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ADH is generally regarded as a transitional lesion within the biological spectrum of breast epithelial proliferations, representing an intermediate step between UDH and low-grade DCIS. One of the most compelling arguments that atypia represents a transitional state is the striking morphologic and structural affinity be... |
Microscopically, ADH is characterized by a relatively uniform (monomorphic) epithelial population showing mild to moderate nuclear enlargement and minimal pleomorphism. The architectural arrangement often mirrors that of low-grade DCIS, displaying cribriform, micropapillary, or solid patterns; however, these features a... |
Because ADH clones possess many (though not all) features of low-grade DCIS, they are anatomically and biologically poised for further progression. Indeed, the threshold from ADH to low-grade DCIS is sometimes subtle and somewhat arbitrary (extent, degree of involvement, continuity) in diagnostic practice [21,40]. Thus... |
#### 3.2.3. Epidemiologic and Clinical Evidence: Progression Risk Linked to Atypia |
From the population and cohort studies, the differential risk conferred by AH (vs. proliferative disease without atypia) is pronounced and consistent. Whereas non-atypical proliferative disease confers a RR of approximately 1.5 to 2, AH confers approximately 4–5× risk of future carcinoma [9]. Moreover, long-term follow... |
Observational cohorts of ADH (particularly “pure ADH” without concurrent lesions) show meaningful rates of later carcinoma, consistent with a subset of these lesions evolving over many years [21]. Additionally, in analyses of biopsy upgrade rates (concurrent, initially occult carcinoma), ADH diagnoses on core biopsy ar... |
In conclusion, atypia is a powerful discriminator of risk and a clinically actionable histologic marker of progression potential. |
#### 3.2.4. Microenvironmental and Epithelial Constraints: How Atypia May Loosen Barriers |
The molecular architecture of atypical lesions (discussed below) suggests a “halfway house”, offering sufficient neoplastic alteration to confer a selective advantage but not yet enough to cross the barrier into invasion. Therefore, while molecular alterations are necessary, they may be insufficient for progression wit... |
The progression from benign epithelial proliferation to neoplasia also depends on changes within the cellular microenvironment that relax usual growth constraints. Atypical epithelial proliferation may represent a stage at which these regulatory barriers begin to fail, allowing for architectural disorganization and ear... |
In parallel, reciprocal communication between the epithelial and stromal compartments is altered. Atypical cells may produce or respond abnormally to growth factors, cytokines, and matrix metalloproteinases (MMPs) that remodel the extracellular matrix, increase matrix stiffness, and recruit fibroblasts and immune cells... |
Additionally, atypical epithelial populations often display increased resistance to apoptosis and enhanced survival under oxidative or replicative stress, enabling their persistence in settings where non-atypical cells would undergo programmed cell death [44]. Overall, atypia may therefore represent not only a morpholo... |
Among untreated DCIS, 25–60% are believed to progress to invasive carcinoma over years to decades, depending on grade, molecular subtype, and microenvironment [45]. However, not all DCIS progress to invasive disease. Microenvironmental suppression, immune surveillance, and stromal constraints continue to play a role. I... |
In AH, reciprocal interactions between epithelial and stromal compartments play a pivotal role in fostering a pro-tumorigenic microenvironment. Atypical epithelial cells secrete or aberrantly respond to growth factors, cytokines, and MMPs, which activate stromal fibroblasts into cancer-associated fibroblasts (CAFs) [47... |
