A2, collagen, type I, alpha-2 chain; COL3A1, collagen, type III, alpha-1 chain; COL5A1, collagen, type V, alpha-1 chain; COL5A2, collagen, type V, alpha-2 chain; CREB3L1, cAMP responsive element-binding protein 3-like 1; CRTAP, cartilage-associated protein; CYP2R1, cytochrome P450, family 2 subfamily R member 1; CYP27B1, cytochrome P450, family 27 subfamily B member 1; DMP1, dentin matrix acidic phosphoprotein 1; DSPP, dentin sialophosphoprotein; ENPP1, ectonucleotide pyrophosphatase/phosphodiesterase 1; FAH, fumarylacetoacetate hydrolase; FBN1, fibrillin 1; FGF23, fibroblast growth factor 23; FGFR3, fibroblast growth factor receptor 3; FKBP10, FKBP prolyl isomerase 10; GALNT3, polypeptide N-acetylgalactosaminyltransferase 3; GPLD1, glycosylphosphatidylinositol-specific phospholipase D1; IFITM5, interferon-induced transmembrane protein 5; LIFR, leukemia inhibitory factor receptor subunit alpha; MGP, matrix Gla protein; MBTPS2, membrane-bound transcription factor peptidase, site 2; NOTCH2, notch receptor 2; P3H1, prolyl 3-hydroxylase 1; P4HB, prolyl 4-hydroxylase subunit beta; PHEX, phosphate-regulating endopeptidase, X-linked; PHOSPHO1, phosphoethanolamine/phosphocholine phosphatase 1; PLOD2, procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2; PLS3, plastin 3; PPIB, peptidylprolyl isomerase B; PTDSS1, phosphatidylserine synthase 1; PTH1R, parathyroid hormone 1 receptor; PTH2R, parathyroid hormone 2 receptor; RUNX2, RUNX-family transcription factor 2; SEC24D, SEC24 homolog D, COPII coat complex component; SERPINF1, serpin family F, member 1; SERPINH1, serpin family H, member 1; SLC34A1, solute carrier family 34, member 1; SLC34A3, solute carrier family 34, member 3; SLC37A3, solute carrier family 37, member 3; SLC39A13, solute carrier family 39, member 13; SLC9A3R1, solute carrier family 9, member 3, regulator 1 (alias NHERF1); SNX10, sorting nexin 10; SOX9, SRY-box transcription factor 9; SP7, transcription factor Sp7; SPARC, secreted protein, acidic and cysteine-rich; SPP1, secreted phosphoprotein 1; TENT5A, terminal nucleotidyltransferase 5A; TMEM38B, transmembrane protein 38B; TNFRSF11A, tumor necrosis family receptor superfamily, member 11a; TNFRSF11B, tumor necrosis family receptor superfamily, member 11b; VDR, vitamin D receptor; WNT1, Wnt family member 1

Selection of target genes

Several databases, including the Online Mendelian Inheritance in Man (OMIM) database, the Kyoto Encyclopedia of Genes and Genomes (KEGG) Pathway Database, and PubMed, were used to identify a panel of candidate genes that may affect ALPL or the HPP phenotype. Specific search terms used to identify genes in peer-reviewed scientific papers and reviews published in the past 30 years included alkaline phosphatase activity, alkaline phosphatase, activity regulation, ALP activity, ALP activity regulation, ALPL regulation, HPP disease, and hypophosphatasia. Genes identified in the search of the OMIM database were added to the candidate gene list in the current study if pathogenic variants with perturbation of ALP were known and the mode of inheritance was specified. The KEGG Pathway Database was reviewed to identify upstream and downstream genes involved in ALP pathways; genes identified in this search were added to the list of candidate genes. From these candidates, a joint steering committee of investigators from Alexion, AstraZeneca Rare Disease and Centogene mutually agreed on a final panel of genes for analysis. Genes included in the analysis and their association with diseases similar to HPP are listed in Table 1.

Data analysis

An end-to-end in-house bioinformatics pipeline, including base calling, primary filtering of low-quality reads and probable artifacts, and annotation of variants was applied. Subsequently, all disease-causing variants reported in the Human Gene Mutation Database [10], ClinVar [11], or CentoMD, as well as variants with minor allele frequency below 1% in the Exome Aggregation Consortium (ExAC) database [12], were considered. In addition to considering all pertinent inheritance patterns, family history and clinical information were used to evaluate identified variants. All identified variants were assessed for their pathogenicity and causality and classified as pathogenic, likely pathogenic, VUS, likely benign, or benign according to the standards and guidelines of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology [13]. All variants related to the patient’s phenotype and CNVs of unknown significance were also reported.

