Genetics and Etiology
HCM is usually characterized by an increase in the asymmetric thickness of the ventricular wall, which is one of the common factors leading to sudden cardiac death (SCD). According to whether the left ventricular outflow tract (LVOT) is obstructed, it can be divided into obstructive and non-obstructive HCM. The ECG mainly shows left ventricular (LV) high voltage and T wave inversion. For obstructive HCM, left ventricular hypertrophy of the ventricular septal outflow tract and anterior mitral systolic anterior motion (SAM) can be seen on echocardiography. Besides, MRI, dynamic monitoring, and exercise electrocardiogram are also helpful for the diagnosis of HCM [
18]. HCM is an autosomal dominant cardiomyopathy. Sixty percent of the patients’ gene mutations are related to sarcomeric genes. The most common are the missense mutations of MYH7 encoding myosin and the nonsense mutations of MYBPC3 encoding myosin binding protein. Although the frequency is not more than 5%, TNNT2, TNNI3, TPM1, MYL2, MYL3, and ACTC1 are also typical gene mutations of HCM. These sarcomere-related mutations will change the myocardial contraction function and cause excessive contraction and poor relaxation. Moreover, some new possible pathogenic genes may be associated with HCM, which cause clinically a typical HCM with mild symptoms, including CSRP3, PLN, CRYAB, TNNC1, MYOZ2, ACTN2, ANKRD1, FLNC, and FHL1 [
19,
20].
In addition to the primary changes, there are still some HCM caused by other genetic diseases. Mitochondrial disease is one of the causes of secondary HCM. Friedreich ataxia can lead to the insufficient synthesis of frataxin protein targeting the mitochondrial matrix, thus hindering the assembly of iron-sulfur-dependent protein and damaging mitochondrial function, which is manifested as HCM and heart failure in the circulatory system damage [
21]. Metabolic diseases can also cause HCM, and 88% of Danon disease patients will show HCM phenotype [
22]. Fabry’s disease is another important factor that causes HCM. It is reported that 1 ~ 3% of adult male HCM patients in the UK are caused by Fabry’s disease [
23] and in a Chinese study, 2 (0.93%) of 217 HCM patients were caused by Fabry disease [
24], showing multiple organ damage. Moreover, the HCM phenotype also exists in Wolff-Parkinson-White syndrome, Noonan/LEOPARD’s syndrome, and Pompe’s disease [
25]. Although HCM and cardiac amyloidosis (CA) are considered to be two different diseases, typical CA is characterized by diffuse hypertrophy and rigidity of the ventricular wall, there are still many cases of HCM caused by amyloidosis [
26], or CA with HCM phenotype [
27]. This may suggest that we may have an intermediate subtype, which is characterized by amyloidosis, but the phenotype is more similar to HCM.
Traditional and Accurate Classification
In the classification of HCM by WHO/ISFC in 1980 and 1995 which mainly depended on morphology, HCM was described as an autosomal dominant myocardial disease with asymmetric ventricular hypertrophy, often involving ventricular septum, which can cause arrhythmia and SCD [
12]. AHA classification further refined the diagnostic basis and classified HCM as primary cardiomyopathy. In this classification, HCM is described as left ventricular hypertrophy without dilation, except for other diseases that cause ventricular wall thickening, such as aortic stenosis. In addition to morphological criteria, AHA mentioned that patients with HCM gene defects do not necessarily have an echocardiographic type, so gene diagnosis for HCM is necessary. These potential gene defects may show symptoms in adulthood [
1]. Because a variety of mitochondrial and metabolic diseases can be characterized by HCM phenotype, which cannot be identified by echocardiography, and myocardial biopsy has the limitation of low clinical applicability, ESC classification no longer distinguishes the cause of myocardial disease, but simply excludes the increased of ventricular wall thickness caused by abnormal blood load, and classifies the cause according to familial/non-familial [
2].
Besides, according to clinical practice, the most mainstream accurate classification of HCM is divided into LVOT obstructive and non-obstructive HCM subtypes. Obstructive HCM is the most common, accounting for 70%. AHA emphasizes the appearance of LVOT obstructive phenotype caused by mitral valve SAM [
28]. Obstructive HCM has many and obvious symptoms, worse cardiac function, higher risk of complications, and poor prognosis. The middle ventricular obstructive HCM (MVOHCM) is a rare subtype, which is related to left ventricular hypertrophy, left ventricular emptying, and apical blood retention caused by an apical aneurysm [
29]. Compared with the common type, MVOHCM has a worse prognosis, which is related to end-stage progression, SCD, and fatal arrhythmia, and the etiology of MVOHCM may also be different from the typical HCM, which may require further genetic research [
30]. Although non-obstructive HCM is relatively rare and has a better prognosis, some studies are still committed to finding the high-risk phenotypes among them. According to echocardiography, non-obstructive HCM is divided into common type, differentiated type, restricted type, and reduced injection fraction type. Gene testing confirms that the rate of sarcomere gene mutations in patients with restricted type is higher, while patients with reduced injection fraction type are more common with multiple sarcomere gene mutations. The common type has the best prognosis and the least symptoms, while the patients with lower ejection fraction have higher cardiovascular mortality [
31]. There are also studies that use machine learning to propose a comprehensive classification method. Non-obstructive HCM is grouped according to a heart murmur, patient characteristics, past history, left ventricular diastolic and systolic function, cardiac imaging phenotype, and so on [
32], suggesting that subtype analysis based on clinical characteristics may be a new direction of HCM precision medicine.