In conclusion, not all atypical lesions are destined to progress. The molecular and morphological heterogeneity within AH means that some lesions may remain indolent, others may regress, and only a subset evolves. The presence of atypia is necessary but not sufficient; additional factors (extent, molecular burden, micr... |
Given the importance of atypia, the pressing translational challenge is to differentiate which atypical lesions will progress from those that will remain stable. Biomarkers such as gene-expression risk scores, mutational burden, spatial heterogeneity metrics, and imaging correlates are being explored [49,50]. |
#### 3.2.5. Specific Molecular Mechanisms from Hyperplasia to Neoplasia |
The progression from MDH to breast cancer entails molecular alterations disrupting homeostasis, fostering genomic instability, and enabling transformation, with variations by subtype risk. Low-risk subtypes, like UDH, show minimal, reversible changes without clonal dominance. Genetic changes are rare, including sporadi... |
## 4. Genetic Causes and Mechanisms of Mammary Gland Hyperplasia: From Atypia to Malignant Transformation |
The likelihood that a given hyperplastic lesion will progress to cancer correlates with the accumulation of specific genetic mutations and epigenetic modifications. Recent research has revealed how these molecular events, from driver gene mutations (e.g., in PIK3CA, BRCA1/2, TP53) to DNA methylation changes and microRN... |
As we have seen, UDH is a proliferation of ordinary ductal epithelium without atypia and is not considered an actual tumor. Generally, it is polyclonal. Molecular analysis shows that UDH lesions typically lack the consistent clonal genetic alterations seen in atypical lesions [48]. Nevertheless, modern sequencing has d... |
From a molecular point of view, ADH is a clonal neoplastic proliferation and is considered a genetically advanced precursor lesion [48]. ADH frequently harbors driver mutations commonly seen in luminal-type breast cancers, such as PIK3CA mutations (the catalytic subunit of PI3K), which are present in a large fraction o... |
[TABLE] Common genetic alterations in subtypes of mammary ductal hyperplasia (MDH). |
Lesion Type | Characteristic Genetic Alterations (Mutations and Copy Number Changes) |
Usual Ductal Hyperplasia (UDH) | Polyclonal proliferation with minimal or no clonal driver mutations. Occasional activating PI3K-AKT pathway mutations (e.g., PIK3CA hotspot mutations) have been detected. It lacks consistent chromosomal aberrations; considered a benign hyperplastic response rather than true neoplasia [4... |
Atypical Ductal Hyperplasia (ADH) | Clonal neoplastic lesion sharing many alterations with low-grade DCIS. Frequent PIK3CA mutations (on the order of 30–40% of cases). Loss of chromosome 16q (harboring CDH1 and other genes) is common. Other recurrent mutations in luminal breast cancer genes (e.g., GATA3, MAP3K1, CBFB g... |
Atypical Lobular Hyperplasia (ALH) | Almost uniformly associated with E-cadherin loss due to CDH1 gene inactivation. This occurs via CDH1 gene mutation or 16q LOH in the majority of cases. Often accompanied by PIK3CA gene mutations similar to those in invasive lobular carcinoma. Occasional mutations in the AKT1 or PTEN... |
Columnar Cell Lesions (CCL) and Flat epithelial atypia (FEA) | High frequency of early oncogenic mutations despite bland histology. PIK3CA-activating mutations are present in 50–60% of cases, making this a hallmark of FEA/CCLs. Some cases show modest copy number changes (e.g., gains on 1q; losses on 16q or 17p). Clonal... |
[TABLE] Key molecular drivers in the malignant progression of atypical mammary hyperplasia. |
Gene/Locus | Pathway/Function | Frequent Alterations in ADH/FEA | Pathological Implication | Reference |
PIK3CA | PI3K/AKT signaling (oncogene) | H1047R, E545K, E542K (missense mutations) | Frequent in FEA/ADH and luminal DCIS/IBC; early driver; sustained proliferation; defines low-grade pathway | [56] |
TP53 | Genome integrity/apoptosis (tumor suppressor) | R175H, R248Q/W, R273H/C(missense mutations) | Subset of atypia/DCIS on progression trajectory; common in high-grade disease; often with ERBB2 amplification | [50] |