To detect structural variants that might affect ALPL but may have been missed with exome sequencing, raw sequence data analysis was performed using the DRAGEN pipeline (Illumina, Inc.); this analysis included base calling, de-multiplexing, alignment to the hg19 human reference genome (Genome Reference Consortium GRCh37), and variant calling. Short-reads were aligned to the GRCh37 (hg19) build of the human reference genome using the DRAGEN aligner algorithm. Variant calling was performed on the alignment files SNVs and indels using Small Variant Caller. Deletions, duplications, inversions, and translocations of at least 50 bp in size were detected using MANTA (Illumina, Inc.) and DRAGEN. Variants were annotated using SnpEff and in-house ad hoc bioinformatics tools.

Results

Patients

Of the 16 patients enrolled, 81.3% (n = 13) were female. The mean (SD; range) age was 45 (5.4; 36 − 54) years overall, 43 (4.9; 36 − 54) years among female patients, and 51 (1.0; 50 − 52) years among male patients. Table 2 summarizes clinical information reported for each patient.

Table 2.

Summary of clinical manifestations reported for each patient

Patient Number Clinical Information
1 Abnormal morphology of thorax, arthralgia, abdominal distension, attention-deficit/hyperactivity disorder, bone pain, decreased bone mineral density, depression, failure to thrive, fatigue, gait disorder, global development delay, headache, joint stiffness, joint swelling, memory impairment, muscle weakness, myalgia, osteopenia, osteoporosis, pain, postural instability, reflux esophagitis, restricted joint mobility, scoliosis, seizure, short stature
2 Arthralgia, abdominal distension, headache, scoliosis
3 Headache, fatigue, muscle weakness, myalgia, postural instability, scoliosis
4 Abnormal thoracic morphology, abdominal distention, arthralgia, bone pain, decreased bone mineral density, fatigue, gait disturbance, headache, joint stiffness, joint swelling, limitation of joint mobility, memory impairment, muscle weakness, myalgia, osteopenia, osteoporosis, pain, pain in joints, postural instability, reflux, reflux esophagitis, scoliosis
5 Abdominal distention, abnormality of teeth, arthralgia, bone pain, chronic pain, delayed ambulatory ability, fatigue, failure to thrive, joint swelling, muscle weakness, myalgia, restriction of joint mobility, scoliosis, seizure
6 Abdominal distention, abnormal alkaline phosphatase level, arthralgia, bone pain, decreased bone mineral density, dyspnea, headache, muscle weakness, pain, postural instability, premature loss of permanent teeth, recurrent fractures
7 Abdominal distention, abnormal alkaline phosphatase level, arthralgia, bone pain, depression, fatigue, gait disturbance, joint stiffness, memory impairment, myalgia, nephrocalcinosis, pain, postural instability, premature loss of teeth, reflux esophagitis, restriction of joint mobility
8 Abdominal distention, arthralgia, body aches, bone pain, fatigue, headache, memory impairment, muscle weakness, myalgia, postural instability
9 Abdominal distention, arthralgia, attention-deficit/hyperactivity disorder, bone pain, fatigue, memory impairment, muscle weakness, myalgia, reflux esophagitis, seizure, scoliosis
10 Abdominal distention, abnormality of calvarial morphology, arthralgia, bone pain, chronic pain, decreased bone mineral density, fatigue, joint swelling, premature loss of deciduous teeth, recurrent fractures, reflux esophagitis
11 Abdominal distention, arthralgia, body aches, depression, joint stiffness, fatigue, muscle weakness, myalgia, premature loss of permanent teeth, reflux esophagitis, restriction of joint mobility, scoliosis
12 Abdominal distention, abnormal thoracic morphology, arthralgia, attention-deficit/hyperactivity disorder, bone pain, chronic pain, decreased bone mineral density, fatigue, headache, joint swelling, kidney stone, limitation of joint mobility, muscle weakness, pain, reflux esophagitis, rickets, scoliosis
13 Abdominal distention, arthralgia, attention-deficit/hyperactivity disorder, bone pain, decreased body weight, depression, dysphagia, fatigue, gait disturbance, headache, joint stiffness, loss of appetite, memory impairment, muscle weakness, myalgia, restricted joint mobility, scoliosis, seizure
14 Arthralgia, body aches, bone pain, decreased bone mineral density, fatigue, joint stiffness, kidney stone, nephrocalcinosis, osteopenia, osteoporosis, pain, recurrent fractures, restriction of joint mobility, seizure
15 Arthralgia, attention-deficit/hyperactivity disorder, body aches, bone pain, depression, fatigue, joint stiffness, memory impairment, muscle weakness, myalgia, pain, postural instability, restricted joint mobility, scoliosis
16 Abdominal distention, arthralgia, bone pain, body aches, decreased bone mineral density, depression, failure to thrive, fatigue, joint stiffness, muscle weakness, myalgia, osteopenia, osteoporosis, pain, restricted joint mobility

