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International Journal of Molecular Sciences logoLink to International Journal of Molecular Sciences
. 2016 Jul 30;17(8):1239. doi: 10.3390/ijms17081239

A Next-Generation Sequencing Approach to Identify Gene Mutations in Early- and Late-Onset Hypertrophic Cardiomyopathy Patients of an Italian Cohort

Speranza Rubattu 1,2,*,†, Cristina Bozzao 1,†, Ermelinda Pennacchini 1,‡, Erika Pagannone 1, Beatrice Maria Musumeci 1, Maria Piane 1, Aldo Germani 1, Camilla Savio 1, Pietro Francia 1, Massimo Volpe 1,2, Camillo Autore 1,*,†, Luciana Chessa 1,†
Editor: William Chi-shing Cho
PMCID: PMC5000637  PMID: 27483260

Abstract

Sequencing of sarcomere protein genes in patients fulfilling the clinical diagnostic criteria for hypertrophic cardiomyopathy (HCM) identifies a disease-causing mutation in 35% to 60% of cases. Age at diagnosis and family history may increase the yield of mutations screening. In order to assess whether Next-Generation Sequencing (NGS) may fulfil the molecular diagnostic needs in HCM, we included 17 HCM-related genes in a sequencing panel run on PGM IonTorrent. We selected 70 HCM patients, 35 with early (≤25 years) and 35 with late (≥65 years) diagnosis of disease onset. All samples had a 98.6% average of target regions, with coverage higher than 20× (mean coverage 620×). We identified 41 different mutations (seven of them novel) in nine genes: MYBPC3 (17/41 = 41%); MYH7 (10/41 = 24%); TNNT2, CAV3 and MYH6 (3/41 = 7.5% each); TNNI3 (2/41 = 5%); GLA, MYL2, and MYL3 (1/41=2.5% each). Mutation detection rate was 30/35 (85.7%) in early-onset and 8/35 (22.9%) in late-onset HCM patients, respectively (p < 0.0001). The overall detection rate for patients with positive family history was 84%, and 90.5% in patients with early disease onset. In our study NGS revealed higher mutations yield in patients with early onset and with a family history of HCM. Appropriate patient selection can increase the yield of genetic testing and make diagnostic testing cost-effective.

Keywords: genetics, gene variants, hypertrophic cardiomyopathy, next-generation sequencing

1. Introduction

Hypertrophic cardiomyopathy (HCM) is a common genetic cardiac disease that affects one out of 500 individuals from the general population [1]. It is a clinically variable and genetically heterogeneous disease. In fact, more than 20 genes were related with HCM and a total number of about 1400 distinct mutations were identified in affected patients [2]. The most frequently encountered mutations fall within myosin heavy chain 7 (MYH7) and myosin binding protein C (MBPC3) [3,4]. Sequencing of sarcomere protein genes in patients fulfilling clinical diagnostic criteria identifies a disease-causing mutation in only 35% to 60% of cases [5,6,7,8]. Identification of an HCM-causing mutation is an important step in the disease’s clinical management, not only to better support the clinical diagnosis in the proband but also to either exclude or confirm the presence of disease-causing mutations in other family members.

Considering the extreme genetic heterogeneity of the disease and the cost of genetic testing, several attempts were made to identify the clinical predictors of an underlying mutation [9,10,11]. In a large study of HCM patients genotyped for mutations in nine genes, the presence of a set of five clinical markers, including age at diagnosis <45 years, accounted for an 80% likelihood of positive genetic testing [11].

In addition, more reliable, precise, and possibly not time-consuming molecular diagnostic approaches are needed. In this regard, Next-Generation Sequencing (NGS), which has already been applied for the diagnosis of hereditary cardiovascular conditions as well as of other diseases [12,13,14,15,16], may represent a suitable tool. Targeted gene panels were shown to generate results with analytical quality identical to Sanger sequencing, and to have the advantage of being faster and cheaper with better coverage and sensitivity than that used in more expanded analyses.

The purpose of the present study was to analyse the yield of NGS applied to the genetic screening of a well-phenotyped Italian HCM cohort, composed of patients with both early- and late-onset diagnosis, also including patients with positive family history, and to explore the ability of NGS to accomplish the molecular diagnostic needs in clinical practice.

2. Results

2.1. Description of Study Population

The clinical characteristics of patients enrolled in the study are shown in Table 1A.

Table 1.

(A) Clinical characteristics of HCM patients with early or late onset of disease; (B) Familial vs. sporadic HCM.

(A)

Variables Early-Onset n = 36 Late-Onset n = 35 p
Age at diagnosis (years) 18.6 ± 8.5 70.4 ± 4.8 0.0001
Male 27 (77.1) 9 (25.7) 0.0001
LV obstruction 14 (40) 24 (68.6) 0.03
Family history of HCM 21 (60) 4 (11.4) 0.0001
NYHA functional class
I 24 (68.6) 4 (11.4) 0.0001
II 9 (25.7) 25 (71.4)
III 2 (5.7) 6 (17.1)
Unexplained syncope 5 (14.3) 6 (17.1) 1
Non sustained ventricular tachycardia 6 (24) 5 (22.7) 1
Left atrial dimension (mm) 39.3 ± 6.2 45 ± 4.5 0.0001
Maximal LV wall thickness (mm) 21.4 ± 6.2 18.7 ± 2.6 0.02
Late gadolinium enhancement 24/29 (82.8) 9/19 (47.4) 0.01
Atrial fibrillation 11 (31.4) 10 (28.6) 1
End stage disease 4 (11.4) 0 (0) 0.11
Myectomy 2 (5.7) 0 (0) 0.49
ICD implantation 12 (34.3) 2 (5.7) 0.006
Death 0 (0) 1 (2.9) 1
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(B)