Accurate classification based on morphology is the most common classification method of HCM. One year after the first WHO/ISFC classification was proposed, Maron proposed the earliest HCM precise classification scheme. According to the hypertrophic structure, HCM was divided into four types: basal septum type; whole septum type; septum, anterior, and anterolateral walls type; the third subtype is more frequent [
33]. More precisely, combined with the shape of the cardiac septum, HCM can also be divided into five types: reverse curvature, sigmoid septum, internal contour, apical, and mid-ventricular form [
34]. HCM can also be divided into septum alone; septum and adjacent segments (not apical segment); apical in combination with other LV; and apical, according to the hypertrophy pattern. The first pattern is the most common and more related to LVOT obstruction, while the third and fourth patterns are more prone to ECG changes [
35]. At present, the more convenient and more applicable morphological classification scheme is to divide HCM into 4 types, namely apical HCM, midventral HCM, basil HCM, and diffuse HCM [
36]. There are clear prognostic differences among these morphological subtypes. Compared with other subtypes, basal HCM has a higher survival rate after surgery [
37]. The apical HCM is a rare subtype. Seventy-five percent of the apical HCM is mutation negative. Among the mutation-positive patients, ACTC1/TPM1 mutations tend to appear as apical phenotype [
38,
39]. In general, apical HCM has a better prognosis and clinical outcome, and myocardial fibrosis and diastolic dysfunction are lighter [
40].
With the deepening of research on HCM, accurate classification for genomics and proteomics has also been proposed. Gene-phenotype research has proved that more than half of HCM patients have classic HCM-related gene mutations, with MYH7 and MYBP3 mutations accounting for the largest proportion. Such patients have worse cardiac pathology [
41]. The onset time of patients with MYH7 mutation is earlier than those without sarcomatous gene mutation, and the probability of ventricular arrhythmia and heart transplantation is higher than with MYBPC3 mutation. Patients with sarcomatous gene mutations are more likely to develop SCD [
42]. All sarcomere gene mutations are associated with diastolic dysfunction and left atrial remodeling [
39], and multiple sarcomere variation has a worse phenotype [
43]. Fifty percent of sarcomere mutation carriers show HCM within 15 years, so predictive gene screening is of great significance [
44]. Generally speaking, according to the sarcomere structure involved by the pathogenic gene, HCM-related gene mutation has been divided into coarse myofilament mutation, fine myofilament mutation, Z-disk mutation, calmodulin mutation, and cytoskeleton mutation [
45]. MHY7 and MYBP3 are the main pathogenic genes of coarse myofilament mutations, which are related to worse clinical features, pathological morphology, onset time, and mortality. TNNT2 and TPM1 are the most common mutations in fine myofilament mutants. Compared with coarse myofilament mutants, they show more related late left ventricular dysfunction, heart failure, and diastolic dysfunction, and lower incidence of outflow tract obstruction [
46]. The penetrance rate is higher in patients with TPM1 mutation [
47]. Z-disk mutations mainly include FHL1 and CSRP3 mutations, and CSRP3 heterozygous carriers often show late-onset and low-risk HCM [
48]. FHL1 is a new mutation associated with type 6 Emery-Dreifuss muscular dystrophy. It still shows more rapid disease progression, but the current research is not sufficient [
49]. Calmodulin-related gene mutations including TNNC1, PLN, and JPH2 are associated with a higher incidence of arrhythmia [
50]. As cytoskeleton-related genes, FLNC and FHOD3 mutations have been confirmed by recent studies to cause HCM, and FLNC mutations prefer SCD events [
51]. At present, there is still a lack of large-scale cohort studies on non-sarcomere proteins, which has the potential to become a new research direction for the pathogenesis and prognosis of HCM.
In addition, some studies have found that HCM-associated mutation is associated with HCM family history, SCD family history, greater left ventricular wall thickness, earlier diagnosis age, reversed partial curvature, higher late gadolinium enhancement, and less static LVOT obstruction, the negative mutation is more likely to cause basal septal hypertrophy, but whether these gene differences can be used as independent risk factors remains to be researched [
52,
53], which reflect the contribution of gene heterogeneity to the differential prognosis of HCM. Proteomic studies classified HCM into four molecular precise subtypes A to D, and there was no difference in the mutation-positive rate between these subtypes. The heart function of subtype D is worse, and it is more prone to heart failure, leading to major adverse cardiovascular events. In subtype D, Ras/MAPK, IP3/Akt, and TGF-β upregulated, indicating that subtype D may be related to myocarditis and fibrosis [
17]. These results suggest that we may have a precise genotyping method of HCM based on the molecular level, which can explain the generation of morphological differences, and uniformly describe the differences between different genetic-phenotype-prognosis, and guide the precise clinical management of HCM.