Clinical chemistry

Figure 1 shows mean (range) serum ALP activity for each patient measured on 2 occasions at least 1 month apart. Mean (SD; range) ALP enzyme activity was 30.81 (5.68; 17.90 − 40.0) U/L for the first measurement and 32.62 (5.80; 23.0 − 39.0) U/L for the second measurement. All measurements were below the normal age- and sex-adjusted range for ALP enzyme activity for adult males (53–128 U/L) and females (42–98 U/L) [14].

Fig. 1.

Fig. 1

ALP enzyme activity.a aNormal range: females 42–98 U/L, males 53–128 U/L. ALP, alkaline phosphatase

Genetic testing

All 16 patients received a genetic report and were confirmed by WGS to be negative for clinically relevant ALPL variants. Table 1 lists genes that were identified through OMIM search as regulators of ALPL or the HPP phenotype. Four patients (25%; patients 3, 4, 7, and 15) had positive WGS findings for variants in other genes that may interfere with ALP enzyme activity or result in some clinical manifestations mimicking HPP (Table 3).

Table 3.

Summary of positive results for genetic variants in genes whose products may regulate ALP activity or modify HPP phenotype

Pt no. Sex Age, y Patient Symptoms Gene and Function Positional Variant and Zygosity Classification Interpretation Clinical Symptoms Associated With Identified Gene Variant
3 F 54 Headache, fatigue, muscle weakness, myalgia, postural instability, scoliosis

COL1A1

Pro-alpha1 chains of collagen triple helix

c.642 + 16 A > G

Heterozygous

VUS Osteogenesis imperfecta types 1–4, Ehlers-Danlos syndrome type VIIA and classical subtype, Caffey disease, idiopathic osteoporosis Low bone density, bone fragility, long bone fractures, fractures with minimal trauma, joint and skin hyperextensibility, soft tissue swelling over affected bones

NLRP12

Attenuating factor of inflammation

c.727 A > G

(p.Arg243Gly)

Heterozygous

VUS Familial cold autoinflammatory syndrome type 2 Episodic and recurrent rash, urticaria, arthralgia, myalgia, headache

SCN9A

Voltage-gated sodium channel involved in nociception signaling

Deletion of chr2:167157718–167462541

Heterozygous

VUS Primary erythromelalgia, paroxysmal extreme pain disorder, channelopathy-associated insensitivity to pain Recurrent episodes of intense, burning pain and redness, warmth, swelling; insensitivity to pain
4 F 43 Abnormal thoracic morphology, arthralgia, abdominal distention, gait disturbance, memory impairment, joint stiffness, joint swelling, bone pain, headache, limitation of joint mobility, fatigue, muscle weakness, myalgia, osteopenia, osteoporosis, postural instability, decreased bone mineral density, reflux, pain in joints, reflux esophagitis, pain, scoliosis

P3H1

Member of the collagen prolyl hydroxylase family

c.1346-1G > A

Heterozygous

Likely pathogenic Osteogenesis imperfecta type 8 autosomal recessive inheritance Connective tissue disorder, increased bone fragility, low bone mass, tendency toward fractures
7 F 36 Abnormal ALP activity, arthralgia, abdominal distention, depression, gait disturbance, memory impairment, joint stiffness, bone pain, restriction of joint mobility, fatigue, myalgia, nephrocalcinosis, postural instability, premature loss of teeth, reflux esophagitis, pain