Patients All n = 70 EO n = 35 LO n = 35 p
Familial HCM 25 (36) 21 (60) 4 (14.4) 0.0001
Sporadic HCM 45 (64) 14 (40) 31 (88.6) 0.0001
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In (A): Continuous variables are expressed as mean ± SD. Qualitative variable are expressed as n (%). HCM: hypertrophic cardiomyopathy; NYHA: New York Functional Class; LV: left ventricular; ICD: implantable cardioverter defibrillator; In (B): Variable are expressed as n (%); EO: early-onset; LO: late-onset.

The patients were divided into two subgroups of 35 patients each, depending on the age at HCM diagnosis: the early-onset (EO) group with a mean age at diagnosis of 18.6 ± 8.5 years and the late-onset (LO) group with a mean age at diagnosis of 70.4 ± 4.8 years. The number of patients with a positive family history for HCM was significantly higher in the EO group (p = 0.0001) (Table 1B). Thirty-four patients were women and 36 were men. The sex distribution of patients was different in the two subgroups, with more males in the EO group (p = 0.0001). The left atrium size was significantly different in the two groups (p = 0.0001), with LO patients more frequently exhibiting left atrial enlargement. The obstructive form of HCM was less frequently observed in the EO as compared to the LO group (p = 0.03). Evolution of the disease towards end stage (left ventricular ejection fraction <50%) was observed only in the EO group. None of the other clinical features considered in the study was significantly different between the two groups.

2.2. Sequencing

The coding region of each of the 17 HCM phenotype causative genes included in the HCM panel was sequenced on Personal Genome Machine (PGM) IonTorrent sequencer. The 17 genes included in the HCM panel used for this analysis are shown in Table 2. Sequencing produced an average of 240,000 reads per patients; the mean read length was 130 bp; the average read depth per sample was 620× with a mean percentage of reads on target of 93.77%; the mean percentage of regions of interest (ROI) covered at least by 20× was 98.6%, and that covered at least by 100× was 94.7%. Details of the sequencing metrics for each patient are reported in Table 3.

Table 2.

Metrics of the 17 genes included into the HCM panel.

#No. Gene Name Ref Seq NCBI Genomic Location (hg19) Description Amplicons Coverage (%) Target (bp) Missed (bp)
1 MYBPC3 NM_000256 chr11:47352958-47374253 myosin binding protein C, cardiac 53 100 5458 105
2 MYH7 NM_000257 chr14:23881948-23904870 myosin, heavy chain 7, cardiac muscle, β 67 98 7746 231
3 TPM1 NM_001018005 chr15:63334838-63364111 tropomyosin 1 α chain isoform 7 23 99.91 2245 2
4 TNNT2 NM_001001430 chr1:201328143-201346805 troponin T type 2, cardiac isoform 1 20 100 2357 0
5 TNNI3 NM_000363 chr19:55663137-55669100 troponin I, cardiac 10 99.9 989 1
6 MYL2 NM_000432 chr12:111348626-111358404 slow cardiac myosin regulatory light chain 2 9 84.8 858 46
7 MYL3 NM_000258 chr3:46899357-46904973 slow skeletal ventricular myosin alkali light 9 94,6 894 136
8 ACTC1 NM_005159 chr15:35080297-35087927 cardiac muscle α actin 1 proprotein 13 100 1440 0
9 LAMP2 NM_002294 chrX:119560004-119603204 lysosomal-associated membrane protein 2 isoform 21 100 2077 0
10 PRKAG2 NM_016203 chr7:151253203-151574316 AMP-activated protein kinase γ 2 subunit 26 84.3 2713 426
11 GLA NM_000169 chrX:100652779-100663001 α-galactosidase A precursor 14 100 1647 0
12 MYH6 NM_002471 chr14:23851199-23877482 myosin heavy chain 6 66 94.52 7707 422
13 TNNC1 NM_003280 chr3:52485108-52488057 troponin C, slow 8 98.2 792 14
14 CSRP3 NM_003476 chr11:19203578-19223589 cysteine and glycine-rich protein 3 8 100 840 0
15 PLN NM_002667 chr6:118869442-118881586 phospholamban 2 100 210 0
16 TCAP NM_003673 chr17:37821599-37822806 telethonin 5 100 606 0
17 CAV3 NM_033337 chr3:8775486-8788451 Homo sapiens caveolin 3 (CAV3), transcript variant 1, mRNA. 4 100 558 0
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Gene symbols: TPM1: tropomyosin 1; ACTC1: actin, α, cardiac muscle 1; LAMP2: lysosomal associated membrane protein 2; PRKAG2: protein kinase AMP-activated non-catalytic subunit γ 2; TNNC1: troponin C 1; CSRP3: cystein and glycine-rich protein 3; PLN: phospholamban; TCAP: telethonin.

Table 3.

Patient sequencing metrics.