BRCA1 | Homologous recombination DNA repair (tumor suppressor) | c.68_69delAG (185delAG), c.5266dupC (5382insC); numerous truncations | High-risk hereditary context; genomic instability in early clones | [57] |
BRCA2 | Homologous recombination DNA repair (tumor suppressor) | Multiple frameshifts/nonsense variants across exons | High-risk hereditary context; facilitates mutation accumulation | [58] |
GATA3 | Luminal Lineage Transcription Factor | Frameshift/truncations; splice-site alterations | Supports luminal differentiation program in HR+ precursors | [59] |
CDH1 | E-cadherin; cell–cell adhesion (tumor suppressor) | Bi-allelic inactivation/truncating mutations | Marks the lobular pathway (atypical lobular hyperplasia to lobular carcinoma) | [60] |
AKT1 | Serine/threonine kinase downstream of PI3K (oncogene) | E17K (PH domain hotspot) | Activates AKT independent of PI3K, specific to HR+ lesions | [61] |
PTEN | PI3K negative regulator; lipid phosphatase (tumor suppressor) | Loss-of-function mutations/deletions; promoter silencing | Cooperates with PIK3CA activation in atypia/DCIS | [48] |
ERBB2 (HER2) | Receptor tyrosine kinase (oncogene) | Amplification; S310F/Y; Y772_A775dup (kinase insertion) | Low-level expression in some early lesions; targetable alterations | [62] |
ERBB3 (HER3) | Receptor tyrosine kinase; dimerizes with HER2 | E928G and kinase-domain substitutions | Co-occurs with ERBB2 mutations; luminal contexts | [59] |
RUNX1/CBFB | Core-binding factor complex; lineage and differentiation | RUNX1 Runt domain loss-of-function; CBFB alterations | Luminal tumors/precursors; tumor-suppressor roles | [48] |
MAP3K1 | MAPK signaling; luminal A-associated | Multiple truncations/missense; pathway-inactivating | Co-mutation patterns with PIK3CA/CBFB in luminal A | [55] |
ESR1 | ER-alpha(nuclear receptor) | Ligand-binding domain: Y537S, D538G (rare in primary) | Rare in early lesions; prevalent in endocrine-resistant metastases | [63] |
FOXA1 | Pioneer transcription factor for the ER program | Wing2/DBD hotspots (e.g., R219); indels affecting DNA binding | Luminal program maintenance in HR+ tumors; early presence reported | [64] |
KMT2C (MLL3)/KMT2D (MLL2) | Histone H3K4 methyltransferases (chromatin modifier) | Truncating mutations across coding exons | Epigenetic deregulation in luminal tumors/precursors | [65] |
ARID1A/ARID1B | SWI/SNF chromatin remodeling complex (tumor suppressor) | Frameshift/nonsense mutations; loss of protein expression | Subset of HR+/basal lesions; chromatin accessibility shifts | [66] |
NCOR1/NCOR2 | Nuclear receptor co-repressors; ER signaling modulation | Truncations and splice variants | Associated with endocrine response and luminal programs | [67] |
CREBBP/EP300 | Histone acetyltransferases; transcriptional co-activators | Loss-of-function mutations | Transcriptional reprogramming in early neoplasia | [65] |
PIK3R1 | PI3K regulatory subunit p85α (tumor suppressor-like) | Truncations/indels disrupting SH2 domains | Cooperates with PIK3CA in PI3K activation | [51] |
NOTCH1 | NOTCH signaling (context-dependent) | PEST-domain truncations; activating mutations | Subset of breast lesions; cross-talk with ER and PI3K | [50] |
Genetically, lobular neoplasia (ADH and ALH) is defined by inactivation of the CDH1 gene, which encodes E-cadherin. E-cadherin loss is observed in more than 90% of ALH/LCIS, either through mutations in the CDH1 gene or 16q22 loss of heterozygosity [45,48]. This loss abrogates cell–cell adhesion and is a defining early ... |
While genetic mutations lay the foundation for neoplastic transformation, epigenetic modifications add an additional layer of dysregulation in MDH. Epigenetic changes, heritable alterations in gene expression that do not stem from DNA sequence mutations, are now understood to occur early in breast lesion development [4... |