SGCE

Transmembrane protein that links the actin skeleton to the extracellular matrix

c.817 C > T

Premature stop codon

Heterozygous

Pathogenic; SGCE locus is known to be imprinted Myoclonus dystonia type 11 autosomal dominant inheritance Rare movement disorder with mild to moderate dystonia
15 M 51 Arthralgia, attention-deficit/hyperactivity disorder, body aches, depression, memory impairment, joint stiffness, bone pain, restricted joint mobility, fatigue, muscle weakness, myalgia, postural instability, pain, scoliosis

VDR

Vitamin D receptor superfamily

c.265 A > G

(p.Met89Val)

Heterozygous

VUS

Vitamin D–dependent rickets type 2 A,

autosomal recessive inheritance

Characterized by hypocalcemia, severe rickets, and alopecia

ALP, alkaline phosphatase; HPP, hypophosphatasia; VUS, variant of uncertain significance

Genes: COL1A1, collagen, type I, alpha-1 chain; NLRP12, nod-like receptor pyrin domain containing 12; P3H1, prolyl 3-hydroxylase 1; SCN9A, sodium voltage-gated channel alpha subunit 9; SGCE, sarcoglycan epsilon; VDR, vitamin D receptor

Patient 3 (54-year-old woman) was found to have a heterozygous intronic VUS in the splice region of the COL1A1 gene on chromosome 17 (genomic location, 48275294T > C; variant, NM_000088.3:c.642 + 16 A > G), which codes for collagen, type I, alpha-1 protein. Additionally, testing outside the requested gene panel showed a heterozygous missense VUS in NLRP12 on chromosome 19 (genomic location, 54314186T > C; variant, NM_001277126.1:c.727 A > G NM_001277126.1 [p.Arg243Gly]), coding for the nucleotide-binding leucine-rich repeat-containing receptor 12, as well as a CNV (heterozygous deletion) of SCN9A, coding for sodium voltage-gated channel alpha subunit 9, from chromosome 2 (genomic location, 167157718–167362541).

Patient 4 (43-year-old woman) was found to be a heterozygous carrier of a likely pathogenic variant (NM_001243246.1:c.1346-1G > A) in the P3H1 gene, which codes for prolyl 3-hydroxylase 1, a member of the collagen prolyl hydroxylase family implicated in autosomal recessive osteogenesis imperfecta [15]. Testing outside the requested gene panel showed that Patient 7 (36-year-old woman) had a heterozygous pathogenic variant (NM_001346713.1:c.817 C > T, leading to a premature stop codon) in SGCE, coding for sarcoglycan epsilon, a transmembrane protein that links the cytoskeleton protein, actin, to the extracellular matrix. This SGCE variant is involved in myoclonus dystonia syndrome (MDS) type 11, a disease with autosomal dominant inheritance that results in rapid, involuntary muscle jerks. No other variant relevant to the described phenotype was found. Patient 15 (51-year-old man) had a heterozygous VUS (positional mutation, NM_001017536.1:c.265 A > G, [p.Met89Val]) in VDR, which codes for the vitamin D receptor.

Discussion

HPP is a disabling condition with wide-ranging impacts on health-related quality of life [16]. However, accurate diagnosis and appropriate treatment are often delayed, owing, in part, to symptom overlap between HPP and more common conditions [1, 17]. Sequencing of the ALPL coding region confirms the diagnosis of HPP if an ALPL variant is identified; however, some patients with clinical and laboratory findings consistent with a diagnosis of HPP have negative genetic testing results [4].

The current study was performed to further elucidate the genetic underpinnings of ALPL and non-ALPL variants beyond what can be detected with standard sequencing of the ALPL coding region, including genes that may interfere with ALPL expression or processing, or activity of ALP, thus modifying the phenotypic presentation of HPP.

With regard to the primary objective of this study, WGS did not reveal any additional variants in ALPL that, according to the current knowledge, were considered pathogenetically relevant and caused reduced ALP activity and HPP symptoms in these patients. However, low ALP activity may be attributed to variation in the non-coding region of ALPL, since there is no definition of what constitutes normal ALPL intronic sequence vs. pathogenic variations in ALPL intronic sequences.