Patients Mapped Reads Reads on Target (%) Uniformity (%) ROI MEAN COVERAGE ROI ≥ 20× (%) n of Amplicons < 20× ROI ≥ 100× (%) n of Amplicons < 100×
EO1 178,727 92.13 93.95 459.94 98.60 5 94.97 18
EO2 178,731 90.57 94.83 452.19 98.88 4 95.53 16
EO3 72,440 91.78 93.75 185.71 96.93 11 83.52 59
EO4 247,711 90.70 93.90 627.61 99.44 2 96.09 14
EO5 111,232 91.03 93.57 282.82 97.21 10 91.62 30
EO6 280,419 93.08 94.15 729.08 99.16 3 96.09 14
EO7 623,594 92.53 93.81 1611.77 99.44 2 98.32 6
EO8 561,715 97.46 92.18 1529.12 99.44 2 97.49 9
EO9 77,846 93.36 93.83 203.00 96.93 11 86.87 47
EO10 381,796 96.33 93.71 1027.32 99.44 2 97.21 10
EO11 311,658 93.28 93.70 812.08 99.44 2 96.09 14
EO12 239,783 93.00 94.15 622.93 98.88 4 95.53 16
EO13 276,453 93.48 94.44 721.90 99.44 2 96.09 14
EO14 215,672 93.30 94.53 562.09 99.44 2 95.81 15
EO15 465,323 94.73 93.01 1231.34 99.44 2 96.65 12
EO16 465,619 97.25 92.65 1264.84 99.44 2 96.93 11
EO17 441,220 95.42 92.96 1176.05 99.72 1 97.49 9
EO18 192,373 98.07 91.50 526.97 98.60 5 94.13 21
EO19 313,968 95.80 93.72 840.19 99.16 3 96.65 12
EO20 192,211 95.35 94.02 517.24 98.60 5 95.81 15
EO21 196,251 95.05 94.02 521.07 98.88 4 95.81 15
EO22 303,435 96.01 93.55 813.79 98.88 4 96.65 12
EO23 322,467 94.14 92.17 847.94 98.88 4 95.81 15
EO24 253,552 95.97 91.35 679.71 98.88 4 95.53 16
EO25 188,696 95.33 93.96 502.45 98.60 5 95.53 16
EO26 182,956 94.99 92.88 485.47 98.88 4 94.13 21
EO27 191,880 94.62 93.58 507.12 98.88 4 94.97 18
EO28 228,313 92.89 93.22 592.43 98.88 4 94.69 19
EO29 199,442 98.07 92.15 546.33 98.32 6 94.97 18
EO30 190,915 97.24 92.66 518.58 98.04 7 94.69 19
EO31 161,793 95.49 92.19 431.54 97.77 8 93.30 24
EO32 245,414 89.57 93.45 613.99 98.32 6 95.81 15
EO33 205,079 95.46 85.54 546.83 96.65 12 89.39 38
EO34 210,900 97.24 93.66 572.83 98.60 5 95.25 17
EO35 147,306 97.24 92.62 402.14 98.32 6 94.13 21
LO1 178,290 90.77 93.68 321.94 97.77 8 91.90 29
LO2 205,008 93.84 93.97 537.36 99.16 3 96.09 14
LO3 159,828 93.15 93.80 502.12 98.88 4 95.53 16
LO4 193,973 93.90 94.09 1062.97 99.44 2 96.37 13
LO5 191,160 93.72 93.21 1097.36 99.16 3 96.65 12
LO6 177,316 94.10 93.42 931.78 99.44 2 96.37 13
LO7 238,812 94.23 93.77 593.70 97.77 8 92.18 28
LO8 158,483 93.67 93.01 708.10 99.44 2 97.49 9
LO9 213,370 93.89 94.34 861.05 99.44 2 97.21 10
LO10 190,285 94.47 93.70 415.86 98.32 6 94.41 20
LO11 182,160 93.99 94.02 505.35 98.32 6 94.97 18
LO12 213,052 92.51 94.69 532.23 98.88 4 94.97 18
LO13 249,591 93.48 93.92 304.45 96.93 11 90.78 33
LO14 195,422 94.82 94.22 378.77 98.04 7 92.74 26
LO15 201,815 95.14 93.81 717.45 98.60 5 95.53 16
LO16 400,156 98.18 90.79 500.41 98.60 5 94.41 20
LO17 274,868 95.75 91.07 423.42 98.04 7 92.74 26
LO18 158,695 92.07 94.09 457.87 98.32 6 94.97 18
LO19 195,752 89.66 93.66 206.68 96.65 12 86.03 50
LO20 83,846 88.25 93.28 490.24 98.32 6 94.13 21
LO21 179,015 91.57 94.00 408.12 98.60 5 94.69 19
LO22 170,180 89.07 93.89 735.17 98.60 5 94.97 18
LO23 161,290 90.82 93.26 680.84 99.16 3 96.09 14
LO24 281,769 92.16 93.07 536.34 98.60 5 95.53 16
LO25 145,219 93.37 93.76 517.60 99.16 3 95.81 15
LO26 390,025 97.57 92.65 508.78 98.88 4 95.53 16
LO27 124,848 87.30 93.29 651.69 99.16 3 95.25 17
LO28 214,031 89.02 93.84 550.53 99.16 3 96.37 13
LO29 203,428 88.93 94.12 479.56 98.88 4 95.53 16
LO30 127,496 90.40 93.91 452.05 98.32 6 94.69 19
LO31 25,524 95.49 93.62 409.17 98.04 7 93.02 25
LO32 329,472 93.56 94.58 559.60 99.16 3 96.09 14
LO33 265,155 95.60 93.94 414.68 98.32 6 94.13 21
LO34 21,736 97.78 89.25 628.59 98.88 4 95.81 15
LO35 352,805 94.55 92.69 466.05 97.77 8 94.69 19
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Two hundred eighty-two variants were identified within the 17 genes analysed: two were ins/del, 175 were intronic, 37 missense, 59 synonimous, five splicing, and four stop mutations. After filtration, 41 variants with a possible clinical effect were selected and confirmed by Sanger sequencing (data not shown). These variants were located in nine of the 17 genes: MYBPC3 (17/41 = 41%); MYH7 (10/41 = 24%); troponin T2 (TNNT2), caveolin 3 (CAV3), and myosin heavy chain 6 (MYH6) (3/41 = 7.5% each); troponin I 3 (TNNI3) (2/41 = 4.8%); and galactosidase alpha (GLA), myosin light chain 2 (MYL2), and myosin light chain 3 (MYL3) (1/41 = 2.5% each). Thirty-four were known variants, whereas seven were novel. Out of the seven new missense mutations, four had uncertain significance, two were likely pathogenic, and one was likely benign. Considering the 34 known variants, 15 were known to have pathogenic effect, six were likely pathogenic, one was likely benign, and 12 were known registered variants but with unknown clinical significance (Table 4). Mutations in sarcomeric genes accounted for 90% of all identified mutations, with MYBPC3 and MYH7 alone accounting for 65% of all mutations. Considering only mutations in MYBPC3, eight missense mutations and nine truncating mutations were identified (Table 4).