Aberrant DNA methylation is one of the earliest and most consistent epigenetic abnormalities observed during breast carcinogenesis. Even at the stage of hyperplasia, especially in AHs, specific genes acquire promoter hypermethylation, leading to transcriptional silencing. For example, the promoters of tumor suppressor ... |
MicroRNAs (miRNAs) are 20–22 nucleotide non-coding RNAs that post-transcriptionally regulate sets of target genes. Altered miRNA expression profiles are well documented in breast cancers, and emerging evidence indicates that miRNA dysregulation starts at pre-cancerous stages [70]. In fact, certain miRNA changes in hype... |
### 4.1. Mechanism of Malignant Transformation |
The process by which ADH transitions into carcinoma involves the accumulation of multiple genetic mutations in key pathways governing cellular growth, survival, and apoptosis [71]. These mutations enable hyperplastic cells to evade normal regulatory controls, fostering an escalating potential for invasive behavior. Thi... |
Additionally, the hypothetical multistep model of breast carcinogenesis proposes progression from normal epithelium to invasive carcinoma through intraductal hyperplasia (IDH), with and without atypia, including ADH and DCIS, and cytogenetic and molecular-genetic analyses reveal accumulation of genetic alterations [50]... |
#### 4.1.1. Gene Expression Changes |
The onset of atypia in ADH is frequently associated with alterations in gene expression of oncogenes (e.g., HER2/ERBB2, Cyclin D1) and tumor suppressor genes (e.g., TP53, BRCA1/2), which histopathologically manifest as proliferative and architectural abnormalities. Transcriptional profiling of matched normal, ADH, and ... |
Matrix metalloproteinase-1 (MMP-1) emerges as a potential progression biomarker, with elevated expression in synchronous versus pure ADH, aligning with basement membrane discontinuities visible on periodic acid-Schiff (PAS) staining [25]. Furthermore, hub gene analyses identify RRM2, TOP2A, PBK, MELK, and NUSAP1 as pro... |
These expression shifts are more pronounced in premenopausal women, in whom higher-grade features, such as punctate necrosis, predict malignant progression, underscoring hormonal influences on histopathological progression [79]. Cyclin D1 gene amplification and protein overexpression occur in benign breast disease and ... |
In vitro hypoxia models show increased Hypoxia-Inducible Factor-1 alpha (HIF-1α), Glucose transporter 1 (GLUT1), and Carbonic Anhydrase IX (CAIX) expression, with in vivo IHC absent in normal, DH, or ADH but present in DCIS (GLUT1 56.8%, CAIX 25.0%) and IDC (GLUT1 44.1%, CAIX 30.5%), higher in high-grade lesions (p = 0... |
#### 4.1.2. Loss of Tumor Suppressor Genes |
The inactivation of genes such as PTEN, p16, and RB often leads to uncontrolled cell division and contributes to the progression of malignancy, as evidenced by histopathological findings including loss of cellular polarity and increased nuclear pleomorphism. Loss of heterozygosity (LOH) at tumor suppressor loci is an e... |
In high-risk families, ADH prevalence reaches 39%, with BRCA1/2 mutations enhancing suppressor loss, as seen in prophylactic mastectomy specimens with multifocal atypia and microcalcifications [90]. These inactivations drive morphological shifts, such as the formation of rigid bridges and solid growth, underscoring the... |
#### 4.1.3. Genetic Instability |
Atypical lesions tend to harbor genomic instability, including chromosomal aberrations and mutations in critical loci, particularly those involved in DNA repair and cell cycle regulation (e.g., BRCA1 and TP53 mutations), which are histopathologically evident as aneuploidy and clonal expansions [94]. Aneuploidy is detec... |
Centrosomal abnormalities, with increased α- and γ-tubulin expression on IHC, are observed in ADH and carcinoma, indicating mitotic spindle defects that correlate with aneuploidy and invasive foci. Molecular profiles reveal advanced changes, including gross chromosomal rearrangements and epigenetic alterations in AH, w... |