Results of this study with respect to the secondary objective of identifying novel genetic variants beyond ALPL that could be pathogenetically relevant do not provide an answer to the long-standing speculation that confounding genes modulate impact on either ALPL expression or ALP activity. Although we have observed variations in several genes that were a priori selected as potentially responsible for low ALP activity, or demonstrated some phenotypic overlap with HPP, we do not claim these to be causative of HPP. The mechanisms involved in this observed association remain unknown. Further research is needed to better understand the molecular basis of HPP in these ALPL variant-negative patients.

All 16 patients with clinical and laboratory results indicative of HPP were confirmed by WGS to be negative for pathogenic ALPL variants, including rare, non-coding variants with potential implications in gene splicing and gross copy number alterations. However, 4 of the 16 patients had findings for genetic variants in other genes with the potential for regulating ALP activity or modifying the HPP phenotype. Variants in COL1A1 are associated with bone disorders, including osteogenesis imperfecta types 1–4 [18]. Osteogenesis imperfecta is a connective tissue disorder associated with bone fragility, making it a potential differential diagnosis of HPP [1, 19]. Variants in SCN9A are associated with primary erythromelalgia, paroxysmal extreme pain disorder, or insensitivity to pain [20], and variants in NLRP12 are associated with familial cold-induced autoinflammatory syndromes [21]. Patient 3 reported history of pain and headache, which could correspond with these conditions but are nonspecific. Since this patient had no history of other, more specific manifestations or clinical presentations associated with these disorders, such as low bone mineral density, rash, redness, warmth, or swelling, the patient was not diagnosed with any of these disorders.

The P3H1 variant NM_001243246.1:c.1346-1G > A is a splice variant known to impair the function of the nearby acceptor splice site, and a variant at this position (with a different nucleotide exchange, c.1346-1G > C, ClinVar variation ID: 284532, clinical testing, listed as pathogenic) has been reported as disease-causing for autosomal recessive osteogenesis imperfecta type 8 [15]. However, the patient’s clinical phenotype was not consistent with osteogenesis imperfecta type 8 and a second relevant variant in P3H1 compatible with an autosomal-recessive trait was not demonstrable. Therefore, the patient is currently deemed to be an asymptomatic carrier of a pathogenic P3H1 variant.

The pathogenic variant in the SGCE gene (NM_001346713.1:c.817 C > T) leads to a premature stop codon and has been described as pathogenic for MDS (OMIM: 159900), a rare movement disorder characterized by dystonia and lightning-like myoclonic jerks [22]. As the SGCE locus is imprinted, approximately 95% of patients with MDS who inherit the variant from their mother will remain healthy, whereas almost all children who inherit the variant from their father will develop MDS [22]. De novo variants are also described. Since the clinical picture of MDS has little overlap with that of the patient and cannot explain her symptoms (e.g., premature loss of permanent teeth), this variant was likely inherited from the mother and thus does not result in the typical phenotype of MDS.

VDR encodes the vitamin D receptor, a member of the nuclear hormone receptor superfamily of ligand-inducible transcription factors [23]. The identified variant NM_001017536.1:c.265 A > G (p.Met89Val) leads to an amino acid exchange from Met to Val at position 89. Pathogenic variants in VDR are associated with vitamin D–resistant rickets type 2A, a disease with an autosomal recessive inheritance [24]. Vitamin D–resistant rickets type 2A or hypocalcemic vitamin D–resistant rickets is an inherited disorder of vitamin D characterized by hypocalcemia, severe rickets, and, in many cases, alopecia [24]. Although not directly related to ALPL, variants in VDR may interact with ALPL at some level and influence the phenotypic presentation of the disease.

The current study is not without limitations. The small study population was overrepresented by females from a single geographic region, reducing generalizability of the results. Accordingly, results of this analysis highlight the importance of future genomic studies in larger cohorts from a variety of geographic regions, including additional genes that may contribute to HPP manifestations, to better understand the genetic basis of HPP. In addition, future studies should investigate epigenetic and extragenomic factors via transcriptomic and metabolomics analyses.

Conclusions

The genetic basis of HPP in patients without ALPL variants remains unclear. In the present analysis, all patients were confirmed via WGS to be negative for variants in ALPL despite low ALP activity. Variants in non-ALPL genes did not conclusively overlap with genes associated with any other disorder and may contribute to the HPP phenotype through unknown mechanisms. These data provide an important rationale for further investigation of molecular mechanisms linking genes beyond ALPL to the HPP phenotype and of the genetic underpinnings of HPP. Nevertheless, diagnosis of HPP remains a clinical diagnosis based on signs and symptoms in affected individuals and does not require detection of ALPL variants [1].