Table 4.

Mutations detected per gene.

Gene ID Chrom Position Exon DNA Change Protein Change Mutation Type dbSNP Prev. Rep. GMAF SIFT POLYPHEN PROVEAN (cutoff = −2.5) Clinical Significance
CAV3 chr3 8787313 2 c.216C>G Cys72Trp MISSENSE rs116840776 yes 0.00100 (G) deleterious 0 probably damaging 0.999 deleterious −6.167 known/uncertain significance
chr3 8787330 2 c.233C>T Thr78Met MISSENSE rs72546668 yes 0.00200 (T) tolerated 0.05 possibly damaging 0.537 neutral −0.833 known/uncertain significance
chr3 8787497 2 c.400G>T Ala134Ser MISSENSE deleterious 0.01 benign 0.07 neutral 0.862 new/uncertain significance
GLA chrX 100653420 6 c.937G>T Asp313Tyr MISSENSE rs28935490 yes 0.0021 (A) deleterious 0 probably damaging 0.952 deleterious −3.183 known/uncertain significance
MYBPC3 chr11 47371426 5 c.553A>T Lys185Ter STOP rs375607980 yes known/pathogenic
chr11 47371414 5 c.565G>A Val189Ile MISSENSE rs11570052 yes 0.00200 (T) tolerated 0.44 benign 0.132 Neutral −0.418 known/likely benign
chr11 47365154 13 c.1112C>G Pro371Arg MISSENSE rs397515887 yes 0.00020 (A) deleterious 0 probably damaging 0.994 deleterious −8.043 known/uncertain significance
chr11 47365147 13 c.1120C>T Gln374Ter STOP rs730880635 yes known/pathogenic
chr11 47364429 15 c.1409G>A Arg470Gln MISSENSE yes deleterious 0.01 probably damaging 0.982 deleterious −3.094 known/uncertain significance
chr11 47364270 16 c.1483C>T Arg495Trp MISSENSE rs397515905 yes deleterious 0 probably damaging 0.999 deleterious −5.228 known/uncertain significance
chr11 47364162 16 c.1591G>C Gly531Arg MISSENSE rs397515912 yes 0.00020 (G) deleterious 0 probably damaging 0.996 deleterious −7.038 known/likely pathogenic
chr11 47364129 16 c.1624G>C Glu542Gln MISSENSE/SPLICING rs121909374 yes 0.00008 (G) known/pathogenic
chr11 47360071 22 c.2308G>A Asp770Asn MISSENSE/SPLICING rs36211723 yes known/pathogenic
chr11 47359347 23 c.2309-2A>G SPLICING rs111729952 yes known/pathogenic
chr11 47359115 24 c.2429G>A Arg810His MISSENSE rs375675796 yes 0.00008 (T) deleterious 0 probably damaging 1 deleterious −4.564 known/likely pathogenic
chr11 47359085 24 c.2459G>A Arg820Gln MISSENSE rs2856655 yes deleterious 0 probably damaging 0.98 deleterious −2.925 known/likely pathogenic
chr11 47356592 26 c.2905+1G>A SPLICING rs397515991 yes known/pathogenic
chr11 47355264 28 c.3034C>T Gln1012Ter STOP rs730880586 yes known/pathogenic
chr11 47354882 29 c.3192_3193insC Lys1065Glnfs INS rs397516007 yes known/pathogenic
chr11 47353801 32 c.3636T>G Ile1212Met MISSENSE deleterious 0 probably damaging 0.918 deleterious −2.498 new/likely pathogenic
chr11 47353662 32 c.3775C>T Gln1259Ter STOP rs730880605 yes known/pathogenic
MYH7 chr14 23900850 8 c.676G>A Ala226Thr MISSENSE deleterious 0 probably damaging 0.985 neutral −1.757 new/uncertain significance
chr14 23896866 16 c.1816G>A Val606Met MISSENSE rs121913627 yes known/pathogenic
chr14 23896042 18 c.1988G>A Arg663His MISSENSE rs371898076 yes 0.00008 (T) known/pathogenic
chr14 23895189 19 c.2146G>C Gly716Arg MISSENSE rs121913638 yes deleterious 0.01 probably damaging 0.995 deleterious −3.728 known/likely pathogenic
chr14 23895179 19 c.2156G>A Arg719Gln MISSENSE rs121913641 yes known/pathogenic
chr14 23894116 22 c.2543_2545 delAAG Lys847del DEL yes known/pathogenic
chr14 23893234 23 c.2804A>T Glu935Val MISSENSE rs730880761 yes known/pathogenic
chr14 23891501 25 c.3133C>T Arg1045Cys MISSENSE rs45611033 yes 0.00020 (A) deleterious 0.03 benign 0.203 deleterious −6.180 known/uncertain significance
chr14 23889413 27 c.3367G>C Glu1123Gln MISSENSE deleterious 0.01 probably damaging 0.968 neutral −2.389 new/uncertain significance
chr14 23887615 30 c.3973G>A Ala1325Thr MISSENSE/SPLICING deleterious 0.02 possibly damaging 0.751 neutral −2.329 new/uncertain significance
TNNT2 chr1 201334751 9 c.281G>C Arg94Thr MISSENSE rs397516452 yes deleterious 0 possibly damaging 0.573 deleterious −5.588 known/uncertain significance
chr1 201330414 14 c.794A>T Lys265Ile MISSENSE rs397516482 yes deleterious 0 probably damaging 0.958 deleterious −6.86 known/uncertain significance
chr1 201328373 16 c.853C>T Arg285Cys MISSENSE rs121964857 yes tolerated 0.06 probably damaging 0.978 neutral −2.09 known/likely pathogenic
MYH6 chr14 23873951 7 c.611G>A Arg204His MISSENSE rs200623022 yes tolerated 0.05 possibly damaging 0.807 neutral −1.327 known/uncertain significance
chr14 23865497 20 c.2425C>T Arg809Cys MISSENSE deleterious 0 probably damaging 0.963 deleterious −5.294 new/likely pathogenic
chr14 23853697 36 c.5519A>G Lys1840Arg MISSENSE rs373629059 tolerated 0.13 probably damaging 0.999 neutral −1.731 known/uncertain significance
MYL2 chr12 111350901 6 c.401A>C Glu134Ala MISSENSE rs143139258 yes deleterious 0.01 possibly damaging 0.755 Deleterious −5.696 known/likely pathogenic
MYL3 chr3 46902303 3 c.170C>A Ala57Asp MISSENSE rs139794067 yes deleterious 0 probably damaging 0.996 deleterious −5.236 known/uncertain significance
TNNI3 chr19 55665561 6 c.385C>G Thr128Ser MISSENSE tolerated 0.186 benign 0.000 neutral 0.61 new/likely benign
chr19 55665516 6 c.431T>A Leu144Gln MISSENSE rs121917760 yes known/pathogenic
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Prev. Rep.: previously reported; GMAF: Global minor allele frequency; Software prediction programs used for sequence variant interpretation: SIFT: Evolutionary conservation; POLYPHEN: Protein structure/function and evolutionary conservation; PROVEAN: Alignment and measurement of similarity between variant sequence and protein sequence homolog.