Genetic alterations are absent in fibroadenomas, even associated with cancer, but LOH in DCIS within fibroadenomas, indicating no role in carcinogenesis [98]. c-erbB-2 amplification is absent in hyperplasia and ADH, but present in high-grade DCIS (10–40% in IDC), highlighting instability in the precursor to invasive tr... |
#### 4.1.4. Microenvironmental Factors |
The surrounding tissue microenvironment, including stromal changes and inflammatory responses, can promote the progression of atypical lesions into invasive cancer, with histopathological evidence of field cancerization and multifocality [100]. Ipsilateral predominance of subsequent breast carcinoma (2:1 ratio) suggest... |
Stromal alterations, such as desmoplasia and calcification, are common in ADH, as revealed by H-E staining, which shows periductal fibrosis and immune infiltrates that promote progression [101]. Lobular involution extent inversely correlates with risk, with absent involution (SIR 7.66) associating with denser stroma an... |
Microenvironmental hormones, like in premenopausal women, heighten the risk of higher-grade carcinomas, with features like punctate necrosis on core biopsy predicting upgrade [104]. Field cancerization explains the multifocal appearance of ADH (higher risk if calcified), with non-clonal origins in cancer-prone tissue, ... |
Overall, these factors, along with genetic changes, drive transformation, advocating for microenvironment-targeted interventions. Extra-tumoral tissue in breast carcinoma patients shows ductal hyperplasia effects similar to non-neoplastic tissue under oral contraceptive use, indicating stromal-inflammatory influences [... |
## 5. Epigenetic Modifications in Atypical Hyperplasia and Their Role in Malignant Transformation |
### 5.1. General Features |
Epigenetic reprogramming plays a key role in mammary tumorigenesis, favoring the progression from AH to invasive carcinoma by altering gene expression, promoting a stem cell-like state, and enabling EMT [110]. The main epigenetic mechanisms comprise alterations in DNA methylation, histone modifications, non-coding RNA ... |
### 5.2. The Main Epigenetic Mechanisms Involved in Breast Tumorigenesis |
#### 5.2.1. Loss of Imprinting |
It was found that genomic imprinting has a pivotal role in development and evolution. Loss of imprinting (LOI) represents an initial phase alteration in neoplasia. Imprinting is defined as the monoallelic expression of genes in a parent-of-origin-specific approach [114]. It was shown that in diploid eukaryotes, the mat... |
LOI develops in a biallelic expression from the stimulation of the dormant allele. Research in mice revealed that demethylation of imprinted genes leading to LOI made cells more sensitive to cellular transformation and carcinogenesis [52]. Initiation of a growth-promoting allele leads to aberrant cell multiplication, a... |
LOI represents an essential abnormality in breast neoplasia growth and evolution, concerning the aberrant expression of imprinted genes linked to epigenetic dysregulation, usually DNA hypomethylation [31]. Reports have discovered imprinted genes such as HM13, KCNQ9, IGF2 and PEG1/MEST that display LOI in breast cancer,... |
Research highlighted that HM13 shows LOI and expression upregulation in mammary tumors, involving DNA demethylation and possibly related to better prognosis in different types of cancer [121]. |
LOI at the KCNQ9 locus has been involved in the overexpression of the TASK3 potassium channel and is linked to triple-negative breast cancer [122]. The connection between KCNQ9 hypomethylation and triple-negative breast cancers indicates that targeting the KCNK9 DMR may represent a probable approach for inhibiting this... |
Persistent LOI of PEG1/MEST has been detected in invasive breast carcinomas, indicating its role in the evolution from hyperplasia to invasive cancer [31]. LOI of the IGF2 gene could determine its overexpression, supporting the growth of tumor-initiating cells and overall chromatin variability [123]. |
#### 5.2.2. Reactivation of Developmental Pathways |
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