Acknowledgements

The authors gratefully acknowledge the contributions of Dr. Anika Knaust, CPM, Kornelia Tripolski, Genetic Scientist, and Dr. Krishna Kumar Kamarswamy, Vice President, Research and Development, Bioinformatics, all from Centogene, Rostock, Germany, and Jasmin Baumann, Study Coordinator of the Clinical Trial Unit at the University of Würzburg for facilitating the conduct of the study. Editorial support was provided by Violet Kiesel, PhD, and Drayton Hammond, PharmD, of Peloton Advantage, LLC, an OPEN Health company, and funded by Alexion, AstraZeneca Rare Disease.

Abbreviations

ALP

Alkaline Phosphatase

CNV

Copy Number Variants

ExAC

Exome Aggregation Consortium

GWAS

Genome-Wide Association Study

HPP

Hypophosphatasia

KEGG

Kyoto Encyclopedia of Genes and Genomes

MDS

Myoclonus Dystonia Syndrome

OMIM

Online Mendelian Inheritance in Man

PLP

Pyridoxal 5ʹ-Phosphate

PPi

inorganic Pyrophosphate

SD

Standard Deviation

SNV

Single Nucleotide Variants

VUS

Variants of Uncertain Significance

WGS

Whole Genome Sequencing

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Felix Reder and Peter Bauer. All authors contributed to development of the first draft of the manuscript, revisions to subsequent manuscript drafts, and read and approved the final manuscript.

Funding

This study was sponsored by Alexion, AstraZeneca Rare Disease, Boston, MA, USA. Editorial support was provided by Violet Kiesel, PhD, and Drayton Hammond, PharmD, of Peloton Advantage, LLC, an OPEN Health company, and funded by Alexion, AstraZeneca Rare Disease.

Open Access funding enabled and organized by Projekt DEAL.

Data availability

Alexion, AstraZeneca Rare Disease will consider requests for disclosure of clinical study patient-level data provided that patient privacy is assured through methods like data de-identification, pseudonymization, or anonymization (as required by applicable law), and if such disclosure was included in the relevant study informed consent form or similar documentation. Qualified academic investigators may request patient-level clinical data and supporting documents (statistical analysis plan and protocol) pertaining to Alexion-sponsored studies. Further details regarding data availability and instructions for requesting information are available in the Alexion Clinical Trials Disclosure and Transparency Policy at https://alexion.com/our-research/research-and-development. Link to Data Request Form (https://alexion.com/contact-alexion/medical-information).

Declarations

Ethical approval

The protocol and an amendment were approved by the ethics committee of the medical faculty of the University of Rostock (Vote number A2020-0169) as well as by the ethics committee of the University of Würzburg (Vote number 221/20). The study was conducted in compliance with guidelines for Good Clinical Practice and the ethical principles of the Declaration of Helsinki. Written informed consent was obtained from patients prior to initiation of study procedures.

Competing interests

Lothar Seefried is a consultant for and has received research funding and honoraria from Alexion, AstraZeneca Rare Disease. Anna Petryk and Guillermo del Angel are employees of and may own stock/options in Alexion, AstraZeneca Rare Disease. Felix Reder is an employee of and may own stock/options in Centogene GmbH, which received funding from Alexion, AstraZeneca Rare Disease for the conduct of the study. Peter Bauer is a board member of and may own stock/options in Centogene GmbH.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

Alexion, AstraZeneca Rare Disease will consider requests for disclosure of clinical study patient-level data provided that patient privacy is assured through methods like data de-identification, pseudonymization, or anonymization (as required by applicable law), and if such disclosure was included in the relevant study informed consent form or similar documentation. Qualified academic investigators may request patient-level clinical data and supporting documents (statistical analysis plan and protocol) pertaining to Alexion-sponsored studies. Further details regarding data availability and instructions for requesting information are available in the Alexion Clinical Trials Disclosure and Transparency Policy at https://alexion.com/our-research/research-and-development. Link to Data Request Form (https://alexion.com/contact-alexion/medical-information).


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