2.3. Group Comparison after Sequencing

The mutation detection rate was 85.7% (30/35) in the EO group and 22.9% (8/35) in the LO group.

The number of patients in which the molecular screening allowed the identification of at least one mutation was significantly different in the two groups of patients with different age at diagnosis (p < 0.0001). The overall detection rate, regardless of the age of onset, was 54.3% (38/70). The NGS analysis confirmed the known mutational status of the 22 controls (seven positive and 15 negative) included in this study. Mutations identified in each patient are listed in Table 5. Considering only patients with positive family history, the detection rate was 88% (22/25), ranging from 75% (3/4) in the LO group to 90.5% (19/21) in the EO group. Considering sporadic cases only, the overall detection rate was 35.5%, with a significant difference between EO (11/14, 78.6%) and LO (5/31, 16%), p < 0.0002.

Table 5.

Mutations detected per patient.

Early-Onset
Patient ID Familiarity Gene ID Exon DNA Change Protein Change Mutation Type Clinical Significance dbSNP Previously Reported Coverage Allele Coverage
EO1 yes MYBPC3 5 c.553A>T Lys185Ter STOP known/pathogenic rs375607980 yes 384 202
EO2 yes MYH7 19 c.2156G>A Arg719Gln MISSENSE known/pathogenic rs121913641 yes 399 204
EO3 CAV3 2 c.233C>T Thr78Met MISSENSE known/uncertain significance rs72546668 yes 124 57
EO4 MYBPC3 23 c.2309-2A>G SPLICING known/pathogenic rs111729952 yes 400 186
EO5 MYH7 16 c.1816G>A Val606Met MISSENSE known/pathogenic rs121913627 yes 383 204
EO6 yes MYH7 8 c.676G>A Ala226Thr MISSENSE new/uncertain significance 399 208
GLA 6 c.937G>T Asp313Tyr MISSENSE known/uncertain significance rs28935490 yes 399 183
EO7 yes MYBPC3 28 c.3034C>T Gln1012Ter STOP known/pathogenic rs730880586 yes 397 194
EO8 yes MYBPC3 23 c.2309-2A>G SPLICING known/pathogenic rs111729952 yes 398 204
EO9 yes MYH7 19 c.2146G>C Gly716Arg MISSENSE known/likely pathogenic rs121913638 yes 354 169
EO11 yes MYBPC3 16 c.1483C>T Arg495Trp MISSENSE known/uncertain significance rs397515905 yes 400 259
CAV3 2 c.216C>G Cys72Trp MISSENSE known/uncertain significance rs116840776 yes 400 182
EO12 yes MYBPC3 23 c.2309-2A>G SPLICING known/pathogenic rs111729952 yes 399 185
EO13 MYBPC3 32 c.3636T>G Ile1212Met MISSENSE new/likely pathogenic 400 201
23 c.2309-2A>G SPLICING known/pathogenic rs111729952 yes 399 187
16 c.1591G>C Gly531Arg MISSENSE known/likely pathogenic rs397515912 yes 400 184
EO14 CAV3 2 c.233C>T Thr78Met MISSENSE known/uncertain significance rs72546668 yes 399 214
EO17 MYBPC3 22 c.2308G>A Asp770Asn MISSENSE/SPLICING known/pathogenic rs36211723 yes 399 195
EO18 yes MYBPC3 13 c.1120C>G Tyr374Ter STOP known/pathogenic rs730880635 yes 400 225
EO19 yes MYBPC3 32 c.3775C>T Gln1259Ter STOP known/pathogenic rs730880605 yes 398 204
EO20 yes MYL2 6 c.401A>C Glu134Ala MISSENSE known/likely pathogenic rs143139258 yes 398 191
EO21 yes MYBPC3 5 c.565G>A Val189Ile MISSENSE known/uncertain significance rs11570052 yes 309 253
MYH7 22 c.2543_2545 delAAG Lys847del DELETION known/pathogenic yes 391 194
EO22 CAV3 2 c.400G>T Ala134Ser MISSENSE new/uncertain significance 330 168
EO23 MYH7 30 c.3973G>A Ala1325Thr MISSENSE/SPLICING new/uncertain significance 400 176
23 c.2804A>T Glu935Val MISSENSE known/pathogenic rs730880761 yes 400 206
EO25 yes MYBPC3 15 c.1409G>A Arg470Gln MISSENSE known/uncertain significance yes 293 130
EO26 yes TNNT2 16 c.853C>T Arg285Cys MISSENSE known/likely pathogenic rs121964857 yes 323 167
EO27 yes TNNT2 14 c.794A>T Lys265Ile MISSENSE known/uncertain significance rs397516482 yes 395 193
EO29 yes MYBPC3 24 c.2429G>A Arg810His MISSENSE known/likely pathogenic rs375675796 yes 400 148
EO30 yes TNNI3 6 c.431T>A Leu144Gln MISSENSE known/pathogenic rs121917760 yes 398 227
EO31 MYBPC3 5 c.565G>A Val189Ile MISSENSE known/likely benign rs11570052 yes 312 152
EO32 MYH7 18 c.1988G>A Arg663His MISSENSE known/pathogenic rs371898076 yes 400 211
EO33 MYBPC3 29 c.3193_3194 insC Lys1065Glnfs INSERTION known/pathogenic rs397516007 yes 398 194
16 c.1591G>C Gly531Arg MISSENSE known/likely pathogenic rs397515912 yes 400 212
13 c.1112C>G Pro371Arg MISSENSE known/uncertain significance rs397515887 yes 235 87
EO34 yes TNNT2 9 c.281G>C Arg94Thr MISSENSE known/uncertain significance rs397516452 yes 400 196
EO35 MYBPC3 26 c.2905+1G>A ex26 SPLICING known/pathogenic rs397515991 Yes 296 139
Late-Onset
Patient ID Familiarity Gene ID Exon DNA Change Protein Change Mutation Type Clinical Significance dbSNP Previously Reported Coverage Allele Coverage
LO1 Yes MYH7 25 c.3133C>T Arg1045Cys MISSENSE known/uncertain significance rs45611033 yes 213 113
LO4 MYH7 27 c.3367G>C Glu1123Gln MISSENSE new/uncertain significance 400 220
LO6 MYH6 20 c.2425C>T Arg809Cys MISSENSE new/ likely pathogenic 299 139
LO8 Yes MYBPC3 16 c.1624G>C Glu542Gln MISSENSE/SPLICING known/pathogenic rs121909374 yes 353 188
TNNI3 6 c.385C>G Thr128Ser MISSENSE new/likely benign 400 204
LO13 Yes MYH7 16 c.1816G>A Val606Met MISSENSE known/pathogenic rs121913627 yes 383 204
LO14 MYH6 36 c.5519A>G Lys1840Arg MISSENSE known/uncertain significance rs373629059 yes 399 196
LO16 MYBPC3 24 c.2459G>A Arg820Gln MISSENSE known/likely pathogenic rs2856655 yes 400 213
LO17 MYH6 7 c.611G>A Arg204His MISSENSE known/uncertain significance rs200623022 yes 398 201
MYL3 3 c.170C>A Ala57Asp MISSENSE known/uncertain significance rs139794067 yes 398 168
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dbSNP: database single nucleotide polymorphisms (www.ncbi.nlm.nih.gov/SNP).

In the EO group, patients EO13 and EO33 carried three different mutations in MYBPC3. One of them was clinically characterized by an unfavourable course with evolution to end stage disease.

Four patients carried two different mutations: EO23 carried two mutations in MYH7, whereas EO6, EO11, and EO21 carried two mutations in two different genes (Table 5). In the LO group, only two patients, LO8 and LO17, harboured two mutations in different genes (Table 5).

The distribution of the identified gene mutations was similar between the two groups with the exceptions of MYH6 and TNNT2. In fact, mutations in MYH6 were identified in the LO group only, whereas mutations in TNNT2 were identified in the EO group only.

3. Discussion

This report describes the results of a genetic screening obtained through NGS approach in an Italian population of unrelated and clinically well characterized HCM cases, divided into two groups according to age at diagnosis. Our population included a good percentage of patients with a family history of HCM. As expected, the prevalence of familial forms was higher in the EO group, whereas the prevalence of sporadic forms was higher in the LO group.

The key finding of our investigation was the higher yield of mutation detection rate in the EO group and in patients with a family history of disease, with 90.5% of cases carrying an identified mutation. The overall yield of genetic testing was close to 50%, and, as previously reported in the literature [4,7,8,9,11], mutations in MYBPC3 and MYH7 accounted for about 65% of all variants. Other mutations were found in six additional sarcomeric genes (TNNT2, CAV3, MYH6, TNNI3, MYL2, and MYL3) and in one non-sarcomeric gene (GLA). Approximately a quarter of all variants were novel, most of them belonging to MYH7. The pathogenicity of novel mutations was verified through appropriate software for analysis.

HCM is a disease characterized by a relevant heterogeneity of both morphological and clinical features. For this reason, despite the growing knowledge on its genetic basis, the establishment of a more precise genotype–phenotype correlation has been difficult to achieve.

The main original aspect of our investigation was to test through NGS a wide range of HCM-causing genes (14 sarcomeric and three non-sarcomeric) while comparing the extreme ages of disease onset and evaluating the impact of familial occurrence of the disease even in patients with late diagnosis. Due to the small sample size of the population, our study could not address the issue of a relationship between genetic variants and phenotypic characteristics of different HCM onset patients. Notably, the presence of double and triple mutations was detected mostly among younger patients, and one of them showed a more severe form of the disease.

The different rate of pathogenic mutations found in HCM patients with early and late onset of the disease was consistent with the literature [17,18,19], confirming that some mutations can be found mainly in young HCM patients (TNNT2) whereas other mutations are detected exclusively in the elderly (MYH6) [17,18,19].

In our study, a majority of patients with young age at diagnosis had a positive genetic testing (80% of cases), four-fold higher than that of the elderly and sporadic HCM cases. These data, together with previous observations, reinforce the concept that age at HCM diagnosis is a powerful predictor of positive genetic testing [11,17,18,19]. We also support the notion that family history of HCM has a key role in appropriately addressing the genetic test. In fact, among HCM patients with a late diagnosis, those with a family history of the disease had a higher rate of mutation detection (75%).

We used an expanded panel of 17 genes in the attempt to improve the mutation detection rate. With this approach we mostly confirmed the type of mutations and the mutation distribution already described in the literature for HCM. In particular, the most frequent sarcomeric gene mutations, namely those in MYBPC3 and MYH7, accounted for the majority of the positive findings. Moreover, six of the seven novel mutations identified in our patients were in the main sarcomeric genes (three in MYH7, one in MYH6, one in MYBPC3, and one in TNNI3). In this regard, the limitations of using a wide diagnostic panel for HCM genetic testing have been recently highlighted in one of the largest clinical genetic studies ever reported for HCM [20]. Consistently, a panel designed only for the main HCM genes (n = 9), was able to successfully screen a large cohort of HCM patients [21]. Our findings support the choice of a limited, well-selected panel of HCM genes as the best tool for diagnostic purposes.

4. Materials and Methods

4.1. Patient Selection

Seventy patients with clinical diagnosis of HCM were included in the study. We selected 35 patients with early diagnosis of the disease (≤25 years, EO-early onset) and 35 patients with a late diagnosis (≥65 years, LO-late onset). All patients underwent a cardiologic evaluation as well as genetic counselling. Clinical data for each patient included a detailed personal and family history and a thorough scrutiny of the age at which HCM was first diagnosed. Both electrocardiographic and echocardiographic examinations were performed at the time of inclusion into the study. The echocardiographic parameters included both structural measurements and resting LV outflow tract gradients derived from the continuous-wave Doppler velocities. The clinical diagnosis of HCM was based on the echocardiographic demonstration of a hypertrophied and not dilated left ventricle (wall thickness >15 mm in adults, or the equivalent wall thickness relative to body surface area in children) in the absence of another cardiac or systemic disease that could produce comparable left ventricular hypertrophy [22,23].

The mutational status for MYH7, MYBPC3, TNNI3, TNNT2, TPM1, and MYL2 genes was already known in 22/70 patients (8 EO and 14 LO patients). All coding exons (±20 bp) of the six genes were previously analysed by Sanger sequencing. The 22 samples were included in our study as positive and negative controls for the six genes also present in our NGS panel. The seven positive controls carried mutations in MYBPC3 (EO7, EO29, EO33, EO35), MYH7 (LO13), TNNI3 (EO30), and MYL2 (EO20). The 15 negative controls for the six genes were: EO10, EO15, LO5, LO6, LO9, LO12, LO19, LO21, LO22, LO25, LO27, LO28, LO29, LO32, and LO33.

This study was conducted in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards (The approval identification number: 42 of 28 September 2007). A signed informed consent for blood sampling was obtained from all patients included in the study.

4.2. DNA Extraction and Quantification

Genomic DNA was extracted from peripheral whole blood using a commercially available kit (Invitrogen, Milan, Italy), and then quantified using Qubitds DNA HS Assay Kit on Qubit 2.0 Fluorometer (Invitrogen).

4.3. Sequencing

Seventeen genes known to be causative of HCM phenotype were selected for targeted sequencing (Table 2). A custom panel for coding DNA (+/−25 bp of intronic flanking regions) analysis of selected genes was designed online using Ion AmpliSeq Designer 2.0.3 (https://www.ampliseq.com/browse.action) [24]. The final custom panel was composed of 358 amplicons divided into two primer pools for a total of 61.89 kb of DNA. The panel covered 96.47% of regions of interest (ROI). Libraries were prepared using Ion AmpliSeq Library Kit v2.0 (Life Technologies, Carlsbad, CA, USA), according to the manufacturer’s instructions. One of 16 barcodes of the Ion Xpress Barcode Adapters1-16 Kit (Thermo Fisher Scientific Life Sciences Solutions, Carlsbad, CA, USA) was added to each sample. Libraries were quantified with Qubit dsDNA HS Assay Kit on Qubit 2.0 Fluorometer (Molecular Probes, Eugene, OR, USA) and equimolar amounts of each library were used to prepare template for clonal amplification. Emulsion PCR with Ion PGM Template OT2 200 Kit (Life Technologies, Carlsbad, CA, USA) was performed on OneTouch2 Systems (Life Technologies, Carlsbad, CA, USA). Templates were enriched using Ion OneTouch ES (Life Technologies, Carlsbad, CA, USA) and prepared for 316v2 chip loading (Life Technologies, Carlsbad, CA, USA). Groups from 12 to 16 sample libraries were sequenced on each chip. Sequencing runs were performed on Ion Torrent Personal Genome Machine (PGM, Life Technologies) using Ion PGM Sequencing 200 Kit v2, according to the manufacturer’s instructions.

4.4. Alignment

Data analysis was performed using the Torrent Suite Software v.4.0.2. (Life Technologies, Carlsbad, CA, USA). Reads were aligned to human reference genome hg19 from UCSC Genome Browser [25] and to a designed bed file from Ion AmpliSeq Designer results. Alignments were visually verified with Integrative Genomics Viewer IGV v.2.3, Broad Institute [26].

4.5. Coverage Analysis

The average read depth and the percentage of reads that mapped on ROI out of the total number of reads (reads on target) was calculated using Coverage Analysis plug-in (Life Technologies, Carlsbad, CA, USA). For each sample the percentage of ROI covered by at least 100× and 20× using amplicon coverage matrix file was calculated.

4.6. Variant Analysis

Variant calling was performed with Variant Caller plug-in configured with germ line-low stringency parameters. Variants were annotated using Ion Reporter 4.0 software (Carlsbad, CA, USA) [27]. Common single nucleotide variants (minor allele frequency MAF>5%, source 1000 Genomes), exonic synonymous variants, and intronic variants were removed from the analysis, while exonic non-synonymous, splice-site, and loss-of-function variants were analysed. The novel variants were analysed by means of three types of prediction software (SIFT, POLYPHEN, and PROVEAN) and classified based on the concordance of the prediction between the three types: “likely pathogenic,” “likely benign” (3/3 concordance), or “uncertain significance” (2/3 concordance).

4.7. Variant Validation

The identified variants were validated by Sanger sequencing using standard protocols. Specific primers were designed for the analysis. Polymerase Chain Reaction (PCR) products were directly sequenced by using the BigDye Terminator v3.1 Cycle Sequencing Kit (Life Technologies Corporation, Carlsbad, CA, USA). Sample analysis was performed on an ABI PRISM 3130xl Genetic Analyser (Applied Biosystems, Carlsbad, CA, USA).

4.8. Statistical Analysis

Statistical analysis was performed with SPSS statistical software (SPSS Inc., Chicago, IL, USA, version 17.0). Continuous variables are expressed as mean±SD. Comparisons between the two groups were performed using a Student’s t-test. The association between the mutational status and the clinical features of the two patient groups was evaluated using Chi-square and Fisher’s exact tests. A p value was considered statistically significant when <0.05.

5. Conclusions

In summary, through NGS, we were able to detect pathogenic mutations responsible for HCM, particularly in patients with early onset of the disease and in those with a family history of HCM. Our findings document the suitability of a novel molecular diagnostic strategy for clinical purposes and the important role of appropriate patient selection in making genetic molecular testing more cost-effective.

Acknowledgments

This work was supported by a 5% grant (Ricerca Corrente) from the Italian Ministry of Health to Massimo Volpe and Speranza Rubattu. The funding sources had no involvement in the study design, in the collection, analyses, and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.

Author Contributions

Speranza Rubattu and Camillo Autore conceived and designed the study. Beatrice Maria Musumeci, Erika Pagannone, Ermelinda Pennacchini, and Pietro Francia collected the study population. Cristina Bozzao, Maria Piane, Camilla Savio, and Aldo Germani performed the genetic analyses. Speranza Rubattu and Camillo Autore drafted and Luciana Chessa and Massimo Volpe finalized the manuscript. All authors closely interpreted all the results, reviewed, and approved the final version of